How to Calculate Square Feet
Square feet is one of those things you need more than you'd expect. Paint a room. Buy a rug. Price out flooring. Figure out if a couch will even fit. The number comes up constantly. And most people forget how to work it out about five minutes after they leave school.
That's fine. The math is easier than you remember, honestly. It's basically one formula with a bunch of variations for different shapes.
I had to measure my whole kitchen last year to order tile. Ended up measuring three other rooms the same week. So this is written from someone who has actually done it, not just read about it.
What a square foot actually is
A square foot is a square that's one foot on each side. That's it. One foot by one foot. Twelve inches by twelve inches.
If you lay down a tile that's exactly 12 inches on every side, you've covered one square foot of floor. Lay ten of them in a row and you have 10 square feet. Lay ten more rows on top of that and you've got 100.
So when somebody says an apartment is 800 square feet, they mean the floor fits 800 of those one-foot tiles. Nothing more complicated than that really.
The short form is sq ft. Or ft². Or sometimes just SF on a floor plan. All the same thing.
The formula everyone uses
For a rectangle, you multiply length by width. Both numbers in feet. That's the whole formula right there.
Length times width. That's it really.
A room that is 10 feet long and 12 feet wide is 120 square feet. A room 15 by 20 is 300. An 8 by 8 closet comes out to 64.
The trick is making sure both numbers are in feet before you multiply. Mix up inches and feet in one equation and the answer is garbage. Won't mean anything useful.
Measuring the room first
Grab a tape measure. A 25-foot tape is fine for most rooms in a house. Longer rooms and open floor plans need a longer tape. Or a laser measurer if you want to spend the money.
Stand in one corner. Run the tape to the opposite wall along the length. Write down the number in feet.
Then do the same along the width.
Multiply the two numbers. You have your square footage.
Pretty much anyone can do this. The hard part is being honest about the numbers. Don't round up to make the math easier or because it feels close enough. Round down if anything.
When the number isn't whole
Rooms are rarely exactly 10 by 12. They're usually something ugly like 10 feet 4 inches by 12 feet 7 inches. Houses settle. Walls aren't straight. Nothing is ever quite square.
You have two options for handling this. Convert everything to feet as a decimal. Or convert everything to inches and convert back at the end.
Here's the easy version. There are 12 inches in a foot. So 6 inches is 0.5 feet. And 3 inches is 0.25 feet. Nine inches works out to 0.75 feet.
A wall that's 10 feet 6 inches is 10.5 feet. A wall 12 feet 3 inches is 12.25 feet.
Multiply 10.5 by 12.25 and you get 128.625 square feet. Round that to 129 if you want. Nobody is going to argue with you over less than a square foot.
If the inch number is weird, like 4 inches, divide 4 by 12. You get 0.333. So 10 feet 4 inches is 10.333 feet. Ugly decimal but it works.
When the numbers are only in inches
Some small projects only need inches. A countertop. A shelf. A bathroom backsplash that's only a couple feet across. Stuff like that.
Multiply the inch measurements together. That gives you square inches. Then divide the total by 144. You're back to square feet.
Why 144? It comes from 12 times 12. A square foot has 12 inches on each side, which means 144 square inches fit inside it.
So a countertop 30 inches by 60 inches comes out to 1,800 square inches. Divide by 144 and you get 12.5 square feet.
Remember that number. 144. It's the conversion you'll use the most for small projects.
Rooms that aren't rectangles
Most rooms aren't perfect rectangles, once you actually look at them. There's a closet jutting out. A nook. A chimney taking up one corner. A bay window that sticks out the front.
The trick is to split the room into rectangles. Measure each rectangle on its own. Then add them up at the end.
Say your main room is 12 by 15, which comes out to 180 square feet. The closet sticking off one side is 3 by 5, which is 15 more. Add them and the total is 195 square feet.
Subtract if something is missing instead. A corner cut out for a support column that measures 2 by 2 means subtracting 4 from the total.
Basically you're doing addition and subtraction. The shapes just happen to be rectangles.
For really weird rooms, draw them out on paper first. Label each section. Then work through them one at a time so you don't miss any.
Circles
Circles come up for patios and pools and weirdly shaped rugs. The formula is the same one you probably learned as a kid and forgot.
Area equals pi times the radius squared.
Pi is 3.14159 if you want to be precise about it. 3.14 is fine for most everyday stuff. The radius is half the diameter, which is the distance across the middle.
A round patio 10 feet across has a radius of 5 feet. Square the 5 and you get 25. Multiply by 3.14. The answer is 78.5 square feet.
A round rug 8 feet across. Radius is 4. Squared is 16. Multiply by 3.14 and you get about 50 square feet of rug.
For a circular above-ground pool, the same formula applies. A 15-foot pool has a radius of 7.5 feet. Square that and you get 56.25. Times pi gives you about 177 square feet of surface area.
Triangles
Triangles are half of a rectangle. The formula shows that.
Base times height. Divide by 2. Done.
A triangle with a 10-foot base and a 6-foot height is 30 square feet. Ten times six. Then half it.
The height has to be measured straight up from the base. Not along the slanted side. People get this wrong a lot. I got it wrong the first time I tried to figure out a weird corner of a room.
If you only know the three sides and not the height, the math is worse. Heron's formula does it. You add the sides, halve it, call that s. Then square root of s times s-a times s-b times s-c.
Honestly nobody does this by hand. Use your phone.
Trapezoids and other weird shapes
A trapezoid has two parallel sides of different lengths. Think of a counter that narrows at one end. Or a patio against an angled fence.
Add the two parallel sides. Divide by 2. Multiply by the height between them.
Say your counter is 4 feet on one side and 6 feet on the other. It's 3 feet deep. Four plus six is ten. Ten halved is five. Five times three is fifteen. So 15 square feet.
Parallelograms are easier. They're pushed-over rectangles. Base times height, same formula.
The height is the straight-up distance. Not the slanted side. People mix that up.
A sector is a slice of a pie. You need the angle. For degrees, the formula is angle over 360, times pi, times the radius squared. A quarter circle with a 10-foot radius. That's 0.25 times 3.14 times 100. So 78.5 square feet.
A ring shape is the outer circle minus the inner. Do both circles. Subtract the small one from the big one. That's the ring.
What the conversions look like
Sometimes the numbers come in other units. Square yards. Square meters. Acres if you're dealing with land.
An acre is 43,560 square feet. Oddly specific. My understanding is it came from English farmers measuring what an ox could plow in a day. Not a round number but that's the real one.
A square yard is 9 square feet. Three by three. Carpet stores still quote in yards sometimes. Annoying but you get used to it.
A square meter is 10.76 square feet. I usually just round to 11 in my head and call it close enough.
A square inch is about 0.00645 square feet. Basically nothing. Which is why you use 144 going the other way. Much easier.
Save these somewhere if you do any work that mixes units. I have them in a note on my phone.
How many tiles or gallons you need
This is where the number starts to matter.
Paint coverage depends on the paint. 200 to 400 square feet per gallon is the usual range. Rough walls eat more. Smooth drywall eats less. Primer is worse than finish paint. And going dark over light always needs two coats. Sometimes three.
Say your room has 400 square feet of wall. A gallon covers one coat. You'll want two gallons for two coats. Which you'll almost always need.
Tile is similar but with waste added. Measure the floor. Buy 10 percent more than that. Some tiles break. Some get cut ugly at the edges. And keep a few boxes in the garage for when one cracks in a few years.
So a 100 square foot floor means buying 110. Or 115 if the room has a lot of corners or you're new at cutting tile. I always over-buy a little. Returns are easier than second trips.
Wallpaper is messier. Rolls usually cover about 56 square feet, but you lose a lot to pattern matching. Add 15 percent minimum. More for busy patterns. A friend of mine ran out halfway through a bedroom because she didn't pad enough.
Flooring costs
Flooring prices are all over the place. A few bucks a square foot up to hundreds. Tile can start at 60 cents. Fancy marble can hit thousands. My neighbor paid more for her kitchen tile than I paid for a used car.
Laminate beats real wood on price and scratches. But it can't be refinished. One bad water spill and it's done.
Engineered wood is the middle option. Real wood on top of plywood. Handles moisture better than solid hardwood. Works in basements.
Bamboo is cheap and easy. But it scratches. Dog claws are the killer. High heels too.
Solid hardwood is the most expensive of the normal options. Lasts forever if you treat it right. Needs sanding and refinishing every ten or twenty years.
Vinyl plank deserves a mention. Looks like wood. Handles water. Cheap. I put it in my laundry room and it's been fine for three years.
Painting a room
For walls, the math changes a little. You need the surface area of the walls, not the area of the floor.
Measure the perimeter of the room first. That's all the walls added together, the distance around. Multiply that number by the ceiling height. That gives you your wall square footage.
A 10 by 12 room has a perimeter of 44 feet. Ten plus twelve is twenty-two, times two is forty-four. If the ceiling is 8 feet high, the total wall area comes out to 352 square feet before subtracting anything.
Subtract for doors and big windows now. A standard interior door is about 20 square feet. A big window is around 15. Small windows you can usually ignore unless there are a lot of them.
So that 10 by 12 room minus one door is about 332 square feet of wall. One gallon of paint will probably cover one coat. Two gallons for two coats, which is what most jobs actually need.
Don't forget the ceiling if you're painting that too. Same square footage as the floor in most cases, assuming the ceiling is flat.
For trim and baseboards, professional painters usually just eyeball it or add a fixed amount per room. There's no clean formula. A quart of trim paint handles a small room's worth of trim most of the time.
Buying a house
When real estate listings say a house is 2,400 square feet, they usually mean the heated indoor living space. Not the garage. Not the deck. Not the basement unless it's finished.
Different cities measure differently though, which is where it gets confusing. Some include finished basements in the total. Some include covered porches. Some don't count anything with a sloped ceiling under 7 feet high.
This matters for price per square foot. Say one house is listed at 2,000 square feet and another nearby is at 2,400. That difference could be a finished basement. Or just how the listing agent decided to measure. Always worth asking what's included in the number.
Price per square foot is just the list price divided by the square footage. A $300,000 house at 2,000 square feet comes out to $150 per square foot.
Compare that to nearby houses. That's how you tell if a place is priced fair or not. Same metric appraisers use when they set the official value.
Common mistakes
The biggest mistake is mixing units. Feet and inches in the same equation. Or feet and yards. It messes up the answer, often by a lot.
Convert everything to the same unit first. Then multiply. Always.
Another common mistake is measuring only one wall and assuming the opposite wall is the same. Houses are rarely that perfect, especially older ones. Walls can be off by an inch or two across a room. Measure both walls. Use the shorter number if they differ. Matters most for flooring and carpet that needs to fit wall to wall.
People also forget about doorways and closets. If you're measuring a room, do you include the closet? Depends on what the number is for. For flooring, yes. For painting, probably not since closets usually aren't painted the same color as the main room.
And don't forget to add waste for flooring. Ten percent minimum. More like fifteen percent if the room has weird angles or diagonal cuts involved.
One more mistake. People measure the outside of a wall instead of the inside. For interior rooms you want the inside measurement. For a whole-house footprint you want the outside. They can be a foot different because of wall thickness.
Using graph paper
For weird rooms, graph paper helps a lot. Let each square equal one foot. Draw the room to scale, including any jogs or closets or jutting corners.
Then just count the squares inside. Whole squares count as one. Partial squares you eyeball and add up as halves or quarters.
My mother does this every time she rearranges furniture. She draws the room on graph paper. Then she cuts out little paper rectangles for the couch and chairs. Moves them around on the paper until something works.
It sounds old-school but the method works. A lot better than moving real furniture back and forth five times. Just to figure out what actually fits.
Decks and patios
Outdoor spaces work the same way as indoor rooms. Length times width for rectangles. Break weird shapes into pieces and add them up.
Decks especially can have odd shapes. A deck wrapping around a corner of the house has two rectangular sections that need measuring separately.
For deck boards, figure out the square footage. Then add about 10 to 15 percent for waste. Boards come in fixed lengths, so there's always some cut-off material. Composite decking costs way more per square foot than pressure-treated pine. But it lasts much longer without needing yearly maintenance.
For a concrete patio, the square footage tells you how much concrete to order. Concrete is usually sold in cubic yards, so you also need the thickness of the slab. A 4-inch-thick patio means the depth is one-third of a foot. Multiply square footage by one-third to get cubic feet, then divide by 27 to get cubic yards.
That's the main time square footage alone isn't enough for outdoor work. Concrete and mulch and gravel all need volume, not just area.
Yards and lawns
For a lawn, measure the grass area like any room. Subtract the house footprint. The driveway. Garden beds. Anything that isn't grass.
Lawn size tells you how much seed or fertilizer or weed killer to buy. Lawn products list coverage in square feet right on the bag.
Say a 40 by 50 backyard. That's 2,000 square feet before you take anything out. Take off 200 for a deck. Another 100 for a garden. You end up at 1,700 square feet of grass.
Sod is weird on pricing. Sometimes per square foot. Sometimes per pallet. A pallet covers about 450 square feet. Figure out your lawn first. Then the store tells you the pallet count.
Mulch and gravel
Mulch bags show coverage at a specific depth. A bag that covers 12 square feet at 2 inches deep covers 24 at 1 inch. Or 6 at 4 inches. Thickness and area scale against each other.
Depth is the variable most people forget. A 100 square foot bed at 3 inches deep needs way more than 1 inch deep. Roughly three times more. I learned this the hard way one spring.
Bulk mulch by the cubic yard covers around 108 square feet at 3 inches. 162 at 2 inches. Same setup as concrete. Different depth.
Landscaping rock is heavier and priced different. Round up always. Running short on a Saturday afternoon project is the worst feeling.
Carpet
Carpet is weird. Older stores still sell it by the square yard. Everything else uses square feet. A square yard is 9 square feet, so divide if you need to compare.
Say a carpet is $30 a square yard. That's about $3.33 a square foot. The yard price always looks bigger but it's the same rug.
Carpet comes on rolls. Usually 12 or 15 feet wide. This is where ordering gets annoying. A 14-foot room and a 12-foot roll means a seam somewhere. Or you buy the 15-foot roll and trash a foot down the length.
Measure your room. Add 10 percent minimum for waste. Then let the carpet store figure out the roll width with you. They do this every day. You don't.
Roofing
Roofs are measured in squares. One square equals 100 square feet. A 30-square roof is 3,000 square feet. It's a weird unit but roofers use it.
The catch is the roof isn't flat. It's sloped. So the real surface area is bigger than the footprint of the house. Steeper pitch means more surface.
A low-slope roof adds maybe 10 to 15 percent to the footprint. A steep one can add 40 percent or more. Roofers use pitch factors to nail the number down. Most of us don't need that.
Shingles come in bundles. Three bundles per square usually. So a 3,000 square foot roof wants about 90 bundles. Add 10 percent for cuts.
Insulation and HVAC
Insulation is sold per square foot at a given R-value. Measure your attic floor or the wall surface. That's the starting number.
Batts come in rolls. Blown-in is sold by the bag. Each bag has a chart on it showing coverage at different depths and R-values. Read the bag.
HVAC sizing starts with square footage. A rough rule is 20 BTU per square foot for cooling. A 1,500 square foot house works out to around 30,000 BTU. Or a 2.5 ton unit.
But that's just a starting point. Windows change it. Ceiling height changes it. Climate changes it a lot. Real HVAC installers do something called Manual J which factors in everything. Don't just size your AC off the rule of thumb.
Rental calculations
In cities where rent is quoted per square foot, the math matters for apartment shopping. A 700 square foot apartment at $3 per square foot per month is $2,100 a month. Same rent in a 900 square foot place is $2.33 per square foot. Better deal on space. Even though the total rent is the same.
Commercial rent often works this way too, usually quoted as annual price per square foot. A 1,200 square foot office at $25 per square foot per year is $30,000 a year, or $2,500 a month.
Sometimes commercial leases add in a percentage for common areas like lobbies and hallways and shared bathrooms. That's called load factor. A 1,000 square foot office with a 15 percent load factor bills you for 1,150 square feet. Worth asking about before signing anything.
When to just use a calculator
For circles with odd numbers, use a calculator. For triangles where you only know the three side lengths, use a calculator. For anything involving pi, use a calculator. Phones all have them built right in now.
There's no shame in it. The point of knowing the formulas is to know what you're doing and why. Not to do long division by hand in a hardware store aisle while the line behind you gets impatient.
There are also free online square footage calculators for every shape you can think of. Rings, sectors, parallelograms, all the rest. Punch in the numbers and they spit out the answer. Useful when you're juggling a lot of measurements at once.
Why the number matters
Square footage is money. That's the short version of why you need to know this stuff at all.
Flooring is sold by the square foot. Paint is priced by coverage in square feet. Rent is quoted per square foot in a lot of cities. Real estate values are tied to it directly. Even property taxes factor it in, since bigger houses pay more.
Get the number wrong and you buy too much or not enough. Either way it costs you money or time or both.
Overbuying is usually better than underbuying, at least for materials. You can return extra tile or paint. You can't get the hardware store to reopen on a Sunday afternoon when you run out halfway through a job.
For cost comparisons across houses or apartments, square footage levels the playing field. A $400,000 house at 2,000 square feet is the same cost per square foot as a $600,000 house at 3,000. That number lets you compare places of different sizes on even footing.
A few tools that help
Laser measuring tools cost around $30 to $80 for a decent one and make the whole job faster. Point at a wall, press a button, read the number off the screen. No stretching a tape across the floor by yourself and watching it sag in the middle.
Graph paper and a pencil still work fine for sketching out a room. Cheap and reliable. Never runs out of battery.
Phone apps can measure rooms with the camera now, using LiDAR or photo analysis. Some of them are accurate. Some are not. Don't trust them for a big flooring order without double-checking with a real tape measure first.
A cheap calculator app handles all the math you'll ever need for square footage. No fancy features required. Just multiplication and division.
Bathrooms
Bathrooms are small but a pain. So many fixtures taking up floor space. Most of the tile work is on walls anyway, not the floor.
Measure the floor first. Then the walls getting tiled. A shower surround means three walls most of the time. Four if there's a tub enclosure too.
A standard tub is 5 feet by 30 inches. That's 12.5 square feet of floor gone. A toilet takes maybe 6 square feet with clearance. Nobody wants to sit jammed against a wall.
Shower tile. Measure each wall. Add them up. A 5 by 3 shower with 8-foot walls. Two long walls are 40 square feet each. Two short walls at 24. Total 128 square feet of tile.
Take out the door or curtain opening. Buy extra for the cuts around the shower head and faucet. And for the niche if you have one.
Bathroom tile waste is always higher. I usually plan for 15 percent. More cuts per square foot when the room is small.
Kitchens
Kitchens mix floor area, wall area, and counter space. Each one measured different.
For the floor, subtract the island and the cabinet footprints. Flooring only goes on the walkable part. So a 12 by 15 kitchen with a 3 by 6 island has 180 minus 18 feet of floor. Which is 162.
Countertops get measured in square feet too. But granite and quartz often get priced by the slab, not per square foot. Slabs run about 55 to 65 square feet each depending on the supplier.
A basic L-shape counter might be 25 to 35 square feet. A big kitchen with an island can hit 60 or 70. Two slabs for a big job. Sometimes three.
Backsplash tile uses the wall footprint. Measure from the countertop up to the bottom of the cabinets. Times the wall length. Most backsplashes work out to 20 to 40 square feet.
Garages and basements
Garage floors are usually easy rectangles. A two-car garage is typically 20 by 20 or 24 by 24. So 400 to 576 square feet.
Sealing or epoxy kits come rated for specific coverage. A one-car kit is usually 250 square feet. A two-car kit covers 500. Buy the bigger kit if you're in between.
Basements are messier. Unfinished basements usually get measured wall to wall, mechanical rooms included. Finished ones only count the living space.
For basement flooring, you have to deal with support posts and stair cutouts. Split the room into rectangles. Subtract whatever's in the way.
Dropped ceilings use square footage too. Same area as the floor. Tile sizes are usually 2 by 2 feet or 2 by 4 feet.
Pools and ponds
Pool surface uses the same formulas. Rectangular pools are length times width. Round pools are pi times radius squared. Kidney-shaped ones get broken into sections or measured on a grid.
Surface area matters for covers and solar blankets. A 16 by 32 pool wants a cover that's 16 by 32 minimum. Usually a foot or two bigger each way. So an 18 by 34 cover is about right.
Pool liners get more complex. The liner has to cover walls and floor together. But surface area is still the starting point. Pool places usually size the liner for you.
Ponds work similar. Surface area tells you the liner size for a new pond. Add twice the depth to each dimension. That's enough to go down the sides and tuck under a rock border.
Whole-house measuring
If you want to measure a whole house yourself, go room by room. Measure each room. Add them up. Don't try to measure the outside of the house and work backwards. Interior walls eat a lot of space.
Include hallways and closets in your total. Don't include the garage or attached storage. Not if it's not heated living space.
Staircases are counted once, not per floor. They take up space on the floor they start on. The opening on the floor above doesn't count again.
Ceilings under 7 feet high don't count in most official measurements. So the corners of an attic bedroom with sloped ceilings get excluded. This is why attic conversions sometimes list lower square footage than you'd expect from walking through them.
My own house measured out about 100 square feet smaller than the listing said when I did it myself. That's normal. Listings round up. Listing agents include things appraisers wouldn't.
Quick reference for common rooms
A small bedroom is usually 10 by 10 to 11 by 12. So 100 to 132 square feet. A master bedroom runs 14 by 16 or bigger. Often 224 square feet and up.
A standard bathroom is about 5 by 8. So 40 square feet. A half bath is smaller, usually 20 to 25 square feet.
Living rooms vary the most. A small one might be 12 by 14, or 168 square feet. A big open-concept living area can be 20 by 25 or more. That's 500 square feet before adding any connected dining or kitchen space.
Apartments run small. A studio is often 400 to 600 square feet total. A one bedroom runs 600 to 900. Two bedrooms start around 900 and go up from there.
Houses are bigger obviously. A small house is under 1,500 square feet. Medium is 1,500 to 2,500. Large is above that. A 4,000 square foot house is properly big. A 6,000 square foot house is getting into luxury territory.
A few last things
Always measure twice. That old advice is still right and there's a reason it gets repeated so often.
Round up for materials and round down for spaces you're trying to fit things into. If your room is 11.5 feet wide, don't order an 11.5 foot couch. Get 11 feet and leave yourself some room to move.
Keep your measurements written down somewhere. You'll need them again, probably sooner than you think. I keep mine in a note on my phone so I can pull them up at the store.
Square footage math isn't hard. It's mostly multiplication with a few conversions. The annoying part is measuring accurately and remembering which formula goes with which shape.
That's really the whole thing.