How to use the paint calculator
Add each room you plan to paint: its length and width along the floor and its height from floor to ceiling. Check "Paint the ceiling" for any room whose ceiling gets painted too. Then count the doors and windows and type their sizes; the boxes start with common-looking sizes, but measure yours. Pick the number of coats and type the coverage from your paint can's label, in square feet per gallon. Add a coat of primer or the baseboard trim if you need them. The answer is the paint to buy in whole cans, chosen so you carry home as few cans as possible without a big leftover, and a table with the walls, ceiling, trim and primer on their own lines, because they are often different paints. Type prices if you want a cost; the page never fills one in. The One wall tab is for a single wall, and What my paint covers tells you how far the cans in your garage will go.
The formula
paint = area × coats ÷ coverage
The wall area is the room's perimeter times its height. Each door and window comes off as width × height. Ceilings are length × width. Paint is area times coats divided by the coverage on the label. Primer is one coat over the walls and ceiling. Baseboard trim is the perimeter, minus the door openings, times the trim's height. The calculator then rounds the paint up to the next quart (or the next half liter in metric) and picks the cans.
A worked example
A bedroom is 12 by 14 feet with 8-foot walls, one door 3 feet by 6 feet 8 inches, and two windows 3 by 4 feet. The walls are 2 × (12 + 14) × 8 = 416 square feet. The door is 3 × 6⅔ = 20 square feet and the windows are 2 × 12 = 24, so 44 come off, leaving 372 square feet. Two coats at 350 square feet per gallon is 372 × 2 ÷ 350 = 2.13 gallons. That rounds up to 9 quarts, which is 2 gallons and 1 quart, with about 0.12 gallon left over. The ceiling is 12 × 14 = 168 square feet; two coats is 0.96 gallon, so one gallon. In metric, a 4 by 5 meter room with 2.5-meter walls, a 0.9 by 2 meter door and two 1.2 by 1.2 meter windows has 40.32 square meters of wall; two coats at 9 square meters per liter is 8.96 liters, so one 10-liter can with about 1 liter left.
How the cans are picked
Paint comes in fixed sizes, so the calculator never tells you to buy 2.13 gallons. It first rounds up to the smallest step, a quart (or half a liter). Then it looks at every way to buy that much or a little more and picks the one with the fewest cans, as long as the leftover stays under one gallon (under 2.5 liters in metric). When two choices have the same number of cans, it takes the one with less left over. So 3 quarts of need becomes one gallon, and 4 gallons and 1 quart becomes a 5-gallon pail. In US stores the sizes are the quart, the gallon and the 5-gallon pail. In metric the page uses 1, 2.5, 5 and 10 liter cans, common sizes in the UK and Europe; check what your store sells. In Canada most paint comes in the US sizes labeled in liters (946 mL and 3.79 L). The picture shows the leftover as a dashed outline in the last can of each line.
Where the numbers come from
The areas are plain geometry, and the gallon and liter are exact units (a US gallon is 3.785411784 liters, and a foot is 0.3048 meter, which is how the Imperial | Metric switch converts what you typed). Coverage is the one number that changes from can to can. It depends on the paint, the color, the surface and how you apply it, which is why the page asks you to read your label. The 350 square feet per gallon (and 9 square meters per liter) filled in to start is only a placeholder, not a standard. It sits inside what makers print: Behr lists 250 to 400 square feet per gallon for its Premium Plus interior paints, and Benjamin Moore lists 400 to 450 for Regal Select, with both saying the real number depends on the surface and how you apply it. No building code covers how much paint to buy. The calculator also doesn't allow for spills, a roller tray or touch-ups; that is what the leftover is for.
What people use it for
- Buying the right number of cans before a weekend repaint.
- Checking a painter's quote for the amount of paint.
- Planning a whole floor of rooms at once, with the ceilings in a different paint.
- Finding out whether the cans already in the garage are enough for a touch-up.
Common mistakes
- Forgetting the second coat. A color change usually needs two; the label says what the maker expects.
- Using the floor area. A 12 by 14 room has 168 square feet of floor but 416 of wall at 8 feet tall.
- Not taking out big openings. A sliding door or a picture window can be 30 square feet or more.
- Rough surfaces. New drywall, bare wood, brick and texture soak up more paint than the label's number.
- Buying all quarts. Four quarts hold the same as one gallon; the calculator never picks 3 quarts when a gallon will do.
Questions
How much paint do I need for a 12 by 12 room?
With 8-foot walls, one door 3 by 6 feet 8 inches and one window 3 by 4 feet, the walls are 384 − 20 − 12 = 352 square feet. Two coats at 350 square feet per gallon is about 2.01 gallons, so 2 gallons and 1 quart. Type your own sizes and coverage for your room.
Should I take out the doors and windows?
Yes, if you know their sizes; it can save a can on a room with big windows. The leftover the calculator allows covers small openings you didn't count.
How much does a gallon cover?
It depends on the paint, so read the can's label. The calculator starts with 350 square feet per gallon only as a placeholder to change.
Can I use metric?
Yes. The Imperial | Metric switch takes meters, square meters per liter and liter cans (1, 2.5, 5 and 10 L). It opens in the system your browser's region uses, converts what you already typed, and remembers your choice on this device.
Can I share my plan?
Yes. Share makes a link with a picture for Facebook, a link you can copy, and a short video for TikTok made on your device. The link keeps the room sizes, the doors and windows, the coats and the coverage for 90 days. It never keeps a price.