Paint Calculator
Work out gallons or litres of paint for walls and ceilings from room size, doors, windows, coats and the coverage on the can, then the cans or tins to buy.
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About this toolWhat is the Paint Calculator?
Paint is sold by volume and used by area, and the conversion between the two is printed on every can as a coverage figure. Most interior wall paints quote 350 to 400 square feet per US gallon for one coat on a smooth, primed surface, or roughly 10 to 12 square metres per litre. This paint calculator measures the area you are painting, multiplies it by the number of coats, and divides by that coverage, so the answer follows the product you are actually buying.
Enter the room length, width and wall height, choose walls, ceiling or both, and subtract doors and windows. The result shows the paint needed for walls and ceiling separately, the total, and the cheapest-looking way to buy it: 5 gallon buckets, gallons and quarts in the US, or 10, 5, 2.5 and 1 litre tins in metric. It never suggests three quarts, because three quarts usually cost more than a gallon.
Optional primer coats, an allowance for touch-ups, several identical rooms and can prices turn the estimate into a shopping list. Everything is calculated in your browser, and the share link reopens the same room.
- Walls, ceiling or both, with doors, windows and other areas subtracted
- Separate coat counts for walls and ceiling, plus optional primer
- Shopping list in 5 gallon, 1 gallon and quart sizes, or 10, 5, 2.5 and 1 litre tins
- Imperial and metric inputs that convert when you switch
- Cost from your own can prices, copy and share link, all in the browser
How to use the Paint Calculator
- Choose imperial (feet, square feet, US gallons) or metric (metres, square metres, litres).
- Enter the room length, width and wall height, and how many identical rooms you are painting.
- Pick walls, walls and ceiling, or ceiling only, then enter the number and size of doors and windows plus any other area you will not paint.
- Set the number of coats and the coverage from your paint can, or pick a surface preset. Add primer coats if the surface is new or stained.
- Read the paint needed and the cans or tins to buy, enter prices to see the cost, and copy or share the result.
Worked examples
12 x 12 ft bedroom, two coats
Input
12 x 12 ft, 8 ft walls, 1 door (21 ft²), 2 windows (15 ft² each), 2 coats, 350 ft²/gal
Output
333 ft² of wall, 1.90 gal of paint; buy two 1 gallon cans
The four walls are 384 ft² gross. Taking out 51 ft² of openings leaves 333 ft², and two coats at 350 ft² per gallon need 666 / 350 = 1.90 gallons.
Same room with the ceiling
Input
Walls and ceiling, 2 coats each
Output
477 ft² painted, 2.73 gal; buy three 1 gallon cans
The 144 ft² ceiling adds 0.82 gallons. Two gallons and three quarts would cover it with less left over, but three quarts cost more than a third gallon, so the calculator rounds to three gallons.
Metric living room
Input
5 x 4 m, 2.5 m walls, 1 door (1.9 m²), 2 windows (1.44 m²), 2 coats, 10 m²/L
Output
40.2 m² of wall, 8.04 L; buy a 5 L, a 2.5 L and a 1 L tin
Two coats over 40.2 m² is 80.4 m² of painting. A 5 L and a 2.5 L tin fall 0.54 L short, so adding a 1 L tin gives 8.5 L, less spare paint than a single 10 L tin.
Background and insight
Coverage is the number that matters most and the one people skip. Manufacturers state a spreading rate for one coat at the film thickness the paint is designed for, usually on a smooth, sealed surface. Rough or porous surfaces, bare drywall, fresh plaster and textured ceilings soak up more, so the real figure can fall to 250 square feet per gallon or lower. Big colour changes, especially light over dark, often need a third coat or a tinted primer. That is why the calculator lets you choose a surface preset or type the figure from your can.
Openings are subtracted by area. A standard interior door of 3 by 7 feet is 21 square feet and a 3 by 5 foot window is 15 square feet, which are the defaults. Painters often ignore small windows because cutting in around them uses nearly as much paint as the glass would have taken; subtracting them gives the leanest honest estimate, and the extra percentage puts some back.
Two coats are the norm for new colour on walls. The first coat hides most of the old colour and evens out porosity; the second gives the uniform sheen and colour the manufacturer tested. Ceilings in flat white over the same white are often fine with one coat.
Common use cases
Repainting a room
Check whether one gallon is enough or you need two before driving to the store, and avoid opening a second batch mid-wall.
New drywall
Add a primer coat with its own coverage figure so the primer and finish paint are bought together.
Whole-house quotes
Price several identical bedrooms at once and compare 5 gallon buckets against single gallons.
Ceilings only
Estimate ceiling white for a whole floor, where one coat over the same colour is often enough.
Tips and limitations
- Read the coverage on your can. A premium paint at 400 ft² per gallon and a contractor grade at 300 differ by a third.
- Buy all the paint for one room in one batch, or box the cans (mix them together) before you start, so the colour is uniform.
- Textured walls, popcorn ceilings and masonry use much more paint; use the rough preset or a figure from the product data sheet.
- The estimate covers walls and ceilings. Trim, doors and cabinets are usually a different sheen and are bought separately, often by the quart.
- Keep a little leftover for touch-ups and label it with the room and the date.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most interior wall paints state 350 to 400 square feet per gallon for one coat on a smooth, primed surface. Rough, porous or textured surfaces can drop that to 250 square feet or less, so use the figure printed on your can.
How much paint do I need for a 12 x 12 room?
With 8 foot walls, one door and two windows, the walls are about 333 square feet. Two coats at 350 square feet per gallon need about 1.9 gallons, so two gallons will do. Adding the ceiling brings it to about 2.7 gallons.
Do I need one coat or two coats of paint?
Two coats for a new colour on walls, because the first coat rarely hides the old colour evenly. One coat can be enough when refreshing the same colour or painting a ceiling white over white with a good paint.
Should I subtract windows and doors when estimating paint?
Yes, subtract their area. A standard 3 x 7 foot door is 21 square feet and a 3 x 5 foot window is 15. In a small room that is 10 to 15 percent of the wall area. Add a small extra percentage if you want a margin for cutting in.
Is it cheaper to buy a 5 gallon bucket of paint?
Usually, per gallon, once you need four or more gallons of the same colour. Buckets also guarantee one batch. The calculator suggests a bucket when the volume justifies it and lets you enter your own prices to compare.
How much primer do I need for new drywall?
One coat over the whole painted area. Drywall primers often cover a little less than paint, around 300 square feet per gallon, because bare paper and joint compound absorb more. Set the primer coats and primer coverage to get a separate primer quantity.
How many litres of paint per square metre?
At a typical 10 m² per litre, one coat needs 0.1 L per square metre and two coats 0.2 L. Smooth surfaces and better paints reach 12 m² per litre; rough plaster may only manage 6 to 8.
Is there an in-depth guide to the Paint Calculator?
Yes. “How Much Paint Do I Need? Coverage, Coats and Cans” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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