Paint Calculator
Wall area and the gallons or litres of paint for a room, minus doors and windows.
Your room
Filled in with example values. Replace them with your own.
Paint
Adds the ceiling area (length × width) with the same number of coats.
Results update as you type.
Result
Area to paint: 299 ft² per coat
Area to paint
299square feet
- Wall area
- 352 ft²
- Doors (1)Not painted
- − 21 ft²
- Windows (2)Not painted
- − 32 ft²
- Area per coat
- 299 ft²
- Total for 2 coats
- 598 ft²
How it’s calculated
Wall area
2 × (12 ft + 10 ft) × 8 ft = 352 ft²
Minus doors and windows
352 ft² − 53 ft² = 299 ft²
Times the coats
299 ft² × 2 = 598 ft²
How the paint calculator works
The walls of a rectangular room form one long strip: its length is the distance around the room, 2 × (length + width), and its height is the wall height. The calculator works out that area, subtracts doors and windows, and adds the ceiling if you want to paint it.
Multiplying by the number of coats gives the total area you will cover. Dividing that by the coverage printed on the paint gives the amount of paint, and dividing by the can size gives the number of cans, rounded up.
The formula
Wall area = 2 × (Length + Width) × Height
Area to paint = Wall area − Doors − Windows (+ Ceiling)
Paint = Area to paint × Coats ÷ Coverage
Cans = Paint ÷ Can size, rounded up
Worked example
A bedroom is 12 × 10 feet with 8-foot walls, one 3 × 7 ft door and two 4 × 4 ft windows. You want 2 coats.
- Wall area: 2 × (12 + 10) × 8 = 352 ft²
- Door and windows: 21 + 2 × 16 = 53 ft²
- Area to paint: 352 − 53 = 299 ft², or 598 ft² for two coats
Suppose the label says the paint covers 400 ft² per gallon (an example; use your label). 598 ÷ 400 =1.5 gallons, so you would buy two 1-gallon cans.
In metric: a 4 × 3 m room with 2.5 m walls and no openings has 35 m² of wall. Two coats at 10 m² per litre (again, an example) need 70 ÷ 10 = 7 litres, or three 2.5-litre tins.
Assumptions and limitations
- The room is a rectangle with walls of one height. Sloped ceilings, alcoves and stairwells need separate calculations.
- No coverage is assumed. It varies by product, colour and surface, so the amount of paint appears only when you enter one.
- The diagram places doors and windows for illustration only; their areas come from your inputs.
- Conversions use exact definitions: 1 US gallon = 231 cubic inches = 3.785411784 litres.
Frequently asked questions
Where do I find the coverage?
On the paint can or the manufacturer’s product sheet, usually as square feet per gallon or square metres per litre. Use the figure for the surface you are painting: rough, porous or previously unpainted surfaces soak up more paint.
How many coats do I need?
It depends on the paint, the surface and how much the colour is changing. Two coats are common; a big colour change or a fresh, unsealed surface may need more. The paint label often gives guidance.
Does this include primer?
No. If you need primer, calculate it separately: use one coat and the coverage printed on the primer.
What about skirting boards, trim and doors?
They are not included. They are usually painted with a different paint and need much less of it. The door and window areas you enter are subtracted from the walls.
How big is a US gallon in litres?
One US gallon is exactly 3.785411784 litres, and one US quart is a quarter of that, about 0.946 litres. The calculator converts between them, so you can enter the can size in the unit printed on it.