How much paint do I need?
Type the size of the room and the coverage printed on your paint can. The paint calculator subtracts doors and windows and shows how many gallons (or litres) you need for the coats you plan.
- Gallons of paint
- 2.3
Paint needed for 366 ft²: 2.3 gal (8.71 L); whole gallons to buy: 3.
- Gallons to buy
- 3
- Litres of paint
- 8.71
- Area to paint, ft²
- 366
- Area to paint, m²
- 34
Gallons of paint: 2.3. Paint needed for 366 ft²: 2.3 gal (8.71 L); whole gallons to buy: 3.
How to calculate
Computes how many gallons (or litres) of paint a room needs from its size, less doors and windows, with the number of coats, the coverage per gallon and an extra share for waste.
Example with the default inputs (Room length 12 ft, Room width 14 ft, Wall height 8 ft, Coats 2, Coverage per gallon 350 ft², Extra for waste 10%, Doors 1, Area of one door 20 ft², Windows 2, Area of one window 15 ft², Paint the ceiling too No): Paint needed for 366 ft²: 2.3 gal (8.71 L); whole gallons to buy: 3.
Method: Area = 2 × (length + width) × height − doors × door area − windows × window area (+ length × width for the ceiling); gallons = area × coats × (1 + extra ÷ 100) ÷ coverage per gallon.
- The room is a rectangle with walls of one height; each coat covers the area printed on the can.
- Doors and windows are not painted. By default a door is 20 ft² (36 × 80 in) and a window 15 ft² (3 × 5 ft).
- Extra for waste is 10% by default, for rough walls, touch-ups and paint left in the tray.
- Gallons are US gallons (3.785411784 L).
Worked examples
Each example is checked against the calculator on every build.
- Room length 12 ft, Room width 14 ft, Wall height 8 ft, Coats 2, Coverage per gallon 350 ft², Extra for waste 10%, Doors 1, Area of one door 20 ft², Windows 2, Area of one window 15 ft², Paint the ceiling too no gives Area to paint, ft² 366, Area to paint, m² 34.002513, Gallons of paint 2.300571, Gallons to buy 3, Litres of paint 8.70861.Source: NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L, 1 ft = 0.3048 m)
- Room length 10 ft, Room width 10 ft, Wall height 8 ft, Coats 1, Coverage per gallon 400 ft², Extra for waste 0%, Doors 0, Area of one door 20 ft², Windows 0, Area of one window 15 ft², Paint the ceiling too yes gives Area to paint, ft² 420, Gallons of paint 1.05, Gallons to buy 2.Source: 2 × 20 × 8 + 10 × 10 = 420 ft² ÷ 400 = 1.05 gal (NIST Handbook 44 units)
- Room length 10 ft, Room width 10 ft, Wall height 8.75 ft, Coats 1, Coverage per gallon 350 ft², Extra for waste 0%, Doors 0, Area of one door 20 ft², Windows 0, Area of one window 15 ft², Paint the ceiling too no gives Area to paint, ft² 350, Gallons of paint 1, Gallons to buy 1.Source: 2 × 20 × 8.75 = 350 ft² ÷ 350 = 1 gal (NIST Handbook 44 units)
- Room length 13.12 ft, Room width 9.843 ft, Wall height 8.202 ft, Coats 2, Coverage per gallon 407.5 ft², Extra for waste 0%, Doors 1, Area of one door 19.38 ft², Windows 1, Area of one window 16.15 ft², Paint the ceiling too no gives Area to paint, m² 31.7, Litres of paint 6.34, Gallons of paint 1.674851.Source: NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L, so 10 m² per litre is 37.85411784 m² per gallon)
How it works
The room is a box. The calculator finds the area to paint and divides by the coverage:
- Wall area = 2 × (length + width) × height
- Openings = doors × area of one door + windows × area of one window
- Area to paint = wall area − openings (+ length × width with the ceiling)
- Gallons = area to paint × coats × (1 + extra ÷ 100) ÷ coverage per gallon
- Gallons to buy = gallons rounded up to a whole number
- Litres = gallons × 3.785411784
The coverage is the area one US gallon covers in one coat. For a coverage in square metres per litre, multiply by 3.785411784 to get square metres per gallon.
Exact arithmetic. Each size is read as the exact decimal you typed, in the unit you typed it in (12 ft 6 in is exactly 150 in). The page stores a size as a number in metres or square metres and reads it back as the decimal with the fewest significant digits, in one of the box's units, that gives the same stored number; a feet-and-inches pair counts by its value in inches (12 ft 6 in counts as 150, and 426.72 cm, exactly 14 ft, stays 426.72 cm), and on a tie the unit listed first in the box's unit menu wins. Every step is exact, so 350 ft² at 350 ft² per gallon is exactly 1 gallon to buy, not 2.
Rounding. Gallons, litres and areas show with at most 2 decimals, each rounded half up from its exact value, with trailing zeros left off. Gallons to buy are rounded up.
No answer. When the doors and windows add up to the whole wall area or more, there is nothing to paint on the walls, and the calculator shows: "The doors and windows add up to at least the whole wall area."
Assumptions
- The room is a rectangle with walls of one height. Doors and windows are not painted.
- A door is 20 ft² and a window 15 ft² unless you type other sizes.
- The extra for waste is 10% by default, from 0% to 100%.
- Lengths are more than 0 and at most 1,000 m; door and window areas from 0 to 10,000 m²; coverage from 1 ft² to 10,000 m² per gallon; 0 to 100 doors and windows; 1 to 10 coats.
Worked examples by hand
A 12 × 14 ft room, 8 ft walls, one door, two windows, two coats, 350 ft² per gallon, 10% extra. Walls: 2 × 26 × 8 = 416 ft². Openings: 20 + 2 × 15 = 50 ft². Area: 366 ft² (34.00 m²). Paint: 366 × 2 × 1.1 ÷ 350 = 2.30 gallons (8.71 L), so buy 3 gallons.
A 10 × 10 ft room with the ceiling, one coat, 400 ft² per gallon, no extra. 2 × 20 × 8 + 100 = 420 ft²; 420 ÷ 400 = 1.05 gallons, so buy 2 gallons.
Exactly one gallon. A 10 × 10 ft room with 8.75 ft walls: 2 × 20 × 8.75 = 350 ft²; at 350 ft² per gallon that is 1 gallon, and 1 gallon to buy.
A 4 × 3 m room, 2.5 m walls, a 1.8 m² door, a 1.5 m² window, two coats of paint that covers 10 m² per litre. 2 × 7 × 2.5 − 1.8 − 1.5 = 31.7 m²; 31.7 × 2 ÷ 10 = 6.34 L (1.67 gallons).
Other questions people ask
How do I calculate how much paint I need for a room?
Add up the wall area: 2 × (length + width) × height. Subtract the doors and windows, multiply by the number of coats, and divide by the area one gallon covers. A 12 × 14 ft room with 8 ft walls has 416 ft² of wall; less one door (20 ft²) and two windows (30 ft²) that is 366 ft². Two coats at 350 ft² per gallon need 366 × 2 ÷ 350 = 2.09 gallons, or 2.30 with 10% extra.
How much area does a gallon of paint cover?
It depends on the paint and the surface, so use the number on the can. The calculator starts at 350 ft² per gallon for one coat. A rough or porous wall, a big colour change or a primer can cover less. In metric, 10 m² per litre is 37.85 m² per gallon (1 gal = 3.785411784 L).
Do I need two coats of paint?
Often, yes: a second coat evens out the colour and the sheen, and many paint makers suggest two coats. Set the number of coats to what you plan; each coat needs the full area again.
Should I subtract doors and windows?
Yes, if they are not painted. The calculator takes 20 ft² for each door (36 × 80 in) and 15 ft² for each window (3 × 5 ft) by default; type the sizes of yours for a closer answer. If you paint the doors too, set the number of doors to 0.
How do I include the ceiling?
Turn on “Paint the ceiling too”. The calculator adds length × width to the area and uses the same coats and coverage. Ceiling paint often differs from wall paint, so run the calculator twice if your coverage differs.
Why buy more paint than the exact amount?
Paint is sold in whole cans, and some is lost in the tray and on the roller. The calculator adds 10% for waste by default and rounds the result up to whole gallons. Keep a little left over for touch-ups.