Work out how much paint to buy. This free paint quantity calculator turns room size or wall area and coats into gallons or litres, subtracting doors and windows and using adjustable coverage.
Enter the room size (the calculator finds wall area) or a wall area directly. Default coverage is about 350 ft² per gallon (≈11 m² per litre) per coat — adjust to match your paint’s label.
Paint needed = wall area × coats ÷ coverage. Coverage varies by paint, surface and colour — porous or dark-to-light changes need more. Buy a little extra for touch-ups and uneven surfaces.
How the Paint Quantity Calculator Works
This tool works out how much paint to buy for a room or wall, using either full room dimensions or a wall area you already know, then applies your number of coats and the paint’s coverage rate.
Formula: In room mode, Wall area = (2 × (Length + Width) × Height) − Doors & windows area. In area mode, Wall area is entered directly. Total paint area = Wall area × Coats. Paint needed = Total paint area ÷ Coverage per unit, given in gallons (feet units, default 350 ft²/gal) or litres (metric, default 11 m²/L), with 1 gallon = 3.785411784 litres for converting between the two.
- Length, Width, Height — the room’s dimensions, used in Room size mode to find wall area automatically
- Wall area — a directly entered area, used instead of room dimensions in Wall area mode
- Doors & windows area — the combined area of openings to subtract from the wall area, optional
- Coats — the number of paint coats planned, defaulting to 2 if left blank
- Coverage / unit — the paint’s coverage rate per coat, defaulting to 350 ft²/gal or 11 m²/L, adjustable to match the label on your paint
Example Scenarios
| Length | Width | Height | Openings | Coats | Coverage | Paint needed |
|---|---|---|---|---|---|---|
| 12 ft | 10 ft | 8 ft | 0 ft² | 2 | 350 ft²/gal | 2.01 gal |
| 15 ft | 12 ft | 9 ft | 40 ft² | 2 | 350 ft²/gal | 2.55 gal |
| 10 ft | 10 ft | 8 ft | 20 ft² | 1 | 300 ft²/gal | 1.00 gal |
| 6 ft | 5 ft | 8 ft | 15 ft² | 3 | 350 ft²/gal | 1.38 gal |
| 4 m | 3.5 m | 2.4 m | 0 m² | 2 | 11 m²/L | 6.55 L |
| 5 m | 4 m | 2.5 m | 2 m² | 2 | 11 m²/L | 7.82 L |
Paint Quantity Calculator FAQ
How many gallons of paint do I need for a 12×10 room?
Enter the room’s length, width and height in Room size mode, subtract any door or window area, and set your number of coats — the calculator finds the wall area and converts it to gallons using your paint’s coverage rate automatically.Should I subtract the ceiling from wall area?
The calculator only estimates wall area (perimeter × height), not the ceiling, so no ceiling adjustment is needed. If you’re painting the ceiling too, measure its area separately and use Wall area mode to add that square footage as its own entry.Why does the default coverage say 350 ft² per gallon?
350 ft² per gallon per coat is a commonly cited average for interior wall paint, but actual coverage varies by brand, sheen, and surface texture. Always check the coverage figure printed on your specific paint can and adjust the Coverage field to match it.How many coats of paint should I use?
Two coats is standard for most colour changes and is the calculator’s default. A single coat may suffice for a matching-colour touch-up, while a dramatic colour change (especially dark-to-light) often needs a third coat or a tinted primer first.Do I need more paint for a dark-to-light colour change?
Usually yes — that’s exactly the kind of change where actual coverage falls short of the can’s stated rate. Enter a lower coverage figure than the default, or add an extra coat, to reflect the additional paint that a dramatic colour change typically needs.How do I convert my paint result between gallons and litres?
Switch the units toggle to Metres to see the result in litres, or Feet to see it in gallons — the calculator shows both figures in the results panel regardless of which unit you’re actively entering measurements in.Related Calculators
Prefer wallpaper to paint for a feature wall? The wallpaper calculator covers roll counts using a similar area-based approach. If you haven’t measured the room yet, start with the square footage calculator, and for the floor once the walls are done, the tile quantity calculator can help plan materials.