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This paint calculator tells you how much paint you need to paint a room. Enter the room size and wall height, subtract doors and windows, choose the number of coats and the paint’s coverage, and get the answer in gallons and quarts or in litres, with a suggestion of which cans or tins to buy.
How to Use the Paint Calculator
- Enter the room length, width and wall height.
- Enter the number of doors and windows. Each door is counted as 21 sq ft (1.95 m²) and each window as 15 sq ft (1.39 m²).
- Choose the number of coats. Two coats are standard for a color change or a fresh finish.
- Enter the coverage from the paint label. Most interior paints cover 350–400 sq ft per gallon or about 10–12 m² per litre per coat.
- Tick the ceiling option if you are painting the ceiling with the same paint.
Paint Formula
Wall area = 2 × (Length + Width) × Height − Doors − Windows
Paint needed = Wall area × Coats ÷ Coverage
If you include the ceiling, its area (length × width) is added to the wall area. Many people use a separate flat white paint for ceilings, in which case it is better to calculate it on its own.
Worked Example: 12 × 12 ft Bedroom with 8 ft Walls
- Wall area: 2 × (12 + 12) × 8 = 384 sq ft
- Minus 1 door (21 sq ft) and 2 windows (30 sq ft) = 333 sq ft
- Two coats: 333 × 2 = 666 sq ft
- Paint: 666 ÷ 350 = 1.9 gallons, so buy 2 gallons
Metric example: a 4 × 3 m room with 2.4 m walls, one door and one window has about 30.3 m² of wall. Two coats at 10 m² per litre need about 6.1 litres, so one 5 L tin plus one 2.5 L tin.
Typical Paint Coverage
| Surface or product | Coverage per coat |
|---|---|
| Interior wall paint on smooth, previously painted walls | 350–400 sq ft per gallon (9–12 m² per litre) |
| Fresh drywall or plaster (with primer) | 250–300 sq ft per gallon (7–9 m² per litre) |
| Textured or rough surfaces | 200–300 sq ft per gallon (5–8 m² per litre) |
| Ceiling paint | 350–400 sq ft per gallon (9–11 m² per litre) |
Always check the label, because coverage changes between brands, sheens and colors. Deep and bright colors often need an extra coat.
Paint Can and Tin Sizes
- US: quart (about 0.95 L), gallon (about 3.8 L) and 5-gallon bucket (about 19 L).
- UK and Europe: 1 L, 2.5 L, 5 L and 10 L tins.
- If you need just over a whole gallon, a gallon plus a quart is usually cheaper than two gallons.
Painting Tips
- Prime new drywall, bare wood, stains and dark colors before applying the finish coat.
- Buy all the paint for one room at the same time and mix the cans together (boxing) for a consistent color.
- Keep a small amount for touch-ups and label it with the room and the color code.
- Hanging new drywall before you paint? Use our drywall calculator.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
A 12 × 12 ft room with 8 ft walls, one door and two windows has about 333 sq ft of wall. Two coats need about 1.9 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover about 350–400 square feet per gallon per coat on smooth, previously painted walls. Rough or new surfaces absorb more paint.
Do I need two coats of paint?
Usually yes. Two coats give even color and better durability, especially when changing colors. Some paint-and-primer products can cover in one coat over a similar color.
Should I subtract doors and windows?
Yes, subtracting them makes the estimate more accurate. The calculator uses standard sizes of 21 sq ft per door and 15 sq ft per window.
How many litres of paint do I need per square meter?
Most emulsions cover about 10–12 m² per litre per coat, so a square meter needs roughly 0.1 litres per coat, or 0.2 litres for two coats.
Related guide: How Much Paint Do I Need for a Room? (Chart by Room Size)
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