How much paint do I need? Stop guessing at the store
Buying paint by eye usually goes one of two ways: half a can drying out in the garage, or running short on the last wall. This calculator uses the method professional painters do: measure the actual wall area, subtract doors and windows, then divide by the coverage rate for the coats you plan to apply. You get the answer in liters and in US gallons.
How to use the calculator
- Pick a mode: Room (length and width; the perimeter is computed for you), or Total wall length if you already measured the linear meters of wall.
- Enter the wall height (most homes fall between 2.4 and 2.7 m).
- Set how many doors and windows the walls have: 1.6 m² per door and 1.2 m² per window are deducted, the standard sizes.
- Adjust the coats (2 is typical) and the coverage rate, in m² per liter or per gallon, whichever your can states.
- Copy the result and take it to the store.
The method and the formula
Four simple operations, chained together:
- Gross area = total wall length × height. In room mode, total wall length is the perimeter: 2 × (length + width).
- Net area = gross area − 1.6 m² per door − 1.2 m² per window.
- Liters = (net area × coats) ÷ coverage in m² per liter.
- Gallons = liters ÷ 3.785 (the US gallon).
A typical water-based wall paint covers about 10 m² per liter per coat on a smooth, sealed wall — roughly 35 to 40 m² per gallon.
Worked example
You want to paint a 4 × 3 meter room with 2.5 m walls, 1 door and 1 window, applying 2 coats of a paint rated at 10 m² per liter:
- Perimeter: 2 × (4 + 3) = 14 m of wall.
- Gross area: 14 × 2.5 = 35 m².
- Net area: 35 − 1.6 − 1.2 = 32.2 m².
- With 2 coats: 32.2 × 2 = 64.4 m² to cover.
- Liters: 64.4 ÷ 10 = 6.44 liters, which is 6.44 ÷ 3.785 = 1.70 gallons.
In practice you would buy 2 gallons and keep the leftover for same-batch touch-ups.
Paint needed by room size
Quick reference assuming 2.5 m walls, 1 door, 1 window, 2 coats and 10 m² per liter:
| Room | Net area | Liters | Gallons |
|---|---|---|---|
| 3 × 3 m | 27.2 m² | 5.44 L | 1.44 |
| 4 × 3 m | 32.2 m² | 6.44 L | 1.70 |
| 4 × 4 m | 37.2 m² | 7.44 L | 1.97 |
| 5 × 4 m | 42.2 m² | 8.44 L | 2.23 |
| 6 × 5 m | 52.2 m² | 10.44 L | 2.76 |
The surface makes or breaks these numbers: smooth, previously painted plaster yields 10-12 m² per liter; raw block or new concrete can drop to 6-8 on the first coat. On unsealed walls, apply a primer-sealer first to restore the rated coverage.
Frequently asked questions
How many square meters does a gallon of paint cover?
Between 35 and 40 m² per coat on a smooth, sealed wall, the figure manufacturers state on the label. On unpainted block or porous surfaces, expect 23-30 m² per gallon for the first coat. The exact number is printed on the can.
How many coats of paint should I apply?
Two coats is the standard for an even finish; a single coat almost always lets the old color show through. Plan on three when going from dark to light, or with deep reds and yellows, which hide poorly. One coat can be enough when repainting the same color over walls in good condition.
Should I subtract doors and windows?
Yes, because they are not painted with the same wall paint. The standard deduction is 1.6 m² per door (0.8 × 2 m) and 1.2 m² per window (1.2 × 1 m). For oversized openings — a floor-to-ceiling window, say — measure the real area and adjust the count.
Does the calculator include the ceiling?
No, walls only. For the ceiling, multiply the room’s length × width (in the example, 4 × 3 = 12 m²), multiply by the coats and divide by the coverage: 12 × 2 ÷ 10 = 2.4 extra liters. Ceilings usually take dedicated flat white paint, so price them separately. And do buy the 10% margin the calculator suggests: real coverage varies with surface and technique, and it leaves you a same-batch reserve for touch-ups.