# Paint calculator

> Paint calculator: litres, US gallons and whole cans of paint and primer for a room, from wall area minus doors and windows, coats and coverage.

Interactive version: https://www.calcopenly.com/construction/paint-calculator
Subject: Construction and home calculators

Paint needed is the area to cover times the number of coats, divided by the coverage printed on the can. Wall area is the room's perimeter times the wall height, 2(length + width) × height, plus the ceiling if you include it, minus each door and window. The result is rounded up to whole cans of the size you pick.

The default 4.5 × 3.5 m room with 2.6 m walls, one door and two windows leaves 36.9 m² to paint. Two coats at 10 m² per litre need 7.38 L, which is two 5 L cans. For US paint, switch coverage to ft² per gallon; Sherwin-Williams puts a gallon at about 350–400 ft² per coat.

Label coverage assumes a smooth, sealed surface. Bare plaster, rough texture and strong colour changes use more paint, so treat the answer as the minimum to buy.

## Inputs

- **Room length**
- **Room width**
- **Wall height**
- **Doors**
- **Area of one door**: A standard interior door is about 1.9 m² (20 ft²).
- **Windows**
- **Area of one window**: A typical window is about 1.4 m² (15 ft²).
- **Paint the ceiling too**
- **Coats**
- **Coverage given as** (options: m² per litre, ft² per gallon)
- **Coverage per coat**: Printed on the can; wall paints typically cover 10–16 m²/L per coat on a smooth surface.
- **Coverage per coat**: Printed on the can; most interior latex paints cover 350–400 ft²/gal.
- **Add one coat of primer**
- **Primer coverage**: Printed on the primer can; leave blank to use the paint's coverage. Sherwin-Williams gives 200–300 ft²/gal (4.9–7.4 m²/L) for primer.
- **Primer coverage**: Printed on the primer can; leave blank to use the paint's coverage. Sherwin-Williams gives 200–300 ft² per gallon for primer.
- **Can size** (options: 1 L, 2.5 L, 4 L, 5 L, 10 L, 20 L, 1 US quart (0.95 L), 1 US gallon (3.79 L), 5 US gallons (18.9 L))

## Results

- Paint cans to buy (cans) — main result
- Paint needed (L)
- Paint needed (US gal)
- Area to paint (m²)
- Area to paint (ft²)
- Primer needed (L)
- Primer cans to buy (cans)

## Formula

$$
V = \frac{\big(2(\ell + w)h + A_{\text{ceiling}} - A_{\text{doors}} - A_{\text{windows}}\big) \times \text{coats}}{\text{coverage}}
$$

## Worked examples

### 4.5 × 3.5 m room, 1 door, 2 windows, 2 coats

- Room length: 4.5 m
- Room width: 3.5 m
- Wall height: 2.6 m
- Doors: 1
- Windows: 2
- Coats: 2
- Coverage given as: m² per litre
- Coverage per coat: 10 m²/L
- Can size: 5 L
- **Area to paint: 36.9 m²**
- **Paint needed: 7.38 L**
- **Paint needed: 1.95 US gal**
- **Paint cans to buy: 2 cans**
- Checked against: Python decimal: 2(4.5 + 3.5) × 2.6 − 1.9 − 2 × 1.4 = 36.9 m²; × 2 ÷ 10 = 7.38 L; 7.38 ÷ 5 → 2 cans

### 12 × 12 ft room, 8 ft walls, US gallons

- Room length: 12 ft
- Room width: 12 ft
- Wall height: 8 ft
- Doors: 1
- Area of one door: 20 ft²
- Windows: 2
- Area of one window: 15 ft²
- Coats: 2
- Coverage given as: ft² per gallon
- Coverage per coat: 350 ft²/gal
- Can size: 1 US gallon (3.79 L)
- **Area to paint: 334 ft²**
- **Paint needed: 1.91 US gal**
- **Paint cans to buy: 2 cans**
- Checked against: Hand calculation: 4 × 12 × 8 = 384 ft² − 20 − 30 = 334 ft²; 668 ÷ 350 = 1.9086 gal → 2 one-gallon cans

### Walls and ceiling with primer, 2.5 L cans

- Room length: 5 m
- Room width: 4 m
- Wall height: 2.5 m
- Doors: 0
- Windows: 0
- Paint the ceiling too: yes
- Coats: 2
- Coverage given as: m² per litre
- Coverage per coat: 10 m²/L
- Add one coat of primer: yes
- Can size: 2.5 L
- **Area to paint: 65 m²**
- **Paint needed: 13 L**
- **Paint cans to buy: 6 cans**
- **Primer needed: 6.5 L**
- **Primer cans to buy: 3 cans**
- Checked against: Python decimal: 45 m² walls + 20 m² ceiling; 130 ÷ 10 = 13 L → 5.2 → 6 cans; primer 6.5 L → 2.6 → 3 cans

### US room with primer at its own 250 ft²/gal coverage

- Room length: 12 ft
- Room width: 12 ft
- Wall height: 8 ft
- Doors: 1
- Area of one door: 20 ft²
- Windows: 2
- Area of one window: 15 ft²
- Coats: 2
- Coverage given as: ft² per gallon
- Coverage per coat: 350 ft²/gal
- Add one coat of primer: yes
- Primer coverage: 250 ft²/gal
- Can size: 1 US gallon (3.79 L)
- **Area to paint: 334 ft²**
- **Primer needed: 5.06 L**
- **Primer cans to buy: 2 cans**
- Checked against: Python decimal: 334 ft² ÷ 250 ft²/gal = 1.336 gal × 3.785411784 L = 5.0573 L → 2 one-gallon cans

### Edge: need is an exact multiple of the can

- Room length: 2.5 m
- Room width: 2.5 m
- Wall height: 2.5 m
- Doors: 0
- Windows: 0
- Coats: 2
- Coverage given as: m² per litre
- Coverage per coat: 10 m²/L
- Can size: 5 L
- **Paint needed: 5 L**
- **Paint cans to buy: 1 can**
- Checked against: Hand calculation: 10 × 2.5 = 25 m² × 2 coats ÷ 10 m²/L = 5 L, exactly one 5 L can

## Questions

### How much paint do I need for a 12 × 12 room?

About 2 US gallons for two coats on the walls. A 12 × 12 ft room with 8 ft walls has 384 ft² of wall; less one 20 ft² door and two 15 ft² windows, 334 ft² remains. Two coats is 668 ft², which at 350 ft² per gallon is 1.91 gallons. Painting the 144 ft² ceiling as well raises the total to 2.73 gallons.

### How many square feet does a gallon of paint cover?

About 350–400 ft² per coat, according to Sherwin-Williams, and a quart covers about 100 ft². In metric, 350–400 ft² per US gallon is 8.6–9.8 m² per litre. Use the figure printed on your own can, because coverage changes with the product, the colour and the surface.

### How many coats of paint do I need?

Two for most walls. Dulux's guide gives one coat for its Simply Refresh one-coat range and two for EasyCare Washable & Tough. Every coat adds the full area again: the default 36.9 m² room needs 3.69 L per coat at 10 m² per litre, so a third coat adds another 3.69 L.

### Should I subtract doors and windows when calculating paint?

Yes. Dulux's method measures each wall as height × width and deducts doors and windows measured the same way. This calculator uses 1.9 m² (20 ft²) per door and 1.4 m² (15 ft²) per window unless you enter your own sizes under More options. In the default room the three openings remove 4.7 m², 11% of the 41.6 m² of wall.

### How much primer do I need?

One coat over the whole area, at the primer's own coverage rate. Sherwin-Williams gives about 200–300 ft² per gallon for primer against 350–400 ft² for paint, so 334 ft² of wall takes 1.1–1.7 gallons of primer. Enter the primer label's figure as Primer coverage under More options; left blank, the paint's coverage is used.

### How accurate is the paint calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “4.5 × 3.5 m room, 1 door, 2 windows, 2 coats” is checked against Python decimal: 2(4.5 + 3.5) × 2.6 − 1.9 − 2 × 1.4 = 36.9 m²; × 2 ÷ 10 = 7.38 L; 7.38 ÷ 5 → 2 cans.

### Where does the method come from?

Dulux UK — how much paint do I need (area ÷ coverage × coats); Sherwin-Williams — paint calculator (paint 350–400 ft² per gallon per coat, primer 200–300 ft²).

## Sources

- [Dulux UK — how much paint do I need (area ÷ coverage × coats)](https://www.dulux.co.uk/en/expert-help/how-much-paint-do-you-need)
- [Sherwin-Williams — paint calculator (paint 350–400 ft² per gallon per coat, primer 200–300 ft²)](https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator)