How the paint calculator works
Paint is sold by volume but used by area, so the whole job is turning the size of your walls into litres or gallons. The calculator first adds up the four walls of the room, then removes the parts you will not paint, then multiplies by the number of coats and divides by the coverage printed on the can.
Paint = (2 × (length + width) × height − doors − windows) × coats ÷ coverage × 1.10A standard interior door is counted as about 1.9 m² (21 ft²) and an average window as 1.4 m² (15 ft²). The final 10% is a margin for the roller tray, touch-ups and the first coat soaking into fresh plaster.
Worked example
A bedroom of 4.5 m × 3.5 m with 2.5 m ceilings has 2 × (4.5 + 3.5) × 2.5 = 40 m² of wall. Take away one door (1.9 m²) and two windows (2.8 m²) and 35.3 m² is left to paint. Two coats of a paint that covers 10 m² per litre need 35.3 × 2 ÷ 10 = 7.06 L, or 7.8 L with the 10% margin. In 2.5 L tins that is four tins (10 L). A 5 L + 2.5 L pair, often cheaper per litre, gives only 7.5 L and falls short; 5 L + 2.5 L + 1 L (8.5 L) covers it.
Coverage rates by surface
| Surface | Typical coverage | Notes |
|---|---|---|
| Smooth, previously painted wall | 10–12 m²/L (375–400 ft²/gal) | Use the can figure |
| New plaster or drywall | 6–8 m²/L (250–300 ft²/gal) | Apply a diluted mist coat or primer first |
| Textured or rough render | 5–7 m²/L (200–275 ft²/gal) | Rougher surfaces hold more paint |
| Ceiling (flat matt) | 10–12 m²/L | Ceiling area = length × width |
How many coats do you need?
Two coats is the right answer for almost every room: the first coat evens out the surface and the second gives the full colour and a uniform sheen. One coat only works when you repaint in the same or a very similar colour with a good-quality paint. Going from a dark colour to a light one, or covering strong reds and yellows, often needs a primer plus two coats, which is why the calculator offers three.
Common mistakes
- Measuring the floor instead of the walls. A 16 m² room has roughly 40 m² of wall at normal ceiling height.
- Trusting the best-case coverage. Cans quote coverage on a smooth, sealed surface. New plaster can absorb twice as much.
- Forgetting trim and ceilings. Skirting boards, door frames and ceilings use different paints and are not included here.
- Buying several batches. Tint can vary slightly between batches, so buy the full quantity at once and mix the cans together for large walls.
How can I check the estimate before buying?
Start with the worked example and compare three separate results: the wall area, the paint volume and the number of tins. For a 4.5 m by 3.5 m room with 2.5 m walls, one door and two windows, the paintable area is 35.3 m². At two coats and 10 m² per litre, the unrounded estimate including the margin is 7.766 L. The displayed 7.8 L is a rounded reading, while the tin calculation rounds upward to four 2.5 L tins. Rounding to the nearest tin would risk buying less than the calculated requirement.
What measurements does this calculator leave out?
The formula assumes a rectangular room with four walls at the entered height. It does not measure alcoves, sloping walls or individual opening sizes. Doors and windows use the standard allowances stated above. If your room has unusual features, calculate the relevant areas separately and compare them with the estimate before shopping. The ceiling, trim and primer are separate quantities; adding them to the wall paint figure without considering their product coverage can hide an incorrect order.
Choose the unit system before entering measurements. The imperial coverage field uses square feet per US gallon; the metric field uses square metres per litre. A number copied from a label in the other system will give the wrong volume even if every room dimension is correct. The extra ten percent is already included once. Apply another margin only when you have a specific reason, rather than adding it again by habit.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12 ft × 12 ft room with 8 ft walls has 384 ft² of wall. After one door and two windows about 333 ft² is left, so two coats at 400 ft² per gallon need roughly 1.8 gallons – buy two gallons.
How many square metres does 1 litre of paint cover?
Most interior emulsions cover 10 to 12 m² per litre per coat on a smooth, sealed wall, and 6 to 8 m² on new plaster or bare drywall.
Should I subtract doors and windows?
Yes for accurate numbers. A door removes about 1.9 m² (21 ft²) and a window about 1.4 m² (15 ft²). In a small room with several windows this saves a whole tin.
Do I need primer?
Use primer or a diluted mist coat on new plaster, bare drywall, stains, or when changing from a dark to a light colour. On sound painted walls a quality paint can be applied directly.
Is it better to buy a little extra paint?
Yes. A 10% margin covers spills and touch-ups, and keeping a small amount of the same batch makes future repairs invisible.