How to use the paint calculator
- Measure the length and width of the floor and the height of the walls (floor to ceiling).
- Count the doors and windows. The calculator removes a typical area for each one.
- Choose how many coats you will apply — two is standard.
- Check the coverage on your paint’s label and change it if it differs from the default.
- Tick paint the ceiling if needed. Walls and ceiling are shown separately because they normally use different paint.
How paint quantity is calculated
Wall area = 2 × (length + width) × height − doors − windows Paint = area × coats ÷ coverage per coat
The result is then rounded up to the containers you can actually buy: whole gallons plus quarts in the US (three quarts are rounded up to a gallon, which is usually cheaper), or the combination of 5 L, 2.5 L and 1 L tins with the least waste in the UK.
Worked example (US)
A 12 × 12 ft bedroom with 8 ft walls has 2 × (12 + 12) × 8 = 384 ft² of wall. Subtract one door (20 ft²) and two windows (30 ft²) to get 334 ft². Two coats is 668 ft², and at 350 ft² per gallon that is 1.91 gallons, so buy 2 gallons.
Worked example (UK)
A 4 × 4 m room with 2.4 m walls has 38.4 m² of wall. Subtract one door (1.6 m²) and two windows (3 m²) to get 33.8 m². Two coats at 10 m² per litre needs 6.76 litres — one 5 L tin and two 1 L tins, or a 5 L and a 2.5 L tin if the larger tin is better value.
Typical paint coverage
| Surface | US (per gallon) | UK (per litre) |
|---|---|---|
| Smooth, previously painted walls | 350–400 ft² | 10–13 m² |
| New drywall / fresh plaster (after primer or mist coat) | 300–350 ft² | 8–10 m² |
| Textured or rough surfaces | 250–300 ft² | 6–8 m² |
| Primer | 200–300 ft² | 6–10 m² |
Fresh plaster and bare drywall soak up paint. In the UK, new plaster is usually sealed with a diluted “mist coat” of emulsion; in the US, bare drywall gets a coat of primer. Either way, count it as an extra coat.
Tips for buying paint
- Buy it all at once. Colours can vary slightly between batches, especially with tinted paint.
- Keep the leftovers. A small amount saved for touch-ups is worth more than a perfect estimate.
- Doors, trim and woodwork use a different paint (gloss, satin or eggshell) and are not included here.
- If you are boarding or patching walls first, the drywall & plasterboard calculator will tell you how many sheets you need.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
For walls 8 ft high with one door and two windows, you have about 334 ft² of wall. Two coats at 350 ft² per gallon need about 1.9 gallons, so buy 2 gallons. Add roughly another gallon if you are also painting a 144 ft² ceiling with two coats.
How many square feet does a gallon of paint cover?
Most interior wall paints cover about 350–400 ft² per gallon for one coat on smooth, primed walls. Rough, porous or textured surfaces can drop that to 250–300 ft². The exact figure is printed on the can.
How much does a 5 litre tin of paint cover?
Typical UK emulsions quote around 10–14 m² per litre per coat, so a 5 litre tin covers roughly 50–65 m² for one coat, or 25–32 m² for two coats. Real-world coverage is usually at the lower end, which is why this calculator defaults to 10 m² per litre.
Do I need two coats of paint?
Usually, yes. Two coats give an even colour and a durable finish. One coat can be enough when refreshing a wall in the same colour, while a big change — such as dark red to white — may need a primer plus two coats.
Should I subtract doors and windows?
Yes for large rooms, as it stops you over-buying. The calculator subtracts about 20 ft² (1.6 m²) per door and 15 ft² (1.5 m²) per window. If you are also painting the door, add its area back on as a separate job with the right paint.
Is ceiling paint calculated separately?
Yes. Ceilings normally use a different paint, so the calculator shows walls and ceiling as separate quantities. Ceiling paint is usually a flat white with lower sheen to hide imperfections.