🎨 Interactive Paint Calculator
Enter your room dimensions to calculate precise wall and ceiling paint volumes.
How to use the paint calculator
- Measure the room: Use a tape measure to get the length and width of the floor, then measure the height from floor to ceiling. Choose feet or metres in the unit box.
- Count doors and windows: These areas will not be painted with wall colour, so the calculator subtracts them. Do not count tiny openings such as air vents.
- Choose the number of coats: Two coats is the standard for most interior walls. Use three when you are covering a dark colour with a light one, or painting raw plaster.
- Check the coverage figure: The default is 10 square metres per litre, which suits most emulsion paints. Your tin usually prints its own coverage figure, so replace ours if yours is different.
- Tick the ceiling box if you plan to repaint the ceiling in the same paint. Leave it unticked if the ceiling gets a different colour.
- Press Calculate paint to see the paintable area and the volume you need.
The formula behind the calculator
Nothing mysterious happens behind the button. The calculation follows four short steps:
- Wall area = 2 × (room length + room width) × wall height
- Paintable area = wall area − (doors × 1.85 m²) − (windows × 1.4 m²), plus room length × room width if you are painting the ceiling
- Total area to cover = paintable area × number of coats
- Paint needed (litres) = total area to cover ÷ coverage per litre
The door and window sizes are averages. A standard interior door is roughly 2.0 m by 0.9 m, which is close to 1.85 m², and a typical window is around 1.4 m². If your openings are unusually large, such as sliding glass doors or a bay window, add extra to the door or window count so the deduction is fair.
Worked example: a 12 ft × 14 ft bedroom
Suppose your bedroom is 12 ft long, 14 ft wide and has a 9 ft ceiling. It has one door and two windows, and you want two coats of paint on the walls only.
| Step | Working | Result |
|---|---|---|
| Wall area | 2 × (12 + 14) × 9 ft = 468 sq ft | 43.5 m² |
| Deduct door and windows | 1.85 + (2 × 1.4) = 4.65 m² | 38.8 m² paintable |
| Two coats | 38.8 × 2 | 77.7 m² |
| Divide by coverage | 77.7 ÷ 10 m² per litre | 7.8 litres (about 2.1 US gallons) |
In practice you would buy a 10-litre tin, or an 8-litre and a 2-litre pair, to leave a little for touch-ups. If you add the ceiling (168 sq ft or about 15.6 m²), the total climbs to roughly 10.9 litres, so a single 10-litre tin would fall just short and a 12 to 15 litre purchase would be sensible.
Tips for buying the right amount
- Add about 10% extra: Rough walls, patched areas and spills absorb more paint than the maths predicts.
- Prime first when needed: New plaster, stained walls and dramatic colour changes need primer. Primer is calculated separately, using its own coverage figure.
- Mix one colour from the same batch: If you need several tins, mix them together in one large bucket (a process called "boxing") to prevent slight colour differences.
- Textured walls drink more paint: Brick effect, stucco or heavy texture can reduce coverage by up to a third. Lower the coverage number to compensate.
- Trim, doors and skirting need a separate estimate. They are usually painted in a different finish, such as satin or gloss, and use much less paint.
- Keep a labelled leftover: Pour a small amount into a jar and write the colour name, brand and date on the lid. It is very handy for future touch-ups.
Common mistakes to avoid
Measuring the floor only: Paint goes on walls, so wall height matters just as much as floor size. Two rooms with the same floor area can need very different amounts of paint if the ceiling heights differ.
Forgetting the second coat: Many people calculate for one coat and then run short halfway through the second. Most paints need two coats for an even, lasting finish.
Trusting the coverage on cheap paint: Budget paints often have thin pigment and lower coverage than the label suggests. If in doubt, calculate with 8 m² per litre instead of 10.
Ignoring the finish: Matt paints hide flaws and often cover slightly better than glossy finishes, which show every brush mark and may require more careful application.
Frequently asked questions
How much paint do I need for a 10 × 12 room?
With a 9 ft ceiling, one door, one window and two coats, a 10 × 12 ft room needs about 6 to 7 litres (roughly 1.7 US gallons). Enter your exact numbers in the calculator above for a closer result.
How many square feet does one gallon of paint cover?
One US gallon usually covers about 350 to 400 square feet with a single coat. That is around 8 to 10 square metres per litre. Always check the label of your specific paint.
Should I subtract windows and doors?
Yes. Subtracting them prevents you from over-buying. If you are unsure or your walls are heavily textured, skip the deduction and let the extra paint act as a safety margin.
Do I need to paint the ceiling with the same paint?
Not necessarily. Ceilings are often painted with a special flat white ceiling paint that hides marks and reflects light softly. If you use it, calculate the ceiling on its own by ticking the box, then subtract that amount from your wall paint.
How long does paint take to dry between coats?
Most water-based emulsions are touch-dry within one hour and ready for a second coat after two to four hours. Oil-based paints can take 6 to 24 hours. Follow the instructions printed on the tin.
Is the result exact?
No calculator can be perfectly exact, because paint absorption depends on the wall surface, application method and paint quality. Treat the result as a reliable planning estimate and add a small buffer.
Plan your colour before you buy
Once you know how much paint to buy, the next big decision is colour. Try our 60-30-10 Colour Palette Generator to build a balanced scheme, then use our Wallpaper Calculator if you want a feature wall instead of paint. Happy decorating!