How Much Paint for a Room, Without Buying Too Much
Paint estimating is one multiplication and one honest assumption: that you will need two coats. Get either wrong and you are back at the shop mid-job, hoping the batch still matches.
By Rohit Sharma, Founder, SEOShouts
Paint is easy to estimate and easy to get wrong, because the two mistakes pull in opposite directions. Underestimate and you run out mid-wall, then risk a visible seam where a new tin, possibly a slightly different batch, takes over. Overestimate and you have half-tins cluttering a shelf for a decade. The fix is a simple calculation done honestly.
The formula
litres = (wall area x number of coats) / coverage per litre
Coverage is on the tin, usually 10 to 12 square metres per litre (roughly 350 to 400 sq ft per US gallon).
Start with wall area: for each wall, height times width, added together. A room 4 by 5 metres with 2.4-metre ceilings has four walls totalling about 43 square metres. At two coats and 11 square metres per litre, that is (43 x 2) / 11 = 7.8 litres, so you buy a 10-litre or two 5-litre tins.
Two coats is the honest default
The single biggest estimating error is planning for one coat. Almost no real repaint covers in one, especially over a different colour, and the coverage figure on the tin is a laboratory best case on primed, ideal surfaces. Plan for two coats unless you are applying the exact same colour over a sound existing finish. If you are going light over dark, or bare plaster is involved, budget for a primer or a third coat.
| Situation | Coats to plan for |
|---|---|
| Same colour, sound surface | 1 |
| New colour, similar shade | 2 |
| Light over dark, or dark over light | 2 to 3 |
| Bare plaster or filler | Primer + 2 |
What to deduct, and what not to
Subtract large openings, doors and big windows, from the wall area, since you are not painting them. Do not bother deducting small windows or switch plates; the area is trivial and the leftover is useful for touch-ups. A common rule is to deduct a standard door as about 1.8 square metres and a standard window as about 1.5, and ignore anything smaller.
Ceilings and trim are separate
A ceiling is its own area, length times width, and usually its own paint. Trim, skirting, and doors use a different product again and a small amount goes a long way, so estimate those separately rather than folding them into the wall figure. Keeping the three surfaces separate is what stops a room estimate from quietly drifting wrong.
- Add up wall area: height times width for each wall.
- Deduct doors and large windows; ignore small openings.
- Multiply by the number of coats you honestly need (usually two).
- Divide by the coverage rate on the tin.
- Add about 10 percent, and buy it all in one batch.
Frequently asked questions
How much paint do I need for a room?
Multiply total wall area by the number of coats, then divide by the coverage rate on the tin (usually 10 to 12 square metres per litre). For most repaints that means two coats. Add about 10 percent for waste and touch-ups.
Do I really need two coats?
Almost always. The coverage figure on the tin is a best case, and one coat rarely hides the old colour evenly. Plan for two unless you are applying the identical colour over a sound existing finish.
Should I buy all the paint at once?
Yes. Tins mixed at different times can vary slightly in shade, and the difference is most visible on a large flat wall. Buying one batch, with a small margin included, avoids a mismatched top-up mid-job.
Calculators from this guide
About the author
Rohit Sharma is the founder of SEOShouts, a search consultancy in India, and has worked in technical SEO and content strategy since 2014. He builds and maintains Calcshark.