Paint calculator
Enter the room and the coats, set the coverage printed on your bucket, and get litres and buckets — with doors and windows deducted.
| Breakdown | Value |
|---|
Estimate. The method and sources are on this page — check them against your own case before you act.
How it is calculated
Paint is sold by volume, not by area, so the job is to turn the surface you are painting into litres. Coverage per litre is the bridge, and it is printed on the bucket.
Net area = 2 × (length + width) × height + ceiling − openings · Litres = net area × coats ÷ coverage
Work out the paintable surface
A room has four walls: 2 × (length + width) × height. Add the ceiling if you are painting it, then subtract the door and window area you are not painting.
Multiply by the number of coats
One coat of a light colour over white may be enough. A colour change, a darker shade or a repaired wall usually needs two; deep colours sometimes need three.
Divide by the coverage on the bucket
A 10 L bucket of interior emulsion labelled 12 m²/L per coat covers 120 m² in one coat. Divide the paint area by that coverage and round up to the bucket sizes sold.
Paint coverage by product
| Product | Coverage per litre | Notes |
|---|---|---|
| Interior emulsion | 12–14 m² per coat | The default used here, about 130–150 sq ft. |
| Exterior emulsion | 10–12 m² per coat | Rougher surfaces and weather exposure. |
| Primer | 10–12 m² per coat | Not included in the result; budget it separately. |
| Enamel / gloss | 14–16 m² per coat | Doors, grills and trims. |
| Wall putty | 1.5–2 kg per m² | Two coats on a fresh plaster wall. |
| Common bucket sizes | 1 L, 4 L, 10 L, 20 L | Buying 10 L or 20 L usually costs less per litre. |
What most pages get wrong
The bucket label beats every rule of thumb
Coverage varies by product, colour and surface. The calculator uses 12 m² per litre by default; type the number from your bucket and the answer follows your paint.
Two coats is not double the paint if the first is thin
Coverage figures assume a normal coat. A heavily thinned first coat on fresh plaster covers more area but hides less, so the second coat often needs more paint than the label suggests.
The result is topcoat only
A new wall needs putty and primer before the finish coats. Neither is included here, so add them from their own coverage figures.
Deduct doors and windows, but not the frames
Only the opening area that will not be painted is deducted. Window frames, grills and skirting are separate items and take enamel, not emulsion.
Frequently asked questions
How much paint is needed for a 4 × 3 m room with 3 m walls?+
Does the calculator include the ceiling?+
How many sq ft does 1 litre of paint cover?+
Should I add extra paint for wastage?+
How much primer do I need?+
Is this calculator for interior or exterior painting?+
Sources, dates and a changelog.
Every number this calculator uses is either an input you control or a value from the sources below. The formula is compiled and tested at build time, so the page cannot ship with a broken calculation.
Sources
This page
- Last verified: 29 October 2026
- Next review: 29 April 2027
- Vertical: Construction & Real Estate
- Build tests: 4 passed
- Changelog: 1 entry
How we handle errors
- Spotted a mistake? Report it — we correct and log it.
- Corrections appear in the changelog below.
- Read the methodology and the disclaimer.