Paint Calculator
How much paint you need for a room.
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Avoid a mid-job trip to the shop. Enter your room's dimensions, deduct the doors and windows, pick how many coats you're applying, and this calculator estimates the paint you'll need in litres and gallons — with the option to include the ceiling.
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How Much Paint? — The Quick Answer
Find the wall area, subtract the openings, divide by the paint's coverage, then multiply by the number of coats. A litre covers roughly 10–12 m² (a US gallon about 350–400 sq ft) per coat.
Shortcut
Paint = (wall area − openings) ÷ coverage × coats
Typical coverage per coat
| Surface | Per litre | Per US gallon |
|---|---|---|
| Smooth, sealed wall | ~12 m² | ~400 sq ft |
| Typical interior wall | ~11 m² | ~350 sq ft |
| Rough or textured | ~10 m² | ~300 sq ft |
These are per-coat figures for a mid-tone on a prepared surface. Most jobs need two coats, and a big colour change (especially dark to light) can need three or more — always multiply by the number of coats.
How It Works
The Paint Coverage Formula
Area, openings, coverage, and coats
Formula
Wall area = 2 × (length + width) × height Paint = (area − openings) ÷ coverage × coats
Variables
Surface to paint
The perimeter (2 × (length + width)) times the height, plus the ceiling (length × width) if you're painting it.
Doors and windows
Subtracted from the area. This tool uses about 21 sq ft per door and 15 sq ft per window as typical values.
Spreading rate
How far the paint goes per litre or gallon — roughly 10–12 m² (350–400 sq ft) per coat on a good surface. Check your tin.
Number of coats
Two is the usual baseline; dramatic colour changes need three or more.
Note: Wall area uses the perimeter times height — not the floor area, a common mix-up. Minor fixtures aren't worth deducting; the coats-and-waste margin absorbs them.
Worked Example: A 12 × 12 ft Room, 8 ft High
Two coats, one door, two windows
Find the wall area
2 × (12 + 12) × 8 = 384 sq ft
Subtract doors and windows
− 21 (door) − 30 (two windows) = 333 sq ft
Divide by coverage, times coats
333 ÷ 350 × 2 coats ≈ 1.9 gallons
Round up and buy a little extra
Buy 2 gallons, keeping some for touch-ups
Reference Guide
| Surface | Coverage per gallon | Note |
|---|---|---|
| Smooth, sealed wall | ~400 sq ft | Best case |
| Typical interior wall | ~350 sq ft | Common estimate |
| Rough or textured | ~300 sq ft | Drinks more paint |
| Dark-to-light change | needs extra coats | Budget 3+ coats |
Why Coverage Varies So Much
What drives how far a tin goes
One coat rarely looks even, so two is the baseline — and a bold colour change can need three or four. Always multiply by coats, since this is where a room that 'should' take two litres quietly needs five.
Best for: Getting the estimate in the right ballpark
Texture, porosity, and colour change all affect how far paint goes. Rough or bare surfaces drink more; smooth, sealed, previously-painted walls get the most from each litre. Published rates assume near-ideal conditions.
Best for: Adjusting the coverage figure realistically
On bare plaster, new drywall, or a big colour change, priming first seals the surface and reduces how much pricey topcoat you use overall, as well as giving a better finish.
Best for: Deciding whether to prime first
How Paint Coverage Really Works
Estimating paint comes down to four things: the surface area, the openings you subtract, how far the paint spreads, and how many coats you apply. A litre typically covers 10–12 m² (a US gallon about 350–400 sq ft) per coat on a prepared surface — but that's a best case. Real walls fall short in predictable ways: texture and porosity drink more, and covering a dark colour with a light one needs extra coats. The single biggest variable people forget is coats: almost no job is one coat, so the realistic amount is area ÷ coverage × coats. This calculator works from paintable wall area (minus doors and windows), not floor area.
Key Features
💡 Pro Tips
- →Buy slightly more than the estimate and keep the colour code — matching a custom tint later is often impossible, and a little spare covers touch-ups.
- →Prime bare, porous, or drastically re-coloured walls first; it seals the surface and cuts the topcoat you need.
- →Measure wall area as perimeter × height, not floor area — mixing these up is the most common estimating error.
- →Check the spreading rate printed on your specific tin rather than assuming a generic figure.
Common Mistakes
Forgetting to multiply by the number of coats
Most jobs need two coats and bold colour changes need more. Estimating a single coat leaves you well short — this is the number one reason people run out of paint.
Using floor area instead of wall area
Walls are the perimeter times the height, not the floor's length times width. Using floor area badly under- or over-estimates depending on the room's proportions.
Trusting the tin's coverage as a guarantee
Published spreading rates assume smooth, sealed, mid-tone walls. Texture, porosity, and colour changes all reduce real coverage, so treat the figure as a best case and add a margin.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
Dulux / major paint manufacturers (2023). Technical datasheets — spreading rates and coat recommendations. Manufacturer product data.
Source of typical coverage figures and coat guidance.
View source - [2]
Painting and Decorating Association (2023). Surface preparation and priming best practice. paintingdecoratingassociation.co.uk.
Guidance on preparation, priming, and coats.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 9, 2026
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Frequently Asked Questions
How much wall does a litre of paint cover?
Roughly 10–12 square metres (about 350–400 square feet per US gallon) for one coat on a smooth, sealed surface. Rough, porous, or dark-to-light surfaces cover less, so always check the tin and build in a margin.
How many coats of paint do I need?
Two is the usual baseline — one coat rarely gives an even, fully opaque finish. A big colour change, especially dark to light, can need three or more. The calculator multiplies by the number of coats you set.
Should I subtract doors and windows?
Yes, for the large openings — a standard door is about 21 sq ft and a window around 15 sq ft. On a small room, leaving them in overestimates significantly. Minor fixtures aren't worth deducting.
Do I need primer?
Often. On bare plaster, new drywall, glossy areas, or a drastic colour change, primer seals the surface and improves adhesion, and it can reduce the amount of topcoat you use overall. On a sound wall in a similar colour you can usually skip it.
Why did I run out of paint following an estimate?
Usually because the estimate didn't account for enough coats, or the surface drank more than the tin's best-case coverage. Rougher, more porous, or bolder-colour walls need more, so buying a little extra is wise.