Paint calculator: walls, coats and cans
Paint math is walls minus openings, divided by coverage, times coats: a 5×4 m room with 2.7 m ceilings needs about 9 litres for two coats at 10 m² per litre. Coverage printed on the can (often 8-12 m²/L, or 350-400 sq ft per US gallon) decides everything, and doors and windows come off the total first. This calculator is built for fast, private, offline use in your browser. Every formula runs locally and is verified against official sources. You enter the key numbers, see the result instantly and check the steps.
TL;DR
- Wall area = 2×(length+width)×height.
- Formulas: L = (walls − openings) ÷ coverage × coats
- Example 1: 5×4 m room, 2.7 m ceiling, 1 door + 1 window, 2 coats at 10 m²/L — Walls: 2×(5+4)×2.7 = 48.6 m²
How it works
Wall area = 2×(length+width)×height. Subtract doors and windows, divide by coverage per litre, multiply by coats. Cans are rounded up to whole 1 L and 5 L sizes. Start with clean data, keep the same units and follow the order in the formula. The tool does the arithmetic, you choose rate, period or factor. A second example with different numbers shows how the result scales.
The math
2×(5+4)×2.7=48.6 m² walls, minus 3.4 m² openings = 45.2 m² ÷ 10 m²/L = 4.52 L per coat × 2 coats = 9.04 L, so 10×1 L or 2×5 L cans. Small room check: 2×(3+3)×2.5=30 m² ÷ 12 = 2.5 L for one coat.
Two coats double one-coat litres. Cans always round up.
Examples
Example 1: 5×4 m room, 2.7 m ceiling, 1 door + 1 window, 2 coats at 10 m²/L
- Walls: 2×(5+4)×2.7 = 48.6 m²
- Net: 48.6 − 3.4 = 45.2 m²
- Paint: 45.2 ÷ 10 × 2 = 9.04 L
- Buy: 10×1 L or 2×5 L cans
Example 2: 3×3 m room, 2.5 m ceiling, no openings, 1 coat at 12 m²/L
- Walls: 2×(3+3)×2.5 = 30 m²
- Paint: 30 ÷ 12 = 2.5 L
- One 2.5 L can covers it with margin
When to use it
✦ Bedroom repaint
Price two coats for a bedroom before the weekend: litres, cans and cost per can from one input.
✦ Rental touch-up
One-coat refresh of a small room: litres for a single coat and the smallest can that covers it.
⚠ Common mistakes
- Forgetting openings: a door plus window (3.4 m² here) is almost a litre of paint.
- One-coat maths for two coats: double the litres, the second coat covers better but still counts full.
- Trusting nominal coverage on rough plaster: cut 8-12 m²/L down by a fifth on textured walls.
FAQ (7)
How much for two coats?
Double one-coat litres: 4.52 L becomes 9.04 L here.
Which cans?
Rounded up to whole cans: 10×1 L or 2×5 L for 9.04 L.
Does the ceiling count?
No, this calculator covers walls only; measure the ceiling separately as length×width.
Do you save the numbers I type for area, volume or conversions?
Not at all. Every conversion — kg to lbs, area, fraction math — runs locally. No form data is posted anywhere. You can convert 1,500 kg to lbs on a shared computer and walk away knowing nothing was logged.
Will it work without internet in class?
Yes — that's why students like it. Load once on Wi-Fi, then fraction, base-converter or standard deviation work offline in class or in an exam hall without signal. It's just math in JavaScript, no server round-trip.
How accurate are the math results?
We use standard definitions (e.g. 1 kg = 2.20462 lbs, BIPM SI) and double-check edge cases via automated tests. For homework or site measurements it's more than enough, but for certified engineering or lab work, cross-check with the primary standard.
Can I share a calculation with a classmate?
Tap 'Share Link' — it puts the inputs in the URL. Your classmate opens it and sees the same 3/4 → 0.75 → 75% breakdown instantly, without retyping.
Sources & References
How this calculator is maintained
This calculator is written and maintained by Davide Coronese, independent developer and founder of CalcTracker.
Methodology: every formula is checked against official publications and validated by an automated test suite before release. Health, tax and finance pages cite the exact sources they rely on.
Found an error? Contact us — verified corrections ship within 24 hours.