Home & construction
Paint Calculator
Enter the room and the coverage printed on the tin. Openings come off, coats multiply, and whole tins are what you buy.
The link carries your figures, so it reopens on exactly these numbers.
What each extra coat costs
Your walls, your paint. Coverage is per coat, so the paint needed is directly proportional — the tins, however, only move when a whole one is crossed.
| Coats | Paint actually needed (gal) | Tins to buy |
|---|---|---|
| One coatOnly ever enough over the same colour. | 1.04 | 2 |
| Two coatsThe normal specification. | 2.09 | 3 |
| Three coatsDark over light, or bare substrate. | 3.13 | 4 |
The formula
Worked example
A 12 × 14 ft room, 8 ft walls, one door and two windows, two coats at 350 ft²/gal
- 52 × 8 = 416 ft² of wall
- 416 − 51 = 365 ft² once the door and windows come off
- 365 × 2 coats = 730 ft² to cover
- 730 ÷ 350 = 2.09 gallons → 3 tins
Where this goes wrong
Treating the coverage on the tin as a property of the paint
It is not. Coverage is the volume solids divided by how thick you lay it on: a gallon of pure solids covers 1,604 ft² at one mil, so 35% solids at 1.6 mils gives 351 ft²/gal and the same paint at 2.5 mils gives 225. That is why one tin claims 400 and another 250 without either being wrong, and why a textured or porous wall eats far more than the label suggests — it is holding a thicker film. Sherwin-Williams publishes both halves of this on its data sheets: 68% solids, 1,089 ft²/gal at one mil, and a spreading rate table that divides straight out of it.
Questions
- How much paint do I need for a room?
- Perimeter times wall height, minus the doors and windows, times the number of coats, divided by the coverage on the tin. For a typical 12 × 14 ft room with 8 ft walls and two coats, that lands near two gallons — but the coverage figure is the part that varies, so read it off the tin rather than trusting a rule of thumb.
- Should I subtract doors and windows?
- Yes. A door and two windows in an average room are around 50 ft², which is more than a tenth of the wall area and can be a whole tin on a larger job. Some calculators skip it on the reasoning that you will want spare paint; that is a decision you should make yourself, not one hidden in the arithmetic.
- Why does the second coat use less paint than the first?
- It usually does, on a porous or previously unpainted surface: the first coat is partly absorbed, the second sits on a sealed film. This page assumes both coats use the same amount, which errs slightly on the generous side. On a previously painted wall in a similar colour, the difference is small.
- What is volume solids?
- The proportion of the tin that stays on the wall after the solvent or water evaporates — the rest goes into the air. It is printed on every professional data sheet and almost never on a consumer tin, which is why consumer tins quote a coverage range instead. Higher solids means more paint left behind per gallon, so more coverage at the same thickness.