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Make It Exact

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Paint Calculator

Enter the room and the coverage printed on the tin. Openings come off, coats multiply, and whole tins are what you buy.

Add the four walls: a 12 × 14 ft room is 52 ft around.

A door is about 21 ft², an average window about 15 ft².

A starting figure, not a fact — read yours off the tin. Tins quote 250 to 400.

The formula

wall area = perimeter × height
net area = wall area − openings
total area = net area × coats
gallons = total area ÷ coverage
(and coverage itself = 1604 × volume solids ÷ dry film thickness in mils)

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.

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