What a paint calculator has to measure
Paint is bought by area, not by room, and the area that matters is what a brush actually touches. Walls go round the perimeter, doors and windows come off, and every coat repeats the whole lot. Most disappointing estimates come from skipping one of those three steps.
- perimeter — twice length plus twice width
- 20 — square feet of a standard door opening
- 15 — square feet of a standard window opening
- coverage — 350 square feet per gallon per coat, a mid-range figure within the published 250 to 400
Worked example: a 12 by 14 foot room, 8 foot ceiling
Two and a tenth gallons means three tins. There is no way to buy the remaining tenth, and running out with one wall left is the worst possible moment to discover the shop has a different batch.
Why coverage is a range, and why 350 is the default
Manufacturers publish 250 to 400 square feet per gallon, and every number in that band is honest — the spread is real, and it is wider than most calculators admit. A sealed, previously painted wall in a similar colour reaches the top of it. Bare plaster, new filler, masonry, heavy texture and any strong colour change fall towards the bottom, and the bottom is a third less paint per tin than the top.
This calculates at 350, which is realistic for a typical repaint and is the figure you can change. The room below shows why it is worth changing: at 400 it takes two gallons, and at 350 or anywhere below it takes three. One step along the range is a whole extra tin. If your walls are new, porous or textured, move the coverage down before you buy rather than after.
Common mistakes
- Forgetting the second coat. It doubles the paint. A single-coat estimate is only right if you are refreshing the same colour.
- Subtracting the whole window wall. Only the glazed opening comes off, not the wall it sits in.
- Ignoring the ceiling. On a 12 by 14 room the ceiling is 168 square feet, comfortably another gallon.
- Forgetting primer entirely. It covers about as much ground as paint — 300 to 400 square feet per gallon — but it is a separate purchase. Enter your primer coats above and it is counted as its own line.
When these numbers do not apply
- Textured surfaces. Woodchip, heavy artex and rough render swallow far more than a flat wall of the same area.
- Dark to light changes. Covering a strong colour with white can take three coats or a tinted primer.
- Sprayed application. Spraying puts more material on the surface and into the air than a roller does.
- Woodwork and trim. Skirting, architrave and doors are painted with a different product and are not counted here.
Before the brush
Paint goes on last. If the walls are new, the drywall calculator works out sheets, screws, tape and compound, and all of that has to be finished and sanded before any of this is worth calculating. If the room is an awkward shape, take the area from the square footage calculator rather than treating a bay or an alcove as a rectangle.
Paint for common rooms — 8 ft ceiling, one door, two windows
| Room | Paintable ft² | 1 coat (gal) | 2 coats (gal) |
|---|---|---|---|
| 10 × 10 ft | 270 | 1 | 2 |
| 12 × 12 ft | 334 | 1 | 2 |
| 12 × 14 ft | 366 | 2 | 3 |
| 14 × 16 ft | 430 | 2 | 3 |
| 16 × 20 ft | 526 | 2 | 4 |
| 20 × 24 ft | 654 | 2 | 4 |
Walls only, at the default coverage rate. Coverage varies with surface porosity and how big a colour change you are making, which moves the answer more than any other input.
