How much paint do I need?
The full calculation, including the deductions and waste most guides skip.
For a standard 12 x 12 ft room with an 8 ft ceiling, two coats and paint covering 350 sq ft per gallon, you need about 2.4 gallons. In practice, two gallons plus a quart, or three one-gallon tins.
Where that number comes from
What you paint is not the room, it is the perimeter times the height. A 12 x 12 ft room has a 48 ft perimeter, which at 8 ft high gives 384 sq ft of wall. Then you take out the openings: a door is about 17 sq ft and an average window 16 sq ft. That leaves 351 sq ft.
Those 351 sq ft get painted twice, so the total painted area is 702 sq ft. Divided by the tin's coverage (350 sq ft per gallon per coat) that is 2.0 gallons, and with 10 % waste, 2.2.
Quick table by room size
With an 8 ft ceiling, two coats, paint covering 350 sq ft/gal, one door and one window:
Swipe to see the whole table
| Room | Net wall area | Gallons (2 coats) | What to buy |
|---|---|---|---|
| 10 x 10 ft | 287 sq ft | 1.8 gal | 2 gallons |
| 12 x 12 ft | 351 sq ft | 2.2 gal | 2 gal + 1 quart |
| 14 x 16 ft | 447 sq ft | 2.8 gal | 3 gallons |
| 20 x 15 ft | 527 sq ft | 3.3 gal | 3 gal + 1 quart |
The three figures that change the answer
1. The coverage on the tin
This varies most and is the only one that does not come from your room. Interior emulsion covers 350-400 sq ft per gallon per coat. Over new drywall, very porous surfaces or textured walls it can drop to 250. It is printed on the tin, and that is the figure to use.
2. The number of coats
Two is normal over a wall already painted a similar colour. Three when covering dark with light, painting unprimed new board, or using a low-hiding colour (intense reds and yellows are the worst offenders).
3. Waste
10 % covers what stays in the tray, the roller and the floor. First time painting? Use 15 %: a spare quart costs little, running out mid-wall costs another trip and a tin that may be from a different batch.
Common mistakes
- Calculating from floor area. A 144 sq ft room has 384 sq ft of wall. Those are not remotely the same number.
- Forgetting it is two coats. Doubling the area to be painted is the single biggest factor in the result.
- Deducting too much. Subtracting fitted wardrobes or furniture is a bad idea: they move, and then the paint runs out exactly where it now shows.
- Buying exactly enough. The spare tin for touch-ups is worth its weight in gold two years later, when that colour no longer exists.
Sources and methodology
The calculation uses elementary geometry (perimeter times height, minus openings) and a configurable coverage figure, because commercial coverage varies between manufacturers and paint types. The defaults (350 sq ft/gal, 17 sq ft per door, 16 sq ft per window) are common values and all are editable in the calculator.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2.2 gallons for two coats with an 8 ft ceiling, one door and one window, using paint that covers 350 sq ft per gallon. Buy two gallons and a quart.
How much does a gallon of paint cover?
350 to 400 square feet per coat on a prepared surface. On porous, textured or unprimed walls it can fall to 250 sq ft per gallon.
Is the ceiling included?
Only if you tick that option. Then the room's length times width is added to the wall area.
How much paint for two coats?
Exactly twice as much as for one. The painted area is multiplied by the number of coats before dividing by the coverage.
Is it better to buy too much or too little?
Too much, without question. A spare tin covers touch-ups for years; going back to the store mid-job costs time, and a new batch may not match the shade exactly.