How to use the paint calculator
Enter the room’s length and width (or the total length of the walls you’re painting) and the wall height. Tell it how many doors and windows to subtract, how many coats you’ll apply, and the coverage printed on your paint can. Tick Include ceiling if you’re painting that too. The result is the number of gallons to buy, with the exact figure alongside so you can decide whether to round down to quarts.
The formula
wall area = perimeter × height − (doors × 21) − (windows × 15)
paint area = (wall area + ceiling area) × coats
gallons = paint area ÷ coverage per gallonA standard interior door is about 20–21 sq ft; a typical window 15 sq ft. Coverage of 400 sq ft per gallon is the usual figure for smooth walls.
Worked example
A 14 × 12 ft living room with 9 ft ceilings, two doors, three windows, two coats at 400 sq ft/gal:
- Perimeter: 2 × (14 + 12) = 52 ft; wall area 52 × 9 = 468 sq ft
- Minus openings: 468 − 42 − 45 = 381 sq ft
- Two coats: 762 sq ft ÷ 400 = 1.9 gallons → buy 2
- Ceiling (14 × 12 = 168 sq ft, two coats): 336 ÷ 400 = 0.84 → 1 gallon of ceiling paint
How much a gallon really covers
| Surface | Coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 400 sq ft |
| Lightly textured (orange peel) | 350 sq ft |
| Heavy texture, popcorn ceiling | 250–300 sq ft |
| Bare drywall or plaster (primer first) | 300 sq ft |
| Rough wood, masonry, stucco | 150–250 sq ft |
Sprayers use 20–30% more paint than rollers; dark colours over light (or vice versa) may need an extra coat.
Buying tips
- Buy all cans of a colour at once and from the same batch; tint can vary slightly between mixes. Box (mix together) multiple gallons before starting.
- Round to the nearest quart for small jobs — a quart covers about 100 sq ft.
- Keep a labelled leftover for touch-ups; sealed paint lasts several years.
- Trim and doors use much less: a quart typically does the trim in an average room.
- Measuring an unusual room? Get the area first with the square footage calculator.
Frequently asked questions
How much paint do I need for a 12×12 room?
With 8-foot ceilings, one door and two windows, the walls are about 333 sq ft. Two coats need 666 sq ft of coverage — just under two gallons at 400 sq ft per gallon. Add another gallon if you're painting the ceiling.
How many square feet does a gallon of paint cover?
Most interior paints cover 350–400 sq ft per coat on smooth, previously painted walls. Textured, porous or unprimed surfaces absorb more — figure 250–300. Check the can; manufacturers print the rate.
Do I need two coats?
Almost always, for an even finish and true colour. One coat can be enough with a paint-and-primer product in a similar colour. Going from dark to light, or painting bare drywall, may need a primer plus two coats.
Should I subtract doors and windows?
Yes — the calculator deducts 21 sq ft per standard door and 15 sq ft per window. Skip the deduction for very small windows or if you want a margin for touch-ups.
Last updated August 27, 2026.