Paint Calculator
Enter the room dimensions, the number of coats and how many doors and windows you are skipping. The result is the number of gallons to put in the cart.
How the paint calculator works
Paint is sold by volume, and coverage is quoted in square feet per gallon, so the whole calculation is an area problem:
- Wall area = perimeter × ceiling height. Perimeter is 2 × (length + width).
- Subtract openings — about 21 sq ft per door and 15 sq ft per window. Skip the trim; it is usually masked or painted separately.
- Multiply by coats, then divide by coverage: 350 sq ft per gallon is the safe planning number.
- Round up to whole gallons. Paint is sold in gallons and quarts, and you cannot buy 1.9 gallons.
Worked example
A 12 ft × 14 ft bedroom with 8 ft ceilings, two doors and two windows, two coats:
- Perimeter = 2 × (12 + 14) = 52 ft
- 52 × 8 = 416 sq ft of gross wall area
- 416 − (2 × 21) − (2 × 15) = 416 − 72 = 344 sq ft
- 344 × 2 coats = 688 sq ft
- 688 ÷ 350 = 1.97 → buy 2 gallons
That leaves about a third of a gallon for touch-ups, which is exactly what you want after the furniture goes back in.
Coverage rates to plan with
| Surface | Coverage per gallon | Notes |
|---|---|---|
| Smooth painted drywall | 400 sq ft | Best case, same or lighter colour |
| Previously painted, typical | 350 sq ft | The planning number this tool uses |
| Textured / orange peel wall | 300 sq ft | More surface, more paint |
| Bare drywall or patched walls | 250–300 sq ft | Absorbs heavily — prime first |
| Primer | 300–400 sq ft | Usually one coat is enough |
Doors, windows and trim
Subtracting openings matters more than people expect. In the example above, four openings removed 72 sq ft — about half a gallon of paint. Count the door and window itself, not the trim: trim is normally cut in with a brush and a separate, harder-wearing paint.
If you are painting a ceiling the same colour as the walls, add length × width. Ceilings usually need one coat if they are already white; two if you are going from a colour.
Frequently asked questions
How much does a gallon of paint cover?
About 350 to 400 square feet in a single coat on smooth, previously painted drywall. Assume 350 for planning. Textured walls drop to about 300, and bare drywall can be as low as 250 because it absorbs so much.
How many gallons of paint for a 12x14 room?
Two gallons, for two coats. The room has about 344 sq ft of paintable wall after subtracting two doors and two windows, which is 688 sq ft across two coats — just under two gallons at 350 sq ft per gallon.
Should I use one coat or two?
Two, for almost everything. One coat only works when you are refreshing the same colour over a sound, similar surface. Any colour change, sheen change, or repair needs two coats to look even in raking light.
Do I need a separate primer?
Yes over bare drywall, joint compound, bare wood, or when going from a dark colour to white. Paint-and-primer products are fine over existing paint, but they are not a substitute for primer on raw substrate.
Does the calculator include the ceiling and trim?
The ceiling is optional — tick “Include ceiling” if you are painting it. Trim, doors and baseboards are excluded because they use a different paint and are normally estimated by linear feet.