Paint Calculator
Wall area minus every door and window, times the number of coats, divided by the real coverage of the paint you bought — for one room or a whole house, in gallons or litres.
Instant answers
Works offline once loaded
Nothing you type is sent anywhere
Doors and windows deducted
Walls, ceiling, trim and primer
Add as many rooms as you like
Rooms
The paint
Square feet per gallon. 350 is typical; check the tin.
Rough surfaces drink paint; the tin's figure assumes smooth.
Buy this much wall paint
—
Net wall area—
Deducted for doors and windows—
Ceiling area—
Trim — linear feet—
Total paint to cover (all coats)—
Estimated cost—
| Job | Area to cover | Coats | Paint needed | Tins to buy |
|---|
Tins are rounded up whole, because paint shops do not sell two-thirds of a gallon. Buying the same batch number for the whole job avoids a visible seam where two tins meet.
Room by room
| Room | Perimeter | Gross walls | Openings | Net walls | Ceiling |
|---|
Instructions
How to use this calculator
Step by step
- Enter each room's length, width and ceiling height in feet. The calculator turns that into a perimeter and multiplies by the height to get the gross wall area.
- Count the doors and windows. Each door comes off at 21 sq ft (3 × 7) and each window at 15 sq ft (3 × 5) — the standard sizes. If yours are much bigger, add an extra door or window to compensate.
- Tick which surfaces you are painting in that room: walls, ceiling, trim, or any combination. A room with only the ceiling ticked contributes no wall area at all.
- Use Add another room for every room in the job. All of them roll up into one shopping figure, which is the point — buying paint once beats three trips.
- Set the coverage from the tin. Most interior latex claims 350–400 sq ft per gallon; premium paints claim more and thin paints deliver less.
- Choose the number of coats. Two is normal. One is only realistic when you are repainting the same colour, and three is for going light over dark or covering a strong colour.
- Set the texture allowance if the surface is not smooth drywall — rough surfaces have more actual area than their measurement suggests.
- Read the buy figure at the top, and the per-job table below it for the ceiling, trim and primer counted separately.
Good to know
- Buy all your paint at once and in the same batch. Two tins mixed weeks apart can differ enough to show as a band on a long wall.
- Do not deduct small windows. The paint you save is less than the paint you waste cutting in around them, and running short mid-wall is far more expensive than an extra half-gallon.
- Ceilings usually need two coats even when the walls need one, because the light rakes across them and shows every thin patch.
- A gallon of primer is not optional over bare drywall, patched repairs, or any strong colour change. Tinting the primer towards your top colour can save a whole coat.
- Trim uses far less paint than people expect but takes far longer. One quart usually does the trim in an average room.
- Keep about half a gallon back for touch-ups and label it with the room name. You will want it within a year.
The maths behind it
- Perimeter P = 2 × (length + width) The distance round the room at floor level.
- Gross wall area A = P × ceiling height Before anything is deducted.
- Net wall area A − (doors × 21 + windows × 15) Standard opening sizes in square feet.
- Paint needed tins = net area × coats ÷ coverage Then rounded up, since tins are sold whole.
- Effective coverage coverage × (1 − texture allowance) A textured wall has more real surface than its measured area.
- Trim length 2 × P + 17 ft per door + 16 ft per window Skirting and cornice both run the perimeter; casing runs round each opening.
How much does a gallon of paint really cover?
Between 250 and 400 square feet for one coat, depending on the paint and the surface. The 350–400 figure printed on the tin assumes a smooth, primed, non-porous wall and a roller in the hands of someone not in a hurry. Over new drywall, textured plaster or a colour change, expect the low end.
Should I really not deduct the windows?
Deduct them here so the estimate is honest, but do not shave your order down to match exactly. This calculator already rounds up to whole tins, which builds in the margin. The rule of thumb among decorators — ignore anything smaller than about 20 square feet — exists because the time lost on a second trip to the shop costs more than the paint.
Do I need primer?
Over bare drywall, new plaster, bare wood, patched repairs, glossy old paint, or any big colour change, yes. Over an existing sound coat of the same sort of colour, no. Primer is cheaper than paint and grips better, so when in doubt it is a good trade.
Why does the calculator suggest fewer coats on the ceiling than the walls?
It caps the ceiling at two, because three coats on a ceiling is almost never what fixes a patchy result — that is usually a primer problem or a paint that is too thin. If your ceiling genuinely needs three, tick trim off and add the ceiling as its own room to control it directly.
Can I use this in metric?
Yes. Switch the unit selector to litres and the coverage box takes square metres per litre — 11 is a good default. Enter the room dimensions in metres and the whole calculation stays consistent. The door and window deductions are metric-equivalent standard sizes.
What about exterior painting?
The same arithmetic works: measure each elevation as a "room" wall, enter zero for the surfaces you are not doing, and set the texture allowance to match the siding. Exterior coverage figures run lower than interior ones, so set the coverage to what the tin says for the substrate you actually have.