How Much Paint for Walls?
One formula works for every wall in every building. Here it is, with the reasoning behind each step.
The formula is: (perimeter × wall height) − openings, × coats, ÷ coverage. For a typical 12x12 room with 8-foot ceilings that gives 2 gallons. The important part is the first term: paint depends on the perimeter, not the floor area.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Wall paint by room size
| Room / surface | Surface | Coats | Paint | Paint cost |
|---|---|---|---|---|
| 10x10 room — perimeter 40 ft | 284 sq ft | 2 | 2 gal | $70 |
| 10x12 room — perimeter 44 ft | 301 sq ft | 2 | 2 gal | $70 |
| 12x12 room — perimeter 48 ft | 333 sq ft | 2 | 2 gal | $70 |
| 12x14 room — perimeter 52 ft | 365 sq ft | 2 | 3 gal | $105 |
| 14x14 room — perimeter 56 ft | 397 sq ft | 2 | 3 gal | $105 |
| 12x16 room — perimeter 56 ft | 397 sq ft | 2 | 3 gal | $105 |
One door and two windows, two coats at 350 sq ft per gallon. Note how rooms with the same perimeter need the same paint.
The formula, step by step
- Measure the perimeter. Add the length of every wall, or measure two adjacent walls and double the sum.
- Multiply by the wall height. That gives gross wall area in square feet.
- Subtract the openings. Use 21 square feet per door and 15 square feet per window — the standard allowances paint manufacturers work from.
- Multiply by the number of coats. Two is the norm.
- Divide by the coverage on the can. Usually 350 to 400 square feet per gallon.
- Round up. Always. You cannot buy part of a gallon.
Put your own numbers into the calculator above and it does all six steps as you type.
The insight that saves the most money
Wall area is perimeter × height. Floor area never enters the calculation, because you are not painting the floor.
This produces results that seem wrong until you see why:
- A 14x14 room and a 12x16 room have different floor areas — 196 against 192 square feet — but the same perimeter of 56 feet, so they need the same 3 gallons.
- An 8x20 room has a floor of just 160 square feet and still needs the same 3 gallons, because its perimeter is also 56 feet.
Long, narrow rooms are the classic estimating trap. Always measure the walls.
Choosing the coverage figure
Coverage is the one input people guess, and it is the one that changes the answer most.
- 400 sq ft/gal — smooth, previously painted walls, quality paint.
- 350 sq ft/gal — the typical figure on an interior wall paint label. Use this by default.
- 300 sq ft/gal — lightly textured walls, or cheaper paint.
- 250 sq ft/gal — bare drywall, heavy texture, or stucco.
The label figure is measured on a smooth sealed surface in laboratory conditions. If your walls are anything other than smooth and already painted, use the lower number.
Frequently asked questions
What is the formula for wall paint?
Perimeter times wall height, minus openings, times the number of coats, divided by coverage per gallon. Round the result up to the next whole gallon.
How many square feet does a gallon of wall paint cover?
350 to 400 square feet on smooth previously painted walls, dropping to 250 to 300 on bare or textured surfaces.
How much wall area does a door or window take up?
The standard allowances are 21 square feet for a door and 15 square feet for a window. Subtract those for each opening.
Does ceiling height matter?
Yes, significantly. Going from an 8-foot to a 10-foot ceiling adds 25 % to the wall area, and therefore 25 % to the paint you need.