Paint Calculator
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Result
Paint to buy
- Paintable wall area
- 397 sq ft
- Area to cover, all coats
- 794 sq ft across 2 coats
- Deducted for openings
- 51 sq ft (1 door, 2 windows)
- You need 2.27 gallons, so 3 leaves most of a tin spare. Buying 2 gallons plus a quart covers it with less waste, if the colour is available in both.
Paint needed by number of coats
| Coats | Area to cover | Gallons needed | Gallons to buy |
|---|---|---|---|
| 1 | 397 sq ft | 1.13 | 2 |
| 2 | 794 sq ft | 2.27 | 3 |
| 3 | 1,191 sq ft | 3.4 | 4 |
How much paint do I need?
To work out how much paint you need, multiply the room's perimeter by the ceiling height for the wall area, subtract 21 square feet per door and 15 per window, then divide by the paint's coverage and multiply by the number of coats. One gallon typically covers 350 square feet.
gallons = ((2(l + w) × h − openings) × coats) ÷ coverage- l, w
- Room length and width in feet
- h
- Ceiling height in feet
- openings
- 21 sq ft per door, 15 per window
- coverage
- Square feet per gallon, from the tin — typically 350
Wall area is the room's perimeter multiplied by its height, which is why a long thin room takes more paint than a square one of the same floor area: a 16 by 12 room has a 56-foot perimeter, while a 14 by 14 room with the same 192 square feet of floor has only 56 feet as well — but a 24 by 8 room has 64 feet and needs about 14% more paint. Doors and windows are deducted at standard sizes because measuring each one individually rarely changes the gallon count. Coverage is the figure most worth checking on the tin rather than assuming: 350 square feet per gallon holds for one coat on primed drywall, but bare drywall, fresh plaster and deep colours over light ones all drink considerably more, and a textured surface can cut coverage by a third. Two coats is the normal specification, and the arithmetic doubles accordingly.
- Add the four wall lengths together to get the perimeter.
- Multiply the perimeter by the ceiling height for wall area.
- Subtract 21 square feet per door and 15 per window.
- Multiply by the number of coats you are applying.
- Divide by the coverage on the tin, usually 350 square feet.
- Round up to whole gallons, or a gallon plus quarts.
Worked example: A 16 × 12 ft room with 8 ft ceilings, one door, two windows, two coats
A 16 × 12 ft room with 8 ft ceilings, one door, two windows, two coats
The perimeter is 2 × (16 + 12) = 56 feet. Multiplied by the 8-foot ceiling that is 448 square feet of wall. Deduct one door at 21 square feet and two windows at 15 each, which is 51 square feet, leaving 397 square feet of paintable wall. Two coats means covering 794 square feet. At 350 square feet per gallon that is 2.27 gallons, so you buy 3 gallons — and since 2.27 leaves most of the third tin unused, two gallons plus a quart is the tidier purchase if the colour comes in both. Adding the ceiling would put another 192 square feet in, taking two coats to 1,178 square feet and 3.37 gallons. Note that ceilings normally take a different, flatter paint, so that is usually a separate purchase rather than part of this total.
What each paint input means
- Room length
- The longer wall, in feet.
- Room width
- The shorter wall, in feet.
- Ceiling height
- Floor to ceiling, in feet. Eight feet is standard in most US homes; older houses are often nine or ten.
- Number of coats
- Two is standard and gives even colour. One coat only works when repainting the same colour over a sound surface.
- Doors
- Each door is deducted at 21 sq ft, the area of a standard 3 × 7 ft opening.
- Windows
- Each window is deducted at 15 sq ft, roughly a 3 × 5 ft opening. Adjust the count up for unusually large glazing.
- Painting the ceiling too
- Adds the floor area to the total. Ceilings usually take ceiling-specific paint, bought separately.
- Coverage per gallon
- From the paint tin. 350 sq ft per gallon is typical for one coat on primed drywall; porous or unprimed surfaces take considerably more.
- Include a primer coat
- Needed on bare drywall, over a dark colour, or on a patched wall. Primer is calculated as one additional coat.
- Price per gallon
- Optional, for a cost total.
What this paint does not account for
- Coverage varies far more than the arithmetic. Bare drywall, new plaster, textured surfaces and deep colours over light ones can all cut 350 sq ft per gallon by a third or more. Check the tin.
- Standard opening sizes are used — 21 sq ft per door, 15 per window. Patio doors, bay windows and floor-to-ceiling glazing are much larger and need the count adjusted upward.
- Trim, doors and window frames are not included. They take a different paint, usually a gloss or satin, and a quart often covers a whole room's trim.
- Ceilings normally take a dedicated flat ceiling paint, so including the ceiling area here mixes two different purchases into one figure.
- A colour change from dark to light may need two coats plus primer, and sometimes three. The primer option adds one coat's worth.
- Sloped ceilings, stairwells and vaulted rooms are not rectangular and this calculation will understate them. Measure the wall faces individually.
Frequently asked questions
How much does a gallon of paint cover?
About 350 square feet for one coat on primed drywall. Bare drywall, textured walls and deep colours cover less, sometimes as little as 250. The tin states the figure for that specific paint.
How many gallons do I need for a 12x16 room?
About 2.3 gallons for two coats on the walls, so buy 3 — or 2 gallons plus a quart. That is a 56-foot perimeter at 8 feet high, less 51 square feet for one door and two windows.
Do I need one coat or two?
Two, in almost every case. One coat only covers when you are repainting the same colour over a sound painted surface. Any colour change, bare drywall or patching needs two for even coverage.
Should I deduct doors and windows?
Yes, at roughly 21 square feet per door and 15 per window. On a small room with several openings that is easily 20% of the wall area, which is a whole gallon on a two-coat job.
Do I need primer?
On bare drywall, new plaster, patched walls or when going light over dark, yes. Over a sound painted wall in a similar colour, a self-priming paint handles it. Primer counts as one extra coat.
How much paint do I need for the ceiling?
The ceiling area equals the floor area, so a 12 by 16 room is 192 square feet — about 0.55 gallons a coat. Buy ceiling paint separately; it is formulated flatter to hide roller marks.
Sources and review
Last reviewed .