Paint Calculator
Find how much paint a room needs from its dimensions, its doors and windows, and how many coats you plan to put on.
21 sq ft each (3 × 7 ft)
15 sq ft each (3 × 5 ft)
Check the can; 350–400 is typical.
301 sq ft × 2 coats at 350 sq ft/gal = 2 gallons
Paint is sold by volume but used by area, so every paint estimate is the same three steps: work out how many square feet you are actually covering, multiply by the number of coats, then divide by how far a gallon goes. This calculator does all three from measurements you can take with a tape measure in about two minutes.
How the wall area is worked out
A rectangular room's four walls are its perimeter times its ceiling height. The perimeter is twice the length plus twice the width, so a 12 × 10 ft room has a 44 ft perimeter, and at an 8 ft ceiling that is 352 square feet of wall. Note that only the length and width change the perimeter — a long, narrow room has more wall than a square room of the same floor area, which is why the calculator asks for both dimensions rather than just the square footage of the floor.
From that gross figure the calculator subtracts the openings you are not painting. It uses the standard sizes carpenters assume: 21 sq ft for a door (a 3 × 7 ft slab) and 15 sq ft for a window (3 × 5 ft). One door and two windows in that same 12 × 10 room deducts 51 sq ft, leaving 301 sq ft of paintable wall. If you tick Paint the ceiling too, the length × width of the floor is added back on — 120 sq ft here, for 421 sq ft in total.
If the openings you enter would add up to more than the wall itself, the deduction is capped at the wall area rather than producing a negative result. That only happens for deliberately silly inputs, but it means the breakdown always adds up on screen.
Coats and coverage
The paintable area is then multiplied by the number of coats. Two coats is the normal answer for a repaint in a similar colour: the first coat evens out the surface and the second delivers the actual colour depth. Go to three when you are covering a dark colour with a light one, painting over a bold accent wall, or using a thin, low-sheen paint. Drop to one coat only if you are refreshing an identical colour over a sound surface — and consider priming instead if you are covering something dramatic, since a coat of primer is cheaper than a third coat of finish paint.
Coverage is how far one gallon goes in a single coat. The rule of thumb is 350 square feet per gallon, and that is what the calculator uses by default, but it is genuinely variable, so the field is editable. Read the number off the can and use that instead when you can. Real coverage drops toward 250–300 sq ft/gal on porous or unprimed drywall, on rough plaster or textured walls, and on masonry; it rises toward 400 when you are rolling a smooth, already-painted surface. Sprayers waste more than rollers, and brushwork on trim goes further per gallon than either.
Why the answer rounds up
The calculator reports both the exact figure and the number of whole gallons to buy, because you cannot buy 1.72 gallons. Rounding up is the right default: running out mid-wall means a second trip, and worse, a second can that may be mixed to a very slightly different batch colour. Leftover paint is not wasted either — you want some for touch-ups, and most stores will match a colour from an existing can far more reliably than from a chip.
One thing the round-up does not account for: if your figure lands just over a whole gallon — say 2.1 gallons — it is often cheaper to buy two gallons plus a quart than three full gallons. A quart is roughly a quarter of a gallon and covers about 85–90 sq ft in one coat.
Worked examples
- Small bedroom, walls only. 12 × 10 ft, 8 ft ceiling, 1 door, 2 windows, 2 coats. 352 sq ft of wall − 51 sq ft of openings = 301 sq ft, × 2 coats = 602 sq ft, ÷ 350 = 1.72 gallons → buy 2 gallons.
- The same room with the ceiling. Add 120 sq ft of ceiling for 421 sq ft paintable, × 2 coats = 842 sq ft, ÷ 350 = 2.41 gallons → buy 3 gallons. (In practice you would buy 2 gallons of wall colour and 1 of ceiling white, which is exactly how the split falls out.)
- Master bedroom. 14 × 12 ft, 9 ft ceiling, 2 doors, 2 windows, 2 coats. 468 − 72 = 396 sq ft, × 2 = 792 sq ft, ÷ 350 = 2.26 gallons → buy 3 gallons.
- Living room including the ceiling. 20 × 15 ft, 9 ft ceiling, 2 doors, 3 windows, 2 coats. 630 − 87 + 300 = 843 sq ft, × 2 = 1,686 sq ft, ÷ 350 = 4.82 gallons → buy 5 gallons.
- Small bathroom. 8 × 6 ft, 8 ft ceiling, 1 door, 1 window, 2 coats. 224 − 36 = 188 sq ft, × 2 = 376 sq ft, ÷ 350 = 1.07 gallons → buy 2 gallons. This is the classic case for buying a gallon plus a quart instead.
- One coat only. The first example at 1 coat is 301 sq ft, ÷ 350 = 0.86 gallons → buy 1 gallon.
What this does not cover
The estimate is for wall and ceiling paint on a rectangular room. Trim, baseboards, door frames, and window casings usually take a different paint in a different sheen, and they are measured by linear foot rather than area — a quart typically does the trim of an average bedroom. Primer is a separate calculation: run the tool again with 1 coat and your primer's own coverage figure. Closets, alcoves, bay windows, and sloped or vaulted ceilings all change the geometry; the simplest fix is to run the calculator once per rectangular section and add the gallons.
Doors themselves, if you are painting them, add roughly 2 × 21 sq ft each because they have two faces. Since the calculator has just deducted that area from the walls, add it back by treating the doors as their own small job rather than adjusting the room figures.
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with 8 ft ceilings has a 48 ft perimeter, so 384 sq ft of wall. Deducting a standard door and two windows (51 sq ft) leaves 333 sq ft, and two coats needs 666 sq ft of coverage — about 1.9 gallons at 350 sq ft per gallon, so you would buy 2 gallons. Add the ceiling and it becomes 477 sq ft paintable, 954 sq ft over two coats, or 3 gallons.
How many square feet does a gallon of paint cover?
The standard rule of thumb is 350 square feet per gallon for one coat, which is what this calculator uses by default. Actual coverage varies: expect 250–300 sq ft/gal on porous, unprimed, or textured surfaces, and up to 400 on smooth, previously painted walls. The number is printed on the can, and the coverage field here is editable so you can use it.
Do I need one coat or two?
Two is the normal answer for a repaint. The first coat evens out the surface and the second gives the colour its real depth and uniformity. Use three coats when going from a dark colour to a light one, or when using a thin low-sheen paint. One coat is only enough when refreshing the exact same colour over a sound, clean surface — and if you are covering something dramatic, a coat of primer is cheaper and more effective than a third coat of finish paint.
Should I subtract doors and windows from the wall area?
Yes, if the room has more than one or two of them. This calculator deducts 21 sq ft per door and 15 sq ft per window, the standard 3 × 7 ft and 3 × 5 ft assumptions. In a small room with several large windows the deduction can be 20% of the wall area, which is easily a whole gallon. If your openings are unusually large — patio doors, picture windows — the estimate will run high, so measure those individually and treat the difference as spare.
Does the estimate include the ceiling?
Only if you turn on "Paint the ceiling too", which adds the floor area (length × width) to the paintable total. It is off by default because ceilings are usually painted in a different product — flat ceiling white rather than the wall colour — so you would generally buy them as separate cans even when doing both in one weekend.
How much paint do I need for trim and baseboards?
Trim is not included here. It is measured by linear foot rather than area and almost always uses a different paint in a semi-gloss or gloss sheen. As a rough guide, a quart covers the baseboards, door frames, and window casings of an average bedroom. Buy trim paint separately from your wall estimate.
Why does the calculator tell me to buy more than it says I need?
Because paint is sold in whole cans. The tool shows the exact figure (say 1.72 gallons) and then the whole gallons to buy (2). Rounding up is deliberate: running out mid-wall costs you a trip and risks a slightly different mixing batch. The leftover is useful for touch-ups. If you land just over a whole gallon, buying gallons plus a quart is usually cheaper than another full gallon.