How to use the paint calculator
Start at the top. Choose Walls if you are only painting the walls, or Walls + ceiling if the ceiling is getting a fresh coat too. Ceiling paint is listed on its own line, because most people use a flat ceiling white rather than the wall color.
Next, enter the room. Measure the length and width of the floor from wall to wall, then the wall height from the floor to the ceiling. You can type 14’ 6” or 14.5, or tap the minus and plus buttons. Switch to centimeters with the unit toggle and the answer comes out in liters and tin sizes instead of gallons and quarts.
Count the doors and the windows. The calculator takes a standard door and an average window off the wall area for each one. Closet doors count as doors. A wide opening with no door, such as an archway into the next room, can be counted as a door or two.
Then set the job:
- Coats: two is normal. Pick three for a big color change, or one only when you are refreshing the same color.
- Wall surface: smooth drywall, a light texture such as orange peel, or a heavy texture such as knockdown or rough plaster. Texture soaks up more paint, so the coverage per gallon drops.
- Primer: turn it on for new drywall, patched walls, stains or a big jump in color.
Painting more than one room? Tap + Add room and enter each one. The totals cover every room, and the drawing shows the room you are editing. Pick a wall color to see it on the unfolded room, then save the picture to take to the paint counter.
The rules behind the numbers
Coverage comes from the label. Interior paint labels promise 350 to 400 sq ft per gallon for one coat on smooth, primed walls. The calculator uses 375, the middle of that range. On a light texture it uses 325, and on a heavy texture 250, because the paint has to fill every bump. Primer covers less, about 250 sq ft per gallon.
Wall area is the perimeter times the height. Add the four wall lengths and multiply by the wall height. A 14 by 12 foot room has 52 feet of wall. At 8 feet high, that is 416 sq ft.
Doors and windows come off. A standard interior door is about 3 by 6 2/3 feet, so 20 sq ft. An average window is about 3 by 4 feet, so 12 sq ft. Painters subtract these because you are not rolling paint over them. Trim around doors and windows is a separate, smaller job, usually in a different sheen.
Multiply by the coats. Two coats of 372 sq ft is 744 sq ft of painting. Divide by the coverage and you have the gallons.
Round up the smart way. Paint is sold in gallons and quarts. Whole gallons come first, then quarts for the remainder. If the remainder needs three quarts or more, the calculator rounds up to another gallon, since three quarts cost about the same as a gallon.
Assumptions
Walls are treated as plain rectangles with a flat ceiling. Vaulted ceilings, stairwells and gable walls are not modeled. For a sloped wall, measure its average height. Doors use a 36 by 80 inch opening and windows a 36 by 48 inch opening. If you have a large picture window or a sliding glass door, count it as two windows or two doors.
The texture you choose applies to every surface you paint, ceiling included. The primer line assumes one coat on every surface you are painting. Trim, doors, cabinets and baseboards are not included.
Coverage on the label assumes a good roller and a primed surface. Bare drywall, very porous plaster and cheap rollers all use more. If you are in doubt, round up. A little leftover paint is handy for touch-ups, as long as you keep the lid tight.
Worked example
Say the room is 14 by 12 feet with 8 foot walls, one door and two windows, and you are painting two coats on smooth drywall.
- Wall area: 2 x (14 + 12) x 8 = 416 sq ft.
- Openings: one door at 20 sq ft and two windows at 12 sq ft each, so 44 sq ft. That leaves 372 sq ft to paint.
- Paint: 372 x 2 coats = 744 sq ft. At 375 sq ft per gallon that is 1.98 gallons. One gallon plus four quarts would do it, but four quarts is a gallon, so buy 2 gallons.
- Ceiling: 14 x 12 = 168 sq ft. Two coats take 0.9 gallons, so 1 gallon of ceiling paint.
- Primer: 372 + 168 = 540 sq ft at 250 sq ft per gallon is 2.16 gallons, so 2 gallons and 1 quart of primer.
In metric, the walls need just over 7.5 liters, shown as 8 L after rounding up to the next half liter. The calculator suggests a 5 liter tin, a 2.5 liter tin and a 1 liter tin, which covers it with about a liter to spare.
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Paint Calculator FAQ
How much paint do I need for a 12 x 12 room?
A 12 x 12 ft room with 8 ft walls has 384 sq ft of wall. Take off about 20 sq ft for a door and 12 for each window and you are near 350 sq ft. Two coats at 375 sq ft per gallon comes to just under 2 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paint labels say 350 to 400 sq ft per gallon for one coat on smooth, primed drywall. Textured walls, bare plaster and new drywall soak up more, so you might get 250 to 325 sq ft. Primer usually covers 200 to 300 sq ft per gallon.
Do I need two coats of paint?
Usually, yes. One coat can look patchy, especially with darker colors or over a different color. Two coats give an even finish and the color you saw on the chip. Going from dark to light, or the other way, may need a coat of primer or a third coat.
Should I buy quarts or another gallon?
If you need three quarts or more on top of your gallons, buy another gallon instead. A quart costs roughly a third of a gallon, so three quarts cost about the same and give you less paint. Leftover paint is useful for touch-ups.
How much paint do I need for a ceiling?
Measure the floor: a ceiling has the same area. A 14 x 12 ft ceiling is 168 sq ft, so two coats take about 0.9 gallons, which means one gallon. Most people use a flat ceiling white, so buy it separately from the wall color.