Paint Calculator: How Much Paint Does Your Room Need?

The estimate runs on four pieces of arithmetic. Measure the room, subtract what you will not paint, then convert the square footage into gallons using the coverage printed on the can.

• Wall area: 2 × (length + width) × ceiling height. A rectangular room has four walls; this totals them all.
• Openings: doors × 20 sq ft + windows × 15 sq ft. An average interior door covers about 3 × 6.8 feet, an average window about 3 × 5 feet.
• Paintable area: wall area − openings (+ length × width if you are painting the ceiling too).
• Gallons needed: paintable area × coats ÷ coverage per gallon. Then round up to whole gallons, because stores do not sell fractions of a can.
• Quarts equivalent: gallons to buy × 4. Useful when a project needs less than one full gallon.

Take a room that is 12 × 15 feet with an 8-foot ceiling, one door, two windows, one coat of paint, and a coverage of 350 sq ft per gallon.

1. Wall area: 2 × (12 + 15) × 8 = 432 sq ft
2. Openings: 1 × 20 + 2 × 15 = 50 sq ft
3. Paintable area: 432 − 50 = 382 sq ft
4. Gallons needed: 382 ÷ 350 = 1.091 gallons
5. Gallons to buy: round 1.091 up = 2 gallons (8 quarts)

You would buy 2 gallons, not 1.091, since paint is sold by the whole can. The extra becomes your touch-up supply.

Measure with a tape measure at floor level — do not guess. Measure each wall's length, the room's width, and the ceiling height in feet, then follow these steps.

Step by step

1. Enter the room length, width and ceiling height in feet. These three are required.
2. Count the doors and windows. Defaults are 1 door and 2 windows; each door subtracts 20 sq ft and each window 15 sq ft.
3. Set the number of coats. The default is 2, the standard for a fresh color change.
4. Set the paint coverage from your paint can label. The default of 350 sq ft per gallon works for most interior latex paints.
5. Choose whether to include the ceiling. Ceiling paint adds length × width to the paintable area.
6. Press Calculate. Read the paintable area, exact gallons needed, gallons to buy, and the quart equivalent underneath.

Coverage is printed on every paint can and its technical data sheet, usually as a range. The number that matters is the coverage of the paint you actually bought — but if the can is not in your hands yet, these manufacturer-typical ranges are a safe starting point.

The safest move is to type the number from your own can's label into the calculator. Manufacturer figures depend on surface texture, porosity, sheen and application method, and thick-film paints cover less per gallon by design.

Once you know the gallon count, the project becomes a shopping trip. A budget calculator helps you fit the paint, primer, rollers and tape into a single project budget before you buy anything. For a deeper look at published coverage figures, see this Benjamin Moore paint cost guide, which lists coverage on smooth versus textured walls.

Room size sets the baseline, but three choices move the answer most: how many doors and windows you subtract, how many coats you apply, and whether the ceiling joins the project.

Standard opening areas

Coats

One coat covers a touch-up or a same-color refresh on walls in good shape. Two coats is the professional standard for a color change — the first coat builds hide, the second builds depth and durability. A third coat is rarely needed unless you are covering a dark color with a light one, and each coat adds the full paintable area to the gallons calculation, so it is the biggest lever you control.

Ceilings

Painting the ceiling adds length × width in one step. A 12 × 15 room gains 180 sq ft — at two coats and 350 coverage that is roughly one extra gallon on its own. If the ceiling just needs freshening, it takes fewer coats than the walls, so consider calculating it as a separate project.

The calculator rounds up for a reason: paint is sold in whole gallons, and a mid-project store run breaks the workflow. A few habits keep the estimate honest.

• Buy the rounded-up number, not the exact number. That 1.091 gallons becomes 2 gallons — the spare quart or so is your touch-up supply for years.
• Add 10% if the walls are textured or damaged. The calculator uses the exact math; your walls may drink a little more than the label promises.
• Prime bare drywall and patches first. Primer is cheaper per gallon than topcoat and stops thirsty spots from flashing through the finish. Coverage figures for primers and topcoats differ — one paint coverage reference compiles the current manufacturer data sheets side by side.
• Keep one unopened can sealed. Stores accept returns of unopened cans, so slight overbuying costs nothing if the plan stays flexible.
• Write the formula on the can. Brand, line, sheen and color code — touch-ups two years from now need the exact mix.
• Mind the recoat time. Most latex paints are dry to the touch in an hour and ready for the second coat in 1–2 hours; rushing it ruins the finish. If you are scheduling the job, a time duration calculator helps you plan coat spacing and drying windows across rooms.