Enter the room size, the number of doors and windows, and how many coats you plan to apply. The calculator subtracts the openings, adds the ceiling if you want it, and tells you how much paint to buy, in gallons or litres. Free and private, it runs in your browser.
Last updated: October 2026
An estimate for smooth, primed walls. Rough surfaces, dark colours and bare drywall can need more.
Paint is sold by volume but used by area, so the job comes down to three steps: measure the surface you will paint, work out how many square feet or metres you need to cover, and divide by the coverage printed on the can.
Add an extra percentage on top (the calculator starts at 10%) to cover touch-ups, uneven surfaces and the paint that stays in the tray and roller.
A room measures 12 × 14 ft with 8 ft ceilings, one door and two windows. The walls are 2 × (12 + 14) × 8 = 416 ft². Subtracting a door (20 ft²) and two windows (2 × 15 = 30 ft²) leaves 366 ft². Two coats cover 732 ft², and at 350 ft² per gallon that is 732 ÷ 350 ≈ 2.09 gallons. With 10% extra the total is about 2.3 gallons, so you would buy 3 gallons, or 2 gallons plus 2 quarts if you prefer smaller cans.
These estimates assume one door, two windows, two coats and 350 ft² per gallon, with no extra percentage added.
| Room | Wall area | Paintable walls | Walls only (2 coats) | Walls + ceiling (2 coats) |
|---|---|---|---|---|
| 10 × 10 ft, 8 ft ceiling | 320 ft² | 270 ft² | 1.54 gal | 2.11 gal |
| 12 × 12 ft, 8 ft ceiling | 384 ft² | 334 ft² | 1.91 gal | 2.73 gal |
| 12 × 14 ft, 8 ft ceiling | 416 ft² | 366 ft² | 2.09 gal | 3.05 gal |
| 14 × 16 ft, 8 ft ceiling | 480 ft² | 430 ft² | 2.46 gal | 3.74 gal |
| 15 × 20 ft, 9 ft ceiling | 630 ft² | 580 ft² | 3.31 gal | 5.03 gal |
Related reading: How much paint do I need? A room-by-room guide. To measure the floor or wall surfaces first, try the Square Footage Calculator.
How much paint do I need for a 12 × 12 room? A 12 × 12 ft room with 8 ft ceilings has about 384 ft² of wall. After subtracting one door and two windows, two coats need roughly 1.9 gallons for the walls, and about 2.7 gallons if you also paint the ceiling. At 350 ft² per gallon, one gallon covers only part of it.
Should I subtract windows and doors? Yes, subtracting them gives a closer estimate. The calculator uses 20 ft² per door and 15 ft² per window by default, and you can change both sizes. Small openings can be ignored, but large windows and patio doors make a real difference.
How many coats of paint do I need? Two coats is the usual choice for walls. One coat can be enough when repainting a similar colour with a quality paint, while new drywall, a big colour change or a dark colour may need a primer plus two coats.
What coverage should I enter? Use the figure printed on your paint can. Many interior paints list about 350 to 400 ft² per gallon (roughly 9 to 10 m² per litre) on a smooth surface. Rough or porous surfaces use more paint, so enter a lower coverage for them.
Does the calculator include trim, baseboards and doors? No. It covers walls and, if you choose, the ceiling. Paint for trim, doors and cabinets is usually bought separately; a quart often covers a few doors and the trim of a small room.