How to work out how much paint you need
The calculation has four steps:
- Wall area: the room's perimeter times the height: 2 × (length + width) × height.
- Subtract doors and windows: a standard door is about 20 square feet, a typical window about 15.
- Add the ceiling if you paint it too: length × width.
- Divide by the coverage and multiply by the number of coats.
gallons = area to paint × coats / coverage (square feet per gallon)
Worked example
A bedroom of 14 × 12 feet with 8-foot walls:
- wall area: 2 × (14 + 12) × 8 = 416 square feet;
- minus 40 square feet for a door and a window: 376 square feet;
- two coats: 752 square feet;
- at a coverage of 350 square feet per gallon: 2.15 gallons, so buy 3 one-gallon cans, or 2 gallons plus a quart or two.
In metric terms, a room of 5 × 4 metres with 2.6-metre walls and 4 m² of openings has 42.8 m² of wall, which takes about 8.6 litres for two coats at 10 m² per litre.
Paint coverage
The coverage (or spread rate) is printed on the can, typically 350–400 square feet per gallon (about 9–10 m² per litre). That figure assumes a smooth, primed surface. You'll get less coverage on:
- rough or porous surfaces, such as new drywall, bare plaster, brick or textured walls;
- a big colour change, such as covering a dark colour with a light one;
- thick-nap rollers, which hold more paint;
- unprimed patches of joint compound.
On new drywall or bare plaster, use a primer first so the topcoat isn't soaked up.
How many coats?
Usually two coats give an even finish. One coat may be enough when repainting the same colour with a good quality paint. For a dramatic colour change or a deep shade, plan on two or three coats, possibly over a tinted primer. A higher-quality paint with better hide can end up cheaper than a budget paint that needs an extra coat.
Buying cans
The calculator rounds up to whole gallon cans and also shows the number of quarts. Buy slightly more rather than too little: a can bought later from a different batch may differ a little in colour, especially with custom tints. Keep leftovers for touch-ups and note the colour code.
Preparation and tips
- Tape and cover: tape skirting boards, trim and outlets, and cover the floor.
- Surface: clean off dust and grease, fill holes and sand rough spots smooth.
- Order of work: ceiling first, then walls; keep a wet edge to avoid lap marks.
- Drying time: follow the recoat time on the can between coats.
Replacing the floor too? Work out the area with the square meter calculator and the materials with the flooring calculator.
Choosing the right sheen
Flat or matte paint hides wall imperfections and suits ceilings and bedrooms, but marks more easily. Eggshell and satin are easier to clean and work well in living rooms and hallways. Semi-gloss and gloss are the most durable and are used for trim, doors, kitchens and bathrooms. The sheen barely affects how much paint you need, but it does affect how visible roller marks and patches are.
Frequently asked questions
How much paint do I need per square foot?
At 350 square feet per gallon, one gallon covers 350 square feet with one coat. Two coats on 700 square feet of wall need about 4 gallons.
How many gallons of paint for a 12 × 12 room?
With 8-foot walls the wall area is 384 square feet, minus doors and windows. Two coats at 350 square feet per gallon need just under 2 gallons.
Should I subtract doors and windows?
Yes, large openings make a real difference. You can ignore small ones as an extra margin.
How many coats of paint do I need?
Usually two. For a big colour change or new drywall, sometimes three, or a primer plus two coats.
What does the coverage on the can mean?
The area one gallon (or litre) covers in a single coat on a smooth surface.
How much paint do I need for a ceiling?
Length × width × coats divided by the coverage. A 14 × 12 ft ceiling (168 square feet) with two coats needs about 1 gallon.
Last reviewed: 2026-10-06. Results are estimates for information only.