How Much Paint Do I Need? (Simple Formula + Calculator)

Work out exactly how much paint you need for any room: the formula, worked examples in gallons and litres, and how to turn it into cans to buy.

Buying paint is a guessing game for most people. Buy too little and you're back at the store mid-job, hoping the new batch matches. Buy too much and you've wasted money on cans that sit in the garage for years.

The good news is that the math is simple. Here's the formula, two worked examples, and the mistakes that throw estimates off.

The formula

1. Wall area = room perimeter × wall height. The perimeter is 2 × (length + width).
2. Subtract openings: about 21 sq ft (1.9 m²) per standard door and 15 sq ft (1.5 m²) per average window.
3. Add the ceiling if you're painting it: length × width.
4. Multiply by coats. Two coats is standard for a color change.
5. Divide by coverage. That's usually 350–400 sq ft per gallon, or about 10–12 m² per litre.
6. Add 10% for waste, then round up to the cans you can buy.

Or skip the arithmetic and use the free paint calculator. It does all six steps and tells you which cans to buy.

Example 1: a 12 × 14 ft bedroom (imperial)

• Perimeter: 2 × (12 + 14) = 52 ft. Walls: 52 × 8 ft = 416 sq ft.
• Minus 1 door and 2 windows: 416 − 21 − 30 = 365 sq ft.
• Two coats: 730 sq ft. Divided by 350 = 2.1 gallons. Add 10% ≈ 2.3 gallons.
• Buy: 2 gallons + 1 quart, or 3 gallons if you want spare for touch-ups.

Example 2: a 4 × 3.5 m living room (metric)

• Perimeter: 2 × (4 + 3.5) = 15 m. Walls: 15 × 2.5 m = 37.5 m².
• Minus 1 door and 1 window: 37.5 − 1.9 − 1.5 = 34.1 m².
• Two coats: 68.2 m². Divided by 10 m²/L = 6.8 L. Add 10% ≈ 7.5 L.
• Buy: one 5 L tin + one 2.5 L tin (or a single 10 L tin, often cheaper per litre).

Mistakes that ruin estimates

• Forgetting the second coat. One coat almost never covers a color change.
• Using the best-case coverage. Cans quote ideal conditions. Use the lower number for new drywall, plaster, textured or porous walls.
• Ignoring primer. New drywall, stains and big color changes need primer first. See how much primer you need.
• Measuring floor area instead of wall area. A 12×12 room has 144 sq ft of floor but about 380 sq ft of walls.

Measure faster

The Paint Calculator & Estimator app for iPhone can scan a room to measure the walls, so you just tap the walls to include. It also saves each room into a job, which is handy when you're painting a whole house.