Paint Calculator

How much paint do you need for that room? Enter dimensions, coats, and coverage to get the exact litres or gallons.

About this calculator

Buying paint without calculating first is how you end up with five half-empty tins in the garage. This calculator turns room dimensions, the number of coats and paint coverage (litres per square metre, or gallons per square foot) into the exact amount you need to buy — no over-ordering, no second trip mid-project.

How it works

The math is straightforward. Wall area equals the perimeter (2 × (length + width)) multiplied by the ceiling height. Subtract the area of doors and windows you don't plan to paint. Multiply by the number of coats. Divide by the coverage rate of your chosen paint.

Coverage rates are printed on every paint tin. Typical interior latex covers about 10 m² per litre (one coat), or roughly 350 ft² per US gallon. Primer covers slightly less; textured or heavily-pigmented paints can cover under 8 m² per litre.

Most rooms need two coats — three if you're going light over dark. One coat is fine for refreshing the same colour. Bare drywall always needs a primer coat first; count that as an additional coat.

Buy 10 % more than calculated. Real coverage is always slightly less than the manufacturer's number (texture, roller absorption, drips), and you'll thank yourself when you need to touch up a scratch six months later from a guaranteed-match batch.

Formula

wall_area  = 2 × (length + width) × height
net_area   = wall_area − doors_and_windows_area
total_paint_area = net_area × coats
paint_needed     = total_paint_area ÷ coverage_per_unit

Examples

12 × 14 ft room, 8 ft ceiling, 2 coats, 350 sq ft/gal coverage

Wall area is 416 sq ft. After subtracting 21 sq ft for a door and window, that's 395 sq ft per coat — 790 sq ft for two coats. At 350 sq ft per gallon, you need about 2.26 gallons. Buy 3 gallons (one will be partial; keep it for touch-ups).

Result: ≈ 2.26 gallons

4 × 4.5 m room, 2.5 m ceiling, 2 coats, 10 m²/L

Walls total 42.5 m². Minus 2 m² for door/window = 40.5 m² per coat = 81 m² for two coats. At 10 m²/L, that's 8.1 litres. Buy 9 or 10 litres for safety.

Result: ≈ 8.1 litres

Frequently asked questions

Should I include the ceiling? +

No — this calculator handles walls only. Ceiling area is just length × width; calculate it separately and add to your paint order if you're painting it too.

How accurate is the coverage figure on the tin? +

Optimistic. Manufacturer numbers are based on smooth, primed surfaces and ideal application. Real coverage is typically 10–20 % less, especially on textured walls or with a roller.

One coat or two? +

Two for any colour change, three if you're going from a strong colour to white. One coat works only when you're refreshing the same colour over a clean, non-stained surface.

Do I need a primer? +

Yes on bare drywall, bare plaster, glossy or oil-based existing paint, water-stained areas, or when going light-over-dark. Count primer as an additional coat in the calculator.

How much extra should I buy? +

10 % extra for the project + keep a small amount for touch-ups. Paint stores can usually colour-match years later, but a sealed tin from the original batch is exact.