Paint Calculator

How this calculator works

The calculator measures the wall area, subtracts your doors and windows, multiplies by the number of coats, then divides by the paint’s coverage (spreading rate):

litres = (wall area − openings) × coats ÷ coverage per litre

Wall area is the room perimeter times the wall height - 2 × (length + width) × height. Tick the ceiling box to add the floor area on top. Because everything runs in exact metric internally, switching between gallons and litres never changes the answer.

Worked example

This example follows the unit system you pick in the calculator above.

A 16 ft × 13 ft room with 8 ft walls, one door and two windows, two coats:

1. Wall area: 2 × (16 + 13) × 8 = 464 sq ft.
2. Subtract openings: one door (21 sq ft) + two windows (12 sq ft each) = 45 sq ft → 419 sq ft to paint.
3. Two coats: 419 × 2 = 838 sq ft of painting.
4. At 350 sq ft per gallon: 838 ÷ 350 ≈ 2.4 gallons → three 1-gallon tins.

Getting the coverage right

Surface	Effect on coverage	What to do
Smooth, previously painted	Best case	Use the tin’s stated figure
Bare plaster or filler	Soaks up the first coat	Add a mist/primer coat
Textured or rough render	Much lower coverage	Drop coverage 20–30%
Strong colour change	Needs extra coats	Use 3 coats, not 2

Interior, ceiling and exterior paint

Walls, ceilings and outside surfaces all use the same area × coats ÷ coverage math, just with different settings:

• Interior walls - two coats over a sound surface of a similar colour; the default coverage figure applies.
• Ceilings - tick the ceiling box (or run a separate job with the walls set to 0) and use ceiling-grade paint, which often covers a little less than wall paint.
• Exterior walls - masonry and render are porous and textured, so drop the coverage 20–30% and plan on two full coats. Measure the outside wall area the same way: perimeter × height.

Set the coverage from your tin and the number of coats to suit the surface, and the calculator adjusts the litres or gallons to match.

Tips for measuring

• Measure the room perimeter and wall height; you don’t need each wall separately for a normal rectangular room.
• For a feature wall only, set the length to that wall and width to 0, then untick the ceiling.
• Ceilings usually need their own tin of ceiling-grade paint - calculate them as a separate job with the ceiling box ticked and the walls set to 0.