Paint Calculator for Rooms & Ceilings

Buy too little and the second coat runs out halfway down the last wall, usually on a Sunday evening. Buy too much and a full tin sits in the shed for a decade. Give this paint calculator the room’s size, its doors and windows, how many coats you plan and the coverage printed on the tin. It tells you how much paint you need and which tins to buy.

It works in metres and litres or in feet and US gallons. It can add the ceiling, adds a waste allowance for rollers, trays and touch-ups, and works out a primer coat separately when you need one.

Worked Example: A 4.5 × 3.5 m Bedroom

The calculator opens on a bedroom 4.5 m long, 3.5 m wide and 2.7 m high, with one door, two windows, two coats and paint that covers 10 m²/L. It reports:

• Wall area: 43.2 m², less 4.77 m² for a 0.9 × 2.1 m door and two 1.2 × 1.2 m windows
• Area to paint: 38.43 m², which takes 3.84 L per coat
• Paint to buy: 8.45 L with the 10% allowance, bought as 1 × 5 L + 1 × 2.5 L + 1 × 1 L = 8.5 L

Load Sample steps through a 12 × 12 ft room with its ceiling (3.04 US gallons, bought as three gallons and a quart), a 6 × 5 m living room that also needs primer, and an 11 × 10 ft room with three coats.

How the Math Turns Wall Area Into Tins

Wall area is the perimeter, 2 × (length + width), times the wall height, minus each door and window. Paint is that area times the number of coats, divided by the coverage, plus the waste allowance. Primer is one coat over the same area at its own coverage, which is usually lower: 8 m²/L by default.

Choosing tins is a small search over the sizes on sale: 10, 5, 2.5 and 1 L, or 5 gallons, 1 gallon and a quart. It looks for the fewest tins that cover the need without overshooting by more than one small tin. So 10.2 L becomes a 10 L and a 1 L tin rather than five small ones. Switching between metric and imperial converts every value you entered instead of reading it in the new units: 10 m²/L becomes 407 sq ft/gal.

Accuracy: What the Tin’s Coverage Figure Leaves Out

The coverage on a tin is measured on smooth, sealed, primed walls. Fresh plaster, bare brick, textured finishes and a dark-to-light colour change all soak up more. Raise the coat count or lower the coverage for those, or tick the primer box. Skirting boards, radiators and built-in wardrobes are not subtracted. On most rooms that is part of what the waste allowance covers. The allowance itself must stay sensible, or the tool says: “Waste allowance must be a percentage from 0 to 50.”