How to Calculate Paint for a Room Accurately

Running out of paint mid-wall is frustrating. Buying three extra litres you never use is wasteful. Getting the paint quantity right before you start saves time, money and the hassle of color-matching a second trip to the store. The calculation itself is straightforward, but several factors affect the final number beyond raw wall area.

Measuring Wall Area

Start by measuring the perimeter of the room. Add up the length of all walls. Then multiply the total perimeter by the ceiling height. A room that is 4 metres by 5 metres has a perimeter of 18 metres. With a standard 2.4-metre ceiling height, the total wall area is 43.2 square metres. This gives you the gross wall area before subtracting openings.

Subtracting Doors and Windows

Doors and windows do not need paint, so subtract their area from the total. A standard interior door is approximately 2 square metres. A typical window is about 1.5 square metres, though sizes vary significantly. If the room has two windows and one door, subtract roughly 5 square metres from the gross wall area. For our example room, that brings the paintable area down to about 38.2 square metres.

Some painters skip this step for small windows, arguing that the paint saved on the window is offset by the extra paint used on trim and cutting in around the frame. This is a reasonable simplification for rough estimates, but for accurate budgeting, subtracting openings gives a better number.

Paint Coverage Rates

Paint coverage varies by product quality, surface texture and application method. Most standard interior latex paints cover approximately 10 to 12 square metres per litre on smooth surfaces. Premium paints with higher pigment loads may cover slightly more. Textured surfaces like stucco or heavy texture paint require 20 to 30 percent more paint because the texture increases the effective surface area.

For our 38.2-square-metre room at 10 square metres per litre coverage, you need approximately 3.82 litres for one coat. Paint is typically sold in 1-litre, 2.5-litre, 4-litre and 10-litre containers. You would buy a 4-litre can for a single coat.

Number of Coats

One coat is rarely sufficient. Two coats are standard for most repainting jobs. If you are covering a dark color with a light one, three coats may be necessary. If you are applying a similar color over an existing painted surface in good condition, two coats will almost always suffice.

For our example room with two coats, the total paint needed is approximately 7.64 litres. A 10-litre can provides enough with some left over for touch-ups, or two 4-litre cans give you 8 litres with minimal waste.

Ceiling Paint

If you are painting the ceiling, calculate its area separately. A 4-by-5-metre room has a ceiling area of 20 square metres. At 10 square metres per litre, that is 2 litres per coat, or 4 litres for two coats. Ceiling paint is usually sold separately from wall paint and is formulated to reduce dripping during overhead application.

Factors That Increase Paint Consumption

Porous surfaces like new drywall, bare wood or unprimed plaster absorb more paint
Rough or textured walls have more surface area per square metre
Dark to light color changes require additional coats for full coverage
Roller application uses slightly more paint than spraying on smooth surfaces
Brush-only application in tight areas uses more paint than roller coverage

When in doubt, a primer coat on porous or previously unpainted surfaces reduces the total paint needed by sealing the surface and providing a uniform base for the topcoat. To get a quick estimate before heading to the store, enter your room dimensions, door and window count, and number of coats into a paint calculator — it does the subtraction and coverage math for you so you buy the right amount the first time.