Paint Calculator: How Many Litres of Paint for a Room

What it calculates

The calculator gives the quantity of finish paint and primer for a room, in litres, along with:

• the surface to paint, walls less doors and windows, plus the ceiling if chosen;
• the area of openings left out;
• a breakdown of how the surface area was built up.

Inputs

Room length, width and wall height are the inside dimensions in feet. Use the height to the false ceiling where there is one, since the wall above it is not painted.

Paint chooses walls only, or walls and ceiling.

Doors and windows are counts; their sizes are in the assumptions panel.

Finish coats and finish paint coverage come from the paint specification and the product datasheet.

The assumptions panel holds primer coats and coverage, the door and window sizes, and a touch-up and wastage allowance.

Method

1. Gross wall area = 2 × (length + width) × height.
2. Openings = doors × door width × door height + windows × window width × window height.
3. Surface to paint = gross wall area − openings, plus length × width if the ceiling is included.
4. Finish paint = surface × coats ÷ coverage × (1 + allowance).
5. Primer = surface × primer coats ÷ primer coverage × (1 + allowance).

Assumptions

Coverage is the input that matters most. Datasheets give it for a smooth surface under good conditions; site coverage on new or textured walls is usually lower.

Limitations

• Rectangular rooms only. Add walls of other shapes separately.
• Openings are averaged. Rooms with full-height glazing or many doors should use their real opening sizes.
• No putty, texture or special finishes. Those have their own coverage and process.
• Paint is sold in fixed pack sizes. Round the litres up to the packs available, and buy the whole quantity from one batch so the shade matches.
• It does not choose the paint. Washable, low-VOC, anti-bacterial and other finishes are a specification decision for the space.

Worked example

A 20 ft × 15 ft room with 10 ft walls, one door and two windows, walls only, two finish coats at 120 sq ft a litre and one primer coat at 100 sq ft a litre:

The room needs about 12 litres of finish paint and 7 litres of primer. Painting the ceiling as well adds 300 sq ft, taking the totals to about 17.4 litres of paint and 10.4 litres of primer.

How engineers use it

On a fit-out, painting quantities are measured from the drawings in square metres for each finish and each surface type, and the specification sets the full system: surface preparation, putty, primer and the number of finish coats. The quantity of material then follows from the manufacturer's coverage for that system. A calculation like this one is the quick check that a supplier's or painter's material estimate is in the right range.

When a professional design must replace it

Use the calculator to estimate how much paint to buy for a room or to check an estimate. Replace it with a measured take-off and a written paint specification for a whole office or building, where surfaces, finishes and colours vary from room to room and where the preparation work is a large part of the job.