Free home project calculator

Paint Calculator

Estimate how many litres of paint you need for a rectangular room by calculating wall area, subtracting doors and windows, allowing for coats, and dividing by product coverage.

Room length (m)

Room width (m)

Wall height (m)

Doors/windows area (m²)

Number of coats

Paint coverage (m² per litre)

Enter your measurements and select Calculate.

How the paint calculation works

Paint required = [2 × (room length + room width) × wall height − openings] × coats ÷ coverage.

Worked example

For a 5 m × 4 m room with 2.4 m walls, the gross wall area is 43.2 m². If 6 m² is occupied by doors and windows, paintable wall area is 37.2 m². Two coats at 10 m²/L require about 7.44 L, before rounding up for purchase.

Planning note: This calculator is a practical estimate. Check product packaging, supplier information, site conditions and applicable local requirements before ordering materials or starting construction.

Guide: using this calculator accurately

For the full explanation, worked examples, measurement tips and FAQs, read the Paint Guide →.

How to measure a room for paint

Measure the length and width of the room at floor level and measure the wall height from floor to ceiling. For a simple rectangular room, the four wall areas can be combined as 2 × (length + width) × height.

How doors and windows change the estimate

Large openings do not normally need the same wall coating, so subtract their combined area. If you have several doors and windows, measure each opening and add the areas together.

Why paint coverage matters

Coverage printed on a paint label is usually a starting point rather than a guarantee. Porous plaster, rough masonry, strong colour changes and application method can affect actual coverage. Use the product label for the final purchasing decision.

How many coats should I use?

The number of coats depends on the product, surface, colour change and preparation. Enter the number recommended by the manufacturer rather than assuming every project needs the same number.

Common paint-calculation mistakes

A common error is multiplying floor area by wall height. Wall paint is based on the perimeter of the room, not the floor area. Another mistake is forgetting to subtract major openings or using a generic coverage figure instead of the product label.

Frequently asked questions

Does this paint calculator include the ceiling?

No. It estimates the four walls of a simple rectangular room. Calculate the ceiling separately as length × width if you plan to paint it.

Should I round the paint amount up?

Yes. The calculator shows an estimated litre requirement and a rounded planning amount. Actual can sizes and product coverage vary, so check the product packaging before buying.

Can I use this for exterior walls?

The same wall-area mathematics can help with a simple rectangular exterior, but exterior projects often have more openings, surfaces and preparation requirements. Measure each wall separately for irregular buildings.