How to Calculate Paint for a Room or House
Learn how to measure wall area, subtract doors and windows, account for coats and use paint coverage without relying on floor area alone.
Start with the surface you will actually paint
For a rectangular room, wall area comes from the room perimeter multiplied by wall height. If you are painting a ceiling, calculate that surface separately. Exterior projects should be measured wall by wall because buildings often have different wall lengths and openings.
Subtract major openings
Doors and windows can represent a meaningful part of the surface. Measure each opening and add the areas together before subtracting them from the gross wall area.
Use the paint label
Coverage is product-specific. A label may give a theoretical coverage figure, but surface porosity, texture, colour changes and application method can change real-world usage.
Plan for coats and purchasing
Multiply paintable area by the number of coats, then divide by the stated coverage. The mathematical result is not necessarily the exact number of tins available in the shop, so round to a practical purchase quantity.
Related calculator
Use the Paint Calculator for the actual room calculation, then review its worked example and FAQs.
A simple measurement checklist
Before calculating, record room length, room width, wall height, the combined area of major doors and windows, the number of coats and the coverage printed on the chosen paint. Keeping those inputs together makes it easier to spot an unrealistic result.
When the room is not rectangular
Do not force an irregular room into one rectangle. Measure each wall section separately and add the paintable areas. The same principle applies to exterior walls, where different elevations can have different heights, lengths and openings.
What can change real paint usage?
Fresh plaster, porous masonry, rough textures, strong colour changes and application method can all change practical coverage. Primer or sealer may also be a separate product with its own coverage rate. Use the calculator for planning, then use the product label for the final purchasing decision.
Quick FAQ
Should I include skirting boards and trim?
Not in this basic wall-area calculation. Trim, doors and other detailed surfaces can be estimated separately if you plan to coat them with a different product.
What if I am painting only one wall?
Measure that wall directly as width × height, subtract major openings, then multiply by the number of coats and divide by coverage.