About Project Quantity
Project Quantity creates simple, free browser-based tools for common DIY planning questions. The paint calculator helps estimate how much paint a rectangular room may need before you visit a shop.
Useful immediatelyNo account, download or email gate.
TransparentThe formula, assumptions and exact result are shown.
Private by designRoom measurements are calculated in your browser.
Practical, not absoluteThe result is guidance and can be fine-tuned.
How the calculator works
The tool calculates the area of four walls using room length, width and height. It subtracts standard-sized doors and windows, adds the ceiling when selected, multiplies by the number of coats, divides by the paint coverage rate and adds the chosen waste allowance.
Assumptions and limitations
- The room is treated as rectangular.
- A standard door is estimated at 1.9 m² and a standard window at 1.4 m².
- Default coverage is 10 m² per litre, or its imperial equivalent.
- Actual coverage varies with paint, surface texture, preparation and application method.
Always check the coverage printed on the paint tin and adjust the calculator before buying. Calculate primer separately.
Privacy
The calculation runs locally and does not send your room measurements to a server. Read the full privacy policy.