Paint Calculator
Estimate how many gallons (or liters) of paint you need, with door/window deductions, multiple coats, and waste factor.
Advanced: Coverage & Price
Default 350 sqft/gallon (10 sqm/L) is typical for good-quality interior latex. Check the can label for your specific paint.
How the Paint Calculator Works
The basic formula is wall area minus door/window openings, multiplied by the number of coats, divided by paint coverage per gallon. Wall area for a rectangular room is 2 × (length + width) × height. If you're painting the ceiling, add length × width on top of that.
Each interior door takes up about 20 sqft (1.86 sqm) and each window about 15 sqft (1.39 sqm). These are industry averages — your actual doors and windows may differ, but using the average gets you within 5-10% of the true paintable area for most rooms. The calculator subtracts these openings so you don't buy paint for surfaces you won't touch.
Most paint cans say 350-400 square feet per gallon (8-10 sqm per liter) for one coat on smooth, primed walls. Real-world coverage drops on textured, porous, or dark surfaces — and most paint jobs need two coats for even color. A 10% waste factor covers spills, brush loading, and corners that need extra paint. Bump to 15-20% if you're working on rough surfaces or have lots of trim work.
Practical workflow: measure the room (length × width × height), count doors and windows, decide on coats (usually 2), and check the paint can for coverage. Then enter price per gallon for a quick budget estimate. If you also need to lay tiles or pour concrete underneath, our tile calculator and concrete calculator round out the project.