About Paint Gallons

Paint Gallons is published by Paint Gallons. It offers a wall paint calculator, ceiling, exterior and cost calculators, and guides that explain the numbers behind them.

The Paint Gallons editorial team prepares the calculators and guides, checks their methods against cited sources, and reviews corrections submitted by readers.

The calculator estimates wall area as 2 × (room length + room width) × wall height, subtracts doors and windows, multiplies the paintable area by the number of coats, then divides by the coverage per gallon printed on the paint can. Surface settings apply planning factors of 1 for a similar-color repaint, 0.8 for new drywall or plaster and 0.75 for a rough or textured wall. These are estimates; the product’s own instructions take priority.

Each door is counted as 20 sq ft and each window as 12 sq ft (1.6 m² and 1.2 m² in metric). They are standard assumed sizes, not measurements of your openings, so measure unusual doors or windows and adjust.

Worked examples in the guides are written to match the calculator exactly. An automated test suite reruns the calculator’s own examples, its can planning and its input checks on every change before it is published.

Last checked: October 10, 2026.

If a figure or explanation looks wrong, please use the contact page. Include the page address, the figure in question, and a source or calculation showing the correction if you have one. That information helps us check the issue and update the page where needed.