How to measure a room for paint
Measure the length and width of the room in feet and the wall height from floor to ceiling. The calculator turns those into the total wall area, then subtracts your doors and windows so you are not paying for paint you will never roll on. Enter the numbers above and you get the gallons to buy and an estimated material cost.
For an L-shaped or open-plan space, split it into rectangles, run each one through the calculator, and add the gallons together. Painting the ceiling too? Add its area (length × width) to your wall total and bump up the gallons.
How the math works
Walls form a band around the room, so the gross area is the perimeter times the height:
- Wall area (gross) = 2 × (length + width) × height
- Paintable area = wall area − doors − windows
- Gallons = paintable area × coats ÷ coverage, rounded up
Because you can only buy whole gallons, the result is always rounded up. A small amount left in the can is normal and useful for touch-ups.
Subtracting doors and windows
Every opening is wall you do not paint. The calculator removes a typical 21 ft² per door and 15 ft² per window. If your openings are unusually large — a sliding patio door or a picture window — subtract more by lowering the door or window count is not enough; instead measure those openings and trim your room dimensions or coats to match. For most rooms the defaults are close.
Coats and coverage
Coverage is how far one gallon goes, printed on the can — usually around 350 ft² per gallon for interior wall paint on a smooth, primed surface. Rough, porous, or previously unpainted drywall drinks more paint, so coverage drops and you will need extra.
How many coats?
- 1 coat: refreshing a similar color over a clean, primed wall
- 2 coats: the standard for an even, durable finish (the default here)
- 3 coats: covering a dark color with a light one, or bold accent walls
Primer counts as its own coat. Going from a deep red to white may need a tinted primer plus two finish coats.
Estimating cost
The figure shown estimates paint only at a national-average per-gallon price, adjusted by region. It does not include primer, brushes, rollers, tape, drop cloths, or trim and ceiling paint, which are often a different product. Buy one extra gallon on bigger jobs so a mid-project run to the store does not leave a visible seam where a fresh batch meets a dry edge. Edit the unit price to match a local quote for your specific brand and sheen.