Estimating paint begins with the area to be covered, not simply the room’s floor area. Wall height, total wall length, openings, ceiling area, number of coats and the paint label’s coverage all affect the result. Use the steps below for a practical preliminary estimate.
1. Calculate gross wall area
Add the lengths of the walls you plan to paint. Multiply the combined wall length by the wall height to estimate gross wall area in square feet. If wall heights differ, calculate each wall separately and add them.
2. Subtract openings and add ceilings
Estimate the combined area of doors and windows that will not be painted, then subtract it from the gross wall area. Add the ceiling area if it is part of the project. Trim, doors, cabinets and other surfaces should be estimated separately if they use a different paint or coverage rate.
3. Multiply by the number of coats
Multiply the net area by the number of coats required. A colour change, surface condition, primer and the manufacturer’s instructions may affect how many coats are appropriate.
4. Convert coverage into gallons
Gallons = (paintable area × coats × (1 + allowance ÷ 100)) ÷ coverage per gallon
For example, 120 feet of combined wall length at 8 feet high gives 960 square feet. Subtract 120 square feet of openings to get 840 square feet. Two coats cover 1,680 square feet. At 350 square feet per gallon and a 10% allowance, the estimate is 5.28 gallons, so a whole-gallon purchase plan would round up to 6 gallons.
Use the Calclio Paint Coverage Calculator to include wall height, openings, ceilings, coats, coverage and optional paint price.
Why actual coverage can vary
- Porous, textured or previously unpainted surfaces can absorb more paint.
- Dark-to-light colour changes may need different preparation or coats.
- Application method, temperature and surface preparation affect coverage.
- Primer is not included in the finish-paint estimate and may need its own calculation.
Frequently asked questions
Should I subtract windows and doors?
Enter their combined area if they will not be painted. Small reveals and trim may still require paint.
Does the calculator include primer?
No. Estimate primer separately using its own stated coverage and the surface area that needs priming.
Why does the calculator round gallons up?
It plans for whole one-gallon containers. Some paint stores offer quarts or other sizes, so check available packaging before purchasing.
Related tools: Flooring Calculator · Tile Calculator.