THE MATH, MADE VISIBLE
How it works
For a rectangular room, wall area = 2 × (length + width) × height. Subtract measured door and window areas. Multiply by coats and allowance, then divide by coverage per coat. Primer is calculated separately.
Real project examples
Walls in a 12 × 10 ft room
At 8 ft high, walls total 352 ft². Subtract one 20 ft² door and two 15 ft² windows: 302 ft². Two coats at 350 ft²/gal plus 10% extra need 1.90 gal; buy 2 gal.
Adding the same paint to the ceiling
The same room adds 120 ft² of ceiling. Total 422 ft² needs 2.65 gal with the same assumptions: either 3 whole gallons or 2 gallons plus 3 quarts.
Before you buy
- Use the coverage on your exact product. The 350 ft²/US gallon default is a planning example within typical manufacturer guidance, not a guarantee.
- Door and window areas are editable examples. Measure large openings; also account for unpainted cabinets or other surfaces in direct-area mode.
- Primer has its own coverage and coat count. Ceiling paint can be calculated separately when it differs from wall paint.
- Whole gallons and gallons-plus-quarts are alternative orders, not quantities to add together. Check available can sizes and batch consistency.
Questions worth asking
How do I calculate paint for a room with doors and windows?
Enter room length, width and wall height, then the number of doors and windows. Replace the opening areas in Product & allowance assumptions with your measured sizes. Choose coats and label coverage before selecting Calculate. Use Wall area mode for irregular walls; enter gross wall area if openings will also be subtracted here.
How many gallons do I need for two coats?
Two coats double the area to cover; they do not double the number of rounded cans. Calculate the total liquid need first, add your allowance, then round once. The 12 × 10 ft example above needs about 1.90 US gallons for two coats, so its whole-gallon order is 2 gallons.
Does floor area equal paintable wall area?
No. Wall area depends on the perimeter and wall height. This calculator uses walls and optional ceiling, not home floor area as paint coverage.
Why might I need more paint than the label suggests?
Texture, porosity, application method and color change can reduce practical coverage. Adjust coverage and coats based on the product and surface.
Do I need to buy primer as extra paint coats?
No. Primer is a separate product. Enable its estimate and enter the primer’s own coverage rate.
Sources & assumptions
NIST · unit conversion factorsSherwin-Williams · paint coverageLast reviewed October 1, 2026. Planning defaults are editable examples. Product labels and supplier information override them.