Estimate paint by measuring the surface to be painted, subtracting openings where they matter, multiplying by the number of coats, and dividing by the coverage shown for the selected paint. Walls and ceilings have different areas, so calculate them separately when using different products or coat counts.
Measure walls and openings
For a rectangular room, wall area before openings is 2 × (length + width) × wall height. A 12-by-10-foot room with 8-foot walls has 352 square feet of gross wall area. Subtract the actual area of doors and windows if you have measured them; preset opening deductions are only approximations.
If the ceiling is also being painted, its area is length × width. In this example that is 120 square feet. Do not count the ceiling as wall area. Irregular rooms, alcoves, built-ins and unpainted surfaces may need to be measured as separate shapes.
Account for coats and product coverage
Suppose the measured openings total 44 square feet. Net wall area is 352 − 44 = 308 square feet. At two coats, that becomes 616 square feet of coating. If the paint label lists 350 square feet per gallon for the relevant surface, 616 ÷ 350 is about 1.76 gallons, which rounds up to two whole gallons in the calculator.
That coverage figure is an example input, not a promise about every paint or wall. Surface texture, porosity, color change, preparation and application affect actual use. Follow the product label and include primer separately when required. Tinting, container sizes and touch-up needs can also affect what to buy.
Make separate estimates when conditions differ
Calculate walls and ceiling separately if they use different paint, coverage or number of coats, then combine the purchase plan with the product packaging in mind. The Paint Calculator lets you enter room dimensions, openings, coats and coverage; use the values for your actual project.
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