Paint Calculator
Estimate wall paint quantity for a room by subtracting doors and windows from wall area and applying the product’s coverage and number of coats.
What is the paint calculator?
Estimate wall paint quantity for a room by subtracting doors and windows from wall area and applying the product’s coverage and number of coats.
How to use the calculator
Enter room length, width, wall height, number of doors and windows, and the coverage shown on the paint can. Coverage varies by product and surface.
How to calculate paintable area
The perimeter of a rectangular room is 2 × (length + width). Multiply the perimeter by wall height, then subtract openings. Ceilings and trim require separate calculations.
Paint coverage and coats
Use the manufacturer’s stated coverage. Porous surfaces, texture, color changes, primer, and application method can reduce coverage. Multiply by the number of coats you actually plan to apply.
Doors, windows, ceilings, and trim
This calculator subtracts standard planning allowances for doors and windows. Measure unusual openings directly. Add ceiling, trim, doors, and other surfaces separately if they will be painted.
Buying paint
Round up to the next package size and consider keeping a small amount for future touch-ups. Different batches can vary slightly in color.
How to improve the accuracy of your estimate
Use the actual product dimensions, package coverage, material density, installed depth, and current local price whenever those values are available. Measure the project in smaller sections when the shape is irregular, and keep fixed costs separate from area-based quantities. For a large purchase, compare the calculator result with the supplier’s own coverage or takeoff before ordering.
Common mistakes to avoid
The most common errors are mixing inches and feet, using nominal dimensions instead of actual product dimensions, forgetting waste or overage, treating volume and weight as interchangeable, and assuming a universal installation depth. Enter the values from your actual product and project specification whenever possible.
Worked planning example
A 12 × 15 ft room with 8 ft walls has 432 sq ft of wall area. After subtracting doors and windows, divide the remaining area by the paint’s actual coverage and multiply by the number of coats.
Useful Paint reference values
| Coats | Coverage multiplier |
|---|---|
| 1 coat | Multiply paintable area by 1 |
| 2 coats | Multiply paintable area by 2 |
| 3 coats | Multiply paintable area by 3 |
Reference values are planning aids. Product specifications and local conditions take precedence.
Frequently asked questions
How much paint do I need for a room?
Calculate wall area, subtract openings, multiply by the number of coats, and divide by the paint’s actual coverage per gallon.
How much does one gallon of paint cover?
Coverage varies by product and surface. Use the number on the label rather than a universal assumption.
Should I subtract doors and windows?
Yes, especially for large openings. This avoids systematically overestimating wall paint.
Can I use this calculator for an irregular project?
Yes, but the best approach is to divide the project into measurable sections and combine the results. Irregular edges, curves, openings, and changes in depth can make a single rectangular measurement less accurate.
What should I verify before buying materials?
Verify the product label or manufacturer coverage, actual dimensions, density or package yield, required installation depth, and supplier delivery terms. For structural or regulated work, also verify the project specification and local requirements.