Paint & Walls
Paint Calculator
Measure one rectangular room, subtract average doors and windows, and estimate the paint needed for every coat.
- Doors & windows deducted
- One to six coats
- Gallons or litres
Enter your room measurements
Use the room’s inside dimensions. Standardized average opening allowances are deducted for each door and window.
Estimate result
How this estimate is calculated
The calculator finds the four-wall area from room perimeter and height, subtracts average door and window areas, then multiplies by the number of coats. It uses a conservative coverage rate of 350 square feet per US gallon.
Paint = ((room perimeter × wall height) − doors − windows) × coats ÷ coverage
Actual coverage changes with surface texture, porosity, colour change, and application method. Check the coverage printed on your chosen paint and keep a small amount for touch-ups.
Paint Calculator FAQs
How much area does one gallon of paint cover?
This estimate uses 350 square feet per US gallon. Many paints list a range near 350 to 400 square feet per gallon, so a rough or porous surface may need more.
Does the calculator subtract doors and windows?
Yes. It deducts 20 square feet for each door and 15 square feet for each window before applying the number of coats.
Should I plan for two coats?
Two coats are common for even colour and durability. A similar colour over a well-prepared surface may need less, while a dramatic colour change may need primer and two finish coats.
Are the litre can counts exact?
The 2.5 L and 5 L counts are separate purchase options, each rounded up. Compare the estimate with the package sizes sold by your local supplier.
Suggested supplies
These non-affiliate Amazon search links can help you compare common project materials. Affiliate tracking can be enabled later from the shared site configuration.
Related project calculators
Continue planning with another calculator from the same BuildingMath site.