Paint Calculator: Estimate Gallons for Your Room
Free paint calculator to estimate gallons for rectangular room walls, accounting for doors, windows, coats and a 10% allowance.
How to calculate wall paint
Measure the length, width and wall height of your room in feet. The calculator uses the perimeter multiplied by height, then subtracts the total area of doors and windows. It multiplies the result by the number of coats, divides by your paint coverage per gallon and adds a 10% allowance.
Example estimate
A room measuring 12 by 10 feet with 8-foot walls has 352 square feet of wall area before openings. Subtract 40 square feet of doors and windows to get 312 square feet per coat. Two coats cover 624 square feet. At 350 square feet per gallon, with 10% extra, plan on approximately 2 gallons.
Before you buy materials
Coverage varies by paint, surface texture, primer and application method. Check the product label and round up to purchasable container sizes. All results are planning estimates, not professional construction advice.
How to measure a room
For a rectangular room, add the lengths of all four walls and multiply by wall height. Subtract the combined square footage of doors and windows. Multiply the remaining wall area by the number of coats. Ceilings and trim should be estimated separately.
Choosing a paint allowance
The calculator includes 10% extra for normal planning. Rough or porous surfaces, major color changes, and application methods can change actual coverage. Use the spread rate on your specific paint label.
Common questions
Does the calculator include the ceiling?
No. It estimates the four walls of a rectangular room. Measure the ceiling separately using length × width.
Should I subtract every window?
You can subtract the combined opening area, but a modest allowance is useful for touch-ups and waste.
Do I need primer?
Some surfaces and color changes benefit from primer. Follow the product label and surface preparation instructions.