Paint Calculator
Estimate how much paint you need for walls and optional ceilings in US or metric units using room dimensions, coats, openings, paint coverage and a waste allowance.
Best for: rectangular rooms where you know the room length, width and wall height. The tool also accounts for standard door and window deductions.
Estimate Your Paint Needs
Estimate how much paint you need for the walls of a rectangular room.
Estimate assumes approximately 21 sq ft per door and 15 sq ft per window. Actual paint coverage may vary by paint type, surface texture, application method, and manufacturer.
A Practical Paint Estimate
Paintable Area
The tool estimates wall area in square feet or square meters, subtracts doors and windows, and can include the ceiling when selected.
Paint Needed
Coverage, number of coats and your waste allowance are used to estimate total paint required in US gallons or liters.
Purchase Quantity
The result gives the required paint volume so you can compare it with the container sizes available for your chosen product and market.
How to Use the Paint Calculator
- Choose US Units or Metric Units, then enter the room length, width and wall height in the units shown.
- Enter the number of doors and windows in the room.
- Choose how many coats of paint you plan to apply.
- Enter the paint coverage shown on your product: square feet per US gallon in US mode, or square meters per liter in Metric mode.
- Add a waste allowance for touch-ups, surface variation and application loss.
- Choose whether to include the ceiling, then select Calculate.
How the Paint Estimate Works
The calculator uses room perimeter and wall height to estimate gross wall area. It then subtracts standard allowances for doors and windows. If the ceiling option is selected, ceiling area is added before coats, coverage and waste are applied.
Core calculation
Wall area = 2 × (length + width) × wall height
Openings deducted = 21 sq ft per door + 15 sq ft per window in US mode, or approximately 1.95 m² per door + 1.39 m² per window in Metric mode
Optional ceiling area = length × width
Paint required = adjusted area × coats ÷ coverage rate, with the selected waste allowance applied. US mode returns US gallons; Metric mode returns liters.
Actual paint coverage depends on the product, surface texture, color change, application method and number of coats. Use the coverage value printed on the paint you plan to buy when possible.
Paint Planning Reference
These assumptions are built into the calculator for planning. Choose the unit system you use and replace the default coverage with the value from your chosen paint whenever possible.
| Item | Calculator assumption |
|---|---|
| Door deduction | 21 sq ft per door / approximately 1.95 m² per door |
| Window deduction | 15 sq ft per window / approximately 1.39 m² per window |
| Wall area | Room perimeter × wall height |
| Ceiling area | Length × width when selected |
| Coverage | User-entered sq ft per US gallon or m² per liter |
| Paint result | US gallons in US mode / liters in Metric mode |
Paint Calculator Example
Suppose a room is 12 ft × 10 ft with 8 ft walls, 1 door, 2 windows, 2 coats, paint coverage of 350 sq ft per gallon, and a 10% waste allowance.
- Estimated paint required for the walls: approximately 1.89 gallons
- Practical purchase quantity shown by the calculator: approximately 2.00 gallons
- If the ceiling is included, estimated paint required becomes approximately 2.65 gallons
- With the ceiling included, practical purchase quantity becomes approximately 2.75 gallons
Metric equivalent: The same room is approximately 3.6576 m × 3.048 m with 2.4384 m walls. Using equivalent coverage of 8.5898 m²/L, the calculator returns approximately 7.16 liters for the walls, or 10.02 liters when the ceiling is included.
This is a planning example. Porous surfaces, strong color changes and product-specific coverage can increase the amount of paint needed.
Frequently Asked Questions
What paint coverage should I enter?
Coverage varies by paint and surface. In US mode, enter the manufacturer’s square feet per US gallon. In Metric mode, enter square meters per liter. Use the coverage stated for the actual paint whenever possible.
Why does the calculator subtract doors and windows?
Doors and windows reduce the wall surface being painted. The calculator uses equivalent planning deductions in either unit system: 21 sq ft (about 1.95 m²) per door and 15 sq ft (about 1.39 m²) per window.
Should I add a waste allowance for paint?
A small allowance can help cover touch-ups, roller and brush loss, surface variation and minor measuring differences. The appropriate percentage depends on the project.
Can I use metric measurements?
Yes. Choose Metric Units to enter room dimensions in meters, paint coverage in square meters per liter, and receive the paint estimate in liters. You can switch between US and metric units without changing the calculation method.
Does the ceiling use the same paint coverage?
The calculator uses the same coverage value you enter for the overall estimate. If your ceiling paint has a different stated coverage, calculate it separately for a more precise plan.
Does the estimate include primer?
No. The result estimates finish paint based on the inputs you provide. Primer should be planned separately when the surface or paint system requires it.
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