Free estimator
Paint Calculator
Use this paint calculator to estimate wall area, paintable area, gallons to buy, and optional paint cost for rooms, garages, and simple exterior wall projects.
Last updated: July 11, 2026
How to measure
Measure room length, width, and wall height. The calculator uses the room perimeter multiplied by wall height, then subtracts estimated door and window area before applying coats and coverage.
For a garage, use the inside wall dimensions and count the overhead garage door as an opening if you are not painting it. For a simple exterior wall estimate, measure each wall face separately, subtract large doors or windows, and add the totals together.
Formula
wall area = room perimeter x wall height
paintable area = wall area - doors area - windows area
gallons = paintable area x coats / coverage per gallon
Tips before buying
- Check the coverage on your paint label before buying.
- Plan extra paint for repairs, texture, dark color changes, and future touch-ups.
- Primer may reduce the amount of finish paint needed on patched or porous surfaces.
- Use lower coverage for rough exterior surfaces, unprimed drywall, masonry, brick, or stucco.
- For garage walls, account for unfinished surfaces, shelving, trim, and large overhead doors.
Paint coverage chart
Paint coverage per gallon depends on the surface, primer, and application method. Use the paint label when available, or start with a conservative range like this:
| Surface type | Typical coverage | Planning note |
|---|---|---|
| Smooth interior wall | 350-400 sq ft/gal | Good default for primed drywall. |
| Unfinished drywall | 250-350 sq ft/gal | Primer usually helps the finish coats go farther. |
| Rough exterior siding | 200-300 sq ft/gal | Weathered or textured surfaces need a safer estimate. |
| Brick, stucco, or masonry | 150-250 sq ft/gal | Porous surfaces often absorb more paint. |
2-car garage paint estimate example
For a simple 20 ft by 20 ft two-car garage with 8 ft walls, the wall area is about 640 sq ft before openings. If you subtract a large garage door and a service door, the paintable wall area may land closer to 480-540 sq ft.
With two coats and 300 sq ft per gallon coverage, that garage could need about 3.2-3.6 gallons before rounding. In practice, many people would buy 4 gallons, and possibly more if the drywall is unfinished or the surface is rough.
Wall paint estimator examples
10 x 12 room
A 10 ft by 12 ft room with 8 ft walls has about 352 sq ft of wall area before doors and windows. Two coats often means buying about 2 gallons.
Single exterior wall
A 30 ft wide wall at 10 ft high is about 300 sq ft before openings. Use lower coverage for rough siding, brick, stucco, or weathered paint.
Garage walls
Garage estimates depend heavily on unfinished drywall, shelving, trim, and whether you paint around or over the overhead door area.
Garage and exterior paint estimates
This calculator works best for interior wall paint, but it can also help with quick garage and exterior planning when the project is mostly flat wall area. Keep the estimate conservative if the surface is rough or porous.
For exterior paint, product coverage can vary widely by siding material, surface prep, sprayer use, and weathered paint. Use the label coverage as your starting point, then add a waste allowance before buying.
FAQ
How much area does one gallon of paint cover?
Many interior paints cover about 350 to 400 square feet per gallon, but the exact coverage depends on the product, surface, color change, and application method.
Should I count doors and windows?
Yes. This calculator subtracts common estimates of 21 sq ft per door and 15 sq ft per window to avoid overestimating wall paint.
Do I need two coats?
Two coats are common for interior walls, especially with a color change. Primer, wall texture, and paint quality can change the final amount.
Can I use this calculator for a garage?
Yes. Enter the garage length, width, and wall height, then adjust doors and windows. For unfinished drywall, masonry, or rough surfaces, lower the coverage per gallon or add extra waste.
Can I estimate exterior paint with this tool?
You can use it as a rough exterior paint estimator for simple wall areas. Exterior siding, trim, stucco, brick, weather exposure, and spraying can change coverage, so always check the paint label and supplier guidance.
Why does paint coverage vary so much?
Coverage changes with surface texture, primer, color change, application method, sheen, and paint quality. A smooth primed wall may cover much farther than raw drywall, stucco, brick, or heavily textured surfaces.