Paint Calculator
Estimate how many gallons of paint to buy. Enter the total wall area, how many doors and windows to skip, the number of coats, and the paint's coverage rate, and the calculator returns the whole gallons you need — rounded up — plus the exact pre-round figure and the paintable area after openings. Add a price per gallon to see an estimated cost.
Calculate
Default result: 3
Result
Gallons needed
- Exact (before rounding up)
- 2.51
- Paintable area
- 439 ft²
How to calculate
Add up the area of every wall you plan to paint (length × height for each, totaled) and enter it as the wall area. Enter the number of doors and windows so the calculator can subtract roughly 15 sq ft for each. Set the number of coats — two is typical — and the coverage rate from your paint label (350 sq ft per gallon is a common default). To see a cost estimate, type your price per gallon; leave it blank to skip cost. The result updates as you type.
gallons = ceil((paintableArea × coats) / coverage), where paintableArea = grossWallArea − openings × 15 sq ft. Variables: grossWallArea is the total wall surface, openings is the count of doors and windows (each deducted at about 15 sq ft), coats is the number of coats, and coverage is the square feet one gallon covers (350 sq ft/gal by default). The exact (unrounded) figure is shown alongside so you can see how close you are to the next whole gallon.
Example calculation
A room with 439 sq ft of paintable wall, painted with 2 coats using a paint that covers 350 sq ft per gallon, needs (439 × 2) ÷ 350 = 2.51 gallons. Because you cannot buy part of a gallon, that rounds up to 3 gallons.
- gallons
- 3 gallons
- exactGallons
- 2.51 gal
- paintableArea
- 439 ft²
Assumptions
- Coverage defaults to 350 sq ft per gallon — a conservative figure for quality interior paint. Actual coverage varies with surface texture, porosity, color change, and sheen, so always check your paint label and use that rate.
- The gallon count is always rounded up — you cannot buy a fraction of a gallon, and running short mid-job risks a visible lap mark.
- Each door or window opening deducts about 15 sq ft of paintable area. This is a nominal average; large picture windows or double doors take off more.
- Primer is not included. Bare drywall, patched areas, or a big color change usually need a separate primer coat — add it as an extra coat or a separate calculation.
- Spray application typically wastes 20–30% more paint than a brush or roller because of overspray. If you are spraying, increase the coverage you enter or add a gallon.
- When you open a shared link the inputs come from the URL first, then from your last-used values saved in this browser, then from the calculator's defaults.
Common mistakes
- Forgetting the second coat. One coat rarely gives even color, especially over a different base color — most jobs need two, which doubles the paint.
- Using the wrong coverage rate. Porous, textured, or bare surfaces cover far less than 350 sq ft per gallon; using the label's stated rate for a smooth, primed wall over-estimates coverage on rough walls.
- Counting the floor or ceiling in the wall area. Enter only the surfaces you are actually painting; the ceiling is usually a separate calculation with its own coats.
Frequently asked questions
How much does a gallon of paint cover?
About 350 to 400 sq ft per gallon for quality interior paint on a smooth, primed wall. Rough, porous, or bare surfaces cover less. The calculator uses 350 sq ft per gallon by default and lets you change it to match your paint label.
How many gallons do I need for two coats?
Roughly double the one-coat amount. The calculator multiplies the paintable area by the number of coats before dividing by the coverage rate, so set coats to 2 and it works out the total for both.
Do I need to subtract doors and windows?
It helps for a tighter estimate. Enter the number of doors and windows and the calculator subtracts about 15 sq ft each. For a small room with one door and one window the difference is minor, but it adds up on larger jobs.
Does this include primer?
No. Primer is a separate product and coat. If you are painting bare drywall, patched areas, or making a big color change, add a primer coat — either count it as an extra coat here or run a separate estimate for the primer.
Why does the calculator round up to whole gallons?
Paint is sold in whole gallons (and quarts), and running out partway through a wall leaves a visible lap mark. The calculator rounds the gallon count up and shows the exact pre-round figure so you can see how close you are to the next can.