Paint Calculator — Gallons Needed for a Room
Enter room dimensions, doors, and windows to calculate how many gallons of paint you need, accounting for coats and standard door/window deductions.
Gallons needed
2 gal
301 sq ft to paint, 2 coats, at 350 sq ft/gal
Gross wall area
352 sq ft
Doors + windows deducted
51 sq ft
Deductions use standard typical sizes: 21 sq ft per door, 15 sq ft per window — override by adjusting the counts if your openings are unusually large or small. Coverage of 350 sq ft per gallon is a common average for one coat on primed drywall; check your specific paint can's label, since coverage varies by brand, sheen, and surface texture (rougher or unprimed surfaces need more paint per square foot).
About this tool
How to use it
- Enter the room's length, width, and wall height in feet.
- Enter how many doors and windows are on the walls you're painting.
- Pick how many coats you're applying and the paint's coverage rate (350 sq ft/gallon is a common default — check your can's label for the exact figure).
- Press Calculate Paint Needed, or just keep typing — the results update live.
Why it matters
Buying too little paint means a second trip mid-project, possibly with a slightly different batch/tint; buying too much wastes money on leftover cans. Getting the number right means correctly computing the room's wall area, subtracting the parts you won't paint (doors, windows), and multiplying by however many coats you're applying — several steps that are easy to get wrong doing by hand, especially the coats multiplication.
When to use it
Useful before a shopping trip for a painting project, when comparing how many coats changes the amount needed, or when checking that a paint store's own estimate matches your room's actual dimensions.
Good to know
Door and window deductions use standard typical sizes (21 sq ft per door, 15 sq ft per window) rather than your exact openings — adjust the door/window counts if yours are unusually large or small, since the calculator can't know your openings' exact dimensions. The coverage rate (350 sq ft/gallon by default) is a common average for one coat on a primed, smooth wall; check your specific paint can's label, since coverage varies meaningfully by brand, sheen, and surface texture — a rough or unprimed wall can need significantly more paint per square foot.
Common questions
Why are door and window deductions fixed sizes instead of asking for exact dimensions?
Standard interior doors and windows cluster around a fairly consistent size, so using a typical average (21 sq ft for a door, 15 sq ft for a window) per opening gives a reasonably accurate result without requiring you to measure every single one. If your openings are unusually large, you can adjust by entering a higher door/window count than you actually have to deduct more area.
Why does the coverage rate matter so much?
Coverage rate (sq ft per gallon) varies meaningfully by paint brand, sheen (flat vs. semi-gloss), and the surface being painted — a rough or unprimed wall soaks up more paint per square foot than a smooth, primed one. Always check your specific can's label rather than trusting the 350 sq ft/gallon default for a precise result.
Should I round up or down to the nearest gallon?
Always round up — paint is sold in whole cans, and running out mid-coat on a wall means an unavoidable trip back to the store, possibly with a slightly different batch. This calculator always rounds the final gallon figure up.
Does this calculate primer separately from paint?
No — enter your primer's own coverage rate and set coats to 1 to get a primer-specific gallon estimate, then run the calculation again with your paint's coverage rate and actual coat count for the paint itself.