Paint Calculator
Estimate how many gallons of paint a room needs based on wall area, coats, and paint coverage, so you can plan your purchase with less guesswork.
Calculate paint needed
How this paint calculator works
The calculation starts with the room’s perimeter (twice the length plus the width), multiplies it by wall height to get total wall area, then subtracts standard allowances for doors and windows since you don’t paint over them. That net area is multiplied by your number of coats and divided by 350 — the standard coverage most interior latex paints get per gallon on smooth, primed walls.
How to use this calculator in 3 steps
- Enter room length, width, and wall height — in feet.
- Enter number of coats, doors, and windows — defaults are set to typical values.
- Add a price per gallon (optional) — to get an estimated total cost, not just gallons.
Why 350 square feet per gallon?
Many interior paints cover roughly 350–400 square feet per US gallon per coat on a smooth surface, although actual coverage depends on the product and wall condition. This calculator uses a fixed estimate of 350 square feet per gallon; it does not let you enter a different coverage rate. Check the coverage printed on your paint can and use the manufacturer’s figure when planning a purchase. For background, see Sherwin-Williams’ paint calculator guidance.
What this estimate includes—and excludes
This tool estimates interior wall paint from room length, width, wall height, number of coats, and standard door and window deductions. It does not calculate ceilings, trim, primer, multiple rooms, or product-specific coverage. The paint quantity is rounded up to the nearest quarter gallon for planning, while the optional cost estimate assumes you buy whole gallons. Treat the result as a starting estimate, not a guarantee of the exact amount needed.
When to round up further
Textured walls, unprimed drywall, or a dramatic color change (like painting over a dark wall with a light color) all reduce effective coverage. In those cases, budget for an extra 10–15% beyond this calculator’s result, or plan on a third coat.
Worked example
Take a 12×14 bedroom with 8-foot ceilings, one door, and two windows, painted in two coats. Perimeter is 2 × (12 + 14) = 52 ft. Gross wall area is 52 × 8 = 416 sq ft. Subtract 21 sq ft for the door and 30 sq ft for the two windows, leaving 365 sq ft of net wall area. Two coats over 365 sq ft, divided by 350 sq ft per gallon, works out to 2.09 gallons — this calculator rounds that up to 2.25 gallons. Since paint is sold by the gallon, you’d actually buy 3 gallons; at $42 a gallon that’s $126 for the room.
Frequently asked questions
Do I need to paint the ceiling separately?
Yes — this calculator covers wall area only. Ceiling area is length × width, and typically needs its own gallon count since ceiling paint often has different coverage and finish.
How much paint does a standard door or window really take up?
A standard interior door is roughly 21 square feet (7 ft × 3 ft), and a standard window is roughly 15 square feet — the defaults used in this calculator. Adjust the counts if your doors or windows are unusually large or small.
Should I always buy an extra gallon just in case?
It’s a reasonable safety margin for touch-ups later, especially since paint can be hard to color-match exactly if you run out mid-project and need to buy more from a different batch.
Does this work for exterior paint or ceilings too?
The same area-based math applies to exterior walls and ceilings — measure the surface instead of interior walls, and use this calculator’s result as your gallon estimate. Exterior surfaces often have lower effective coverage due to texture (stucco, brick), so budget slightly higher than the standard 350 sq ft per gallon.
Can I use this to estimate spray paint instead of rollers?
Spray application typically uses more paint per square foot than a roller due to overspray, so treat this calculator’s gallon estimate as a minimum and add roughly 20–30% extra when spraying.