Estimate exactly how many gallons of paint you need for your room's walls, accounting for doors and windows.
Room Dimensions
Paint Needed
0gallons
For 0 sq ft of wall space
Assuming 350 sq ft per gallon.
Round up to the next full gallon or quart when buying — paint isn't sold in exact fractional amounts.
Last updated: July 2026
Take the Guesswork Out of Painting
Nothing is more frustrating than running out of paint when you're three-quarters of the way through a wall, or conversely, ending up with three extra expensive gallons you don't need. Properly calculating your wall surface area—and subtracting space for doors and windows—is the key to a stress-free painting project.
How to Use This Tool
Enter the dimensions of your room (or individual walls) along with the number of standard doors and windows. The calculator will determine the exact square footage to be painted and tell you precisely how many gallons of paint to buy for optimal coverage. Note: this estimate covers walls only — it does not include the ceiling or trim.
Worked Example
Using the calculator's default numbers — a 12ft × 12ft room, 8ft ceilings, 1 door, 1 window, 2 coats:
Wall area = (2 × 12 × 8) + (2 × 12 × 8) = 384 sq ft
Deductions = (1 × 21) + (1 × 15) = 36 sq ft
Paintable area = 384 − 36 = 348 sq ft
Total coverage needed (2 coats) = 348 × 2 = 696 sq ft
Gallons = 696 ÷ 350 ≈ 1.99, rounded up to 2.0 gallons
Frequently Asked Questions
One gallon of standard interior wall paint typically covers between 350 and 400 square feet for a single coat. This calculator uses 350 sq ft per gallon, the more conservative end of that range, so you're less likely to run short.
Yes, standard practice requires two coats for an even finish and true color representation, especially if you are painting over a darker color or using a lower quality paint.
It's highly recommended to keep a quart or half-gallon of leftover paint for future touch-ups to cover scuffs or wall repairs.
No, this tool calculates wall area only. If you're painting the ceiling too, add its square footage (length × width) separately — ceilings often use a different paint (flat, stain-blocking) than walls anyway.
No, paint is sold in fixed sizes (typically quarts, gallons, and 5-gallon buckets), not arbitrary fractions. Round up to the next full gallon, or buy a gallon plus a quart to cover the extra. Never round down, since running short mid-wall means an inconvenient paint store trip mid-project.
The calculator subtracts 21 sq ft per standard door and 15 sq ft per standard window. If your doors or windows are noticeably larger or smaller (e.g. sliding glass doors, picture windows), your actual paintable area will differ from the estimate.
No, this estimate is for wall paint only. Trim and molding are usually painted with a separate, smaller can of trim/semi-gloss paint, and typically need far less than a full gallon for an average room.