A paint calculator tells you how many gallons of paint you need for a room based on its dimensions, the number of doors and windows (which don't get painted), and the number of coats. Enter the room size and feature counts, the calculator subtracts the average door (21 sq ft) and window (15 sq ft) area from the total wall area, multiplies by your coat count, and rounds up to the next whole gallon.
Most interior latex paint covers about 350 square feet per gallon, a number that's been stable for decades and matches the labels on cans from Sherwin-Williams, Benjamin Moore, and other major brands. Always round up; running out of paint mid-project means a second store trip and a risk of color batch mismatch.
Key takeaway
Paint coverage math is just total wall area divided by 350. Everything else, accounting for doors and windows, two coats vs. one, primer layers, is small adjustments to that core number. The single biggest gotcha isn't the math, it's forgetting that two coats means double the paint. Most projects need at least two coats of color (or one coat of primer plus one coat of color) for a clean, even finish.
How it's calculated
The wall area is the perimeter of the room times the ceiling height: 2 × (length + width) × ceiling. From that, subtract the area of doors (21 sq ft each, standard 80" × 36") and windows (15 sq ft each, average residential window). Multiply by number of coats. Divide by 350 (sq ft per gallon coverage rate). Round up.
Two notes on coverage: textured walls absorb more paint than smooth walls (closer to 250 sq ft/gallon for heavy texture). Dark colors over light walls, or vice versa, often need a third coat or a primer layer first.
Source: Standard 350 sq ft per gallon coverage rate
Examples
12'×14' bedroom, 9' ceilings, 1 door, 2 windows, 2 coats
- Room length 14 ft
- Room width 12 ft
- Ceiling height 9 ft
- Doors 1
- Windows 2
- Number of coats 2
Wall area: 2 × (14 + 12) × 9 = 468 sq ft. Subtract one door (21) and two windows (30) → 417 sq ft. Two coats → 834 sq ft total. At 350 sq ft per gallon, that's 2.4 gallons, round up to 3 gallons. You'll have a bit left for touch-ups, which is what you want.
20'×18' living room, 10' ceilings, 2 doors, 3 windows, 2 coats
- Room length 20 ft
- Room width 18 ft
- Ceiling height 10 ft
- Doors 2
- Windows 3
- Number of coats 2
Wall area: 2 × (20 + 18) × 10 = 760 sq ft. Subtract two doors (42) and three windows (45) → 673 sq ft. Two coats → 1,346 sq ft. That's 3.85 gallons, rounded up to 4 gallons. For a high-traffic living space where you might want a third touchup later, buying 5 is a reasonable hedge.
Frequently asked questions
Does this calculator include trim and ceiling paint?
No, this calculates wall paint only. Trim and ceilings use different paint types (satin/semi-gloss for trim, flat for ceilings) and have their own coverage math. As a rough rule, a typical room's ceiling needs about 1 gallon and the trim needs about 1 quart.
How much paint for ceiling only?
Multiply length × width to get ceiling square footage, then divide by 350 (one coat) or 250 (two coats with a flat ceiling paint that doesn't always cover well in one pass). A 12×14 ceiling is 168 sq ft, about 1 gallon for two coats.
Do textured walls need more paint?
Yes. Heavy texture (popcorn ceilings, knockdown walls) typically gets 250-300 sq ft per gallon instead of 350, so add 15-25% to your calculated gallons. Smooth walls get the standard 350. Some labels on the can will note "for textured surfaces" coverage figures explicitly; check before buying.
Can I just round down to save money?
Don't. Running short of paint mid-project is a much bigger problem than buying an extra gallon. Returning to the store for a second batch risks color match failure, even cans of the same SKU mixed weeks apart can vary subtly, and the difference shows on the wall. Either round up to the next whole gallon, or buy one more gallon than the calculator suggests for any project larger than a single bedroom.