Paint Calculator
Calculate how many gallons of paint you need for a room's walls, plus total cost at your price per gallon.
How this calculator works
Paint coverage is measured per gallon over a wall’s total surface area. This calculator multiplies wall length by wall height to get total square footage, then divides by 350 square feet per gallon — the standard rate for one coat on a smooth interior wall.
Formula: Gallons = (Total wall length ft × Wall height ft) ÷ 350 sq ft per gallon.
Worked example
A room with 40 ft of total wall length and an 8 ft ceiling (320 sq ft), $35/gallon:
- Wall area: 40 × 8 = 320 sq ft
- Gallons: 320 ÷ 350 ≈ 0.91 gallons
- At $35/gallon, that’s $32.00 for one coat.
Coverage reference
| Total wall length | Wall height | Gallons needed (1 coat) |
|---|---|---|
| 30 ft | 8 ft | 0.69 gallons |
| 40 ft | 8 ft | 0.91 gallons |
| 60 ft | 8 ft | 1.37 gallons |
How to measure your room
Walk the perimeter of the room and add up the length of every wall you’re painting to get your total wall length, then use your ceiling height (commonly 8 ft) as the width figure. This gives total wall square footage for one coat — multiply the resulting gallons by however many coats you’re planning, and remember primer or heavily textured surfaces will pull actual coverage below the standard rate.
Frequently asked questions
How much wall area does one gallon of paint cover?
About 350 square feet for one coat on a smooth interior wall. This calculator divides your total wall area by that rate to get gallons for a single coat.
What do the length and width fields mean for a room?
Length is the total combined length of all the walls you're painting (add up each wall's width as you go around the room), and width is the wall height — 8 ft is standard for most residential ceilings. Multiplying the two gives your total paintable wall area.
How many coats of paint does a room actually need?
Two coats is standard for a full color change or a wall going from a much lighter to much darker shade, while a single coat can work for a minor refresh in a similar color. This calculator gives you the gallons for one coat — multiply the result by your planned coat count.
Does primer change how much paint I need?
Primer itself is a separate product calculated the same way (350 sq ft/gallon is a reasonable primer estimate too), but skipping primer on a drastic color change or bare drywall can mean the topcoat needs an extra coat to fully cover, effectively increasing your paint total.
Does textured or unfinished drywall need more paint?
Yes — heavily textured surfaces like orange peel or knockdown finish have more actual surface area than their flat square footage suggests, and unprimed new drywall is more absorbent, both of which reduce effective coverage below the standard 350 sq ft/gallon rate.
Should I subtract doors and windows from my wall area?
For a single door or a couple of windows, it's usually not worth the trouble since the difference is small relative to a can of paint. For a wall with a large opening — like a big picture window or sliding door — subtract its square footage for a more accurate gallon count.
Estimates only. Verify quantities with your supplier before purchasing.