Construction & DIY
Paint Calculator
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How to use it
Using the paint calculator
- 01
Add up wall area
Multiply each wall’s length by its height and sum them all. Ceiling-height changes count separately.
- 02
Subtract openings
Count doors and windows; the calculator removes their area automatically.
- 03
Pick coats and read gallons
Two coats is normal over existing color; use three when covering reds or dark blues. Buy the rounded-up gallon figure.
Good to know
Coverage claims vs reality
A “350 sq ft per gallon” rating assumes smooth primed surfaces applied at ideal spread rate. Textured walls, bare drywall soaking primer, and dramatic color changes can cut real-world coverage 20% or more; which is exactly why the coat selector exists.
Buy once, tint once
Gallons are mixed by machine, and a rematch never matches perfectly. Buying the full quantity up front; including the extra partial gallon; keeps every wall the same shade. Leftover paint stores well for touch-ups.
Primer counts as a coat of coverage
Over new drywall or drastic colors, tinted primer buys you cheap square footage. Counting primer as one of your coats often saves a full finish gallon on big rooms.
How it's calculated
The math behind this calculator
Gallons = ⌈(Wall area − Doors×21 − Windows×15) × coats ÷ coverage⌉Each door opening removes about 21 sq ft and each window about 15 sq ft from the paintable surface. Multiply the remaining area by the number of coats, divide by the paint’s quoted coverage per gallon, and round up to whole gallons.
Assumptions & limitations
- Standard door (≈21 sq ft) and window (≈15 sq ft) opening sizes.
- Smooth interior walls in good repair; rough or heavily dark walls need more paint than the can quotes.
- Trim, ceilings and primers are priced separately.
Worked example
For 400 sq ft of wall with 2 doors and 4 windows painted twice at 350 sq ft/gallon: 298 net sq ft × 2 coats = 596 sq ft, which rounds up to 2 gallons.
FAQ
Frequently asked questions
- How much wall area is a typical bedroom?
- A 12×12 room with 8-ft ceilings has about 384 sq ft of wall before openings; roughly what the calculator’s defaults show.
- Does one coat ever work?
- With paint-and-primer products over a similar light color, sometimes. Dark-to-light transitions almost always need two coats minimum.
- Should ceilings be included?
- No; this tool models walls only. Estimate ceilings separately using floor area × 1.15 for sloped or cut-in losses.
- What about trim and doors?
- Trim work uses far less paint per linear foot; a single quart usually covers an average room’s trim and door casings.
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