Exterior Paint Calculator
To paint a 40 ft long, 9 ft tall exterior wall with 2 coats you need about 720 sq ft of coverage (with 10% waste) — 5 gallons, based on 300 sq ft of coverage per gallon per coat.
Reviewed by The ponderloop teamUpdated 2026-07-11How we calculate
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0 sq ft + 0% waste = 0 sq ft
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How this number is calculated
0 sq ft (no waste added) ÷ 150 sq ft per gallon = 0 gallons
Shopping list
- Exterior acrylic latex paint
- Airless paint sprayer
- Exterior primer
- Painter's tape (exterior grade)
- Canvas drop cloth
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Methodology & assumptions
Wall area is wall length times wall height, minus any door/window openings. That area is divided by coverage per gallon per coat — a 300 sq ft/gal baseline (lower than typical interior figures because exterior siding is rougher and more porous), divided by your number of coats — then multiplied by your chosen waste percentage and rounded up to the next whole gallon.
- Manufacturer coverage specs (~300 sq ft/gal per coat, exterior acrylic latex on typical siding)
- Typical US exterior paint retail price range, 2026
Worked examples
| Room | Result |
|---|---|
| 40 × 9 ft wall, 2 coats, +10% waste | 360 sq ft + 10% waste ≈ 396 sq ft → 3 gallons |
| 60 × 10 ft wall, 1 coat, exact | 600 sq ft, no waste → 2 gallons |
| 25 × 8 ft garage wall, 2 coats, +10% waste | 200 sq ft + 10% waste ≈ 220 sq ft → 2 gallons |
Frequently asked questions
- How many gallons of exterior paint do I need?
- Multiply the wall length by height in feet to get the square footage, subtract any doors and windows you will not paint, then add 10% for touch-ups if you want a buffer. Divide by the coverage per gallon per coat (about 300 sq ft) divided by your number of coats, and round up to the next whole gallon.
- Why is exterior coverage lower than the 350-400 sq ft/gal on the can?
- Can labels usually list coverage for smooth, primed, interior-like surfaces. Exterior siding is typically rougher and more porous — stucco, wood lap siding, brick, and weathered or unprimed wood all soak up more paint per square foot. This calculator uses 300 sq ft per gallon per coat as a more realistic baseline for typical siding; very rough or heavily textured surfaces (raw stucco, rough-sawn cedar) can drop well below that, so buy extra in those cases.
- How many coats of exterior paint do I need?
- Two coats is standard for most exterior repaints and is this calculator's default — it evens out coverage and gives better UV and weather protection. One coat can work over a similar existing color in good condition; three coats may be needed for a drastic color change or bare/patched wood.
- Does the number of coats change how much paint I need?
- Yes, directly — coverage per gallon is divided by the number of coats, since each coat needs its own full pass. Two coats roughly doubles the gallons versus one coat for the same wall area.
- Should I round up or down when figuring out how many gallons to buy?
- Always round up to the next full gallon. Exterior paint is mixed in dye lots, and running short mid-wall can leave a visible seam or force a rush order that may not match exactly.