Exterior Paint Calculator: How Much Paint Does Your House Need?

Nothing stalls a weekend exterior paint job like realizing you're a gallon short with the sun going down — or staring at three leftover gallons you'll never open again. Exterior paint costs more than interior paint, so getting the math right matters even more. Here's how the pros do it.

Step 1: Measure each wall (not the floor plan)

Unlike interior rooms, you measure the outside of the house wall by wall. For each wall, multiply width × height (to the eaves, not the peak — we'll handle gables separately).

Example: a 40 ft wide × 10 ft tall front wall = 400 sq ft. Do this for all four walls and add them up. A typical 1,500 sq ft ranch might have roughly 1,600–2,000 sq ft of paintable siding.

Gables: those triangular wall sections under the roof peak count too. Area of a triangle = ½ × base × height. A 30 ft wide gable rising 6 ft = ½ × 30 × 6 = 90 sq ft.

Step 2: Subtract windows, doors, and anything unpainted

Measure every window and door and subtract them — on exteriors these add up fast:

• Standard window: ~15 sq ft each
• Entry door: ~21 sq ft
• Garage door (two-car): ~130 sq ft — this one surprises people
• Brick or stone sections you aren't painting: subtract the full area

A house with 12 windows, 2 doors, and a two-car garage door loses roughly 12 × 15 + 2 × 21 + 130 = 352 sq ft from the total. That's real money in exterior paint.

Step 3: Decide on coats (exteriors usually need two)

Exterior repaints almost always need two coats — sun, rain, and temperature swings punish thin coverage, and most manufacturers' warranties assume two coats. Going from dark to light? Still two. Bare wood or stucco? Definitely two, possibly with a primer coat first.

Multiply your net wall area by 2.

Step 4: Divide by coverage — exterior paint covers less

Here's the part most people get wrong: exterior paint covers less per gallon than interior paint. Rough or textured siding (stucco, rough-sawn wood, brick) drinks paint. Use these realistic numbers:

| Surface | Coverage per gallon | |---|---| | Smooth siding (vinyl, aluminum, smooth wood) | ~350 sq ft | | Wood siding / shingles | ~300 sq ft | | Stucco (smooth) | ~250 sq ft | | Stucco (rough) / brick | ~200 sq ft |

Example: 1,800 sq ft of smooth siding × 2 coats = 3,600 sq ft ÷ 350 = 10.3 gallons.

Step 5: Add 10% for waste and touch-ups

Exterior jobs waste more paint — wind, overspray, ladder drips, thick coats on rough spots. Add 10% minimum. Our example: 10.3 × 1.1 = 11.3 → buy 12 gallons.

Don't forget the trim (separate calculation)

Trim — fascia, soffits, window casings — usually gets painted a different color (often white) in a glossier finish. Estimate trim separately: measure linear feet of trim boards, multiply by board width, and calculate with the same method. A typical house needs 2–4 extra gallons just for trim.

Quick checklist before you buy

• [ ] All walls measured (including gables)
• [ ] Windows, doors, garage door subtracted
• [ ] Two coats planned
• [ ] Right coverage number for your siding type
• [ ] 10% waste added
• [ ] Trim calculated separately
• [ ] Same color batch: buy all gallons at once and box (mix) them together for consistent color

Want to skip the math? Use our free paint calculator — enter your measurements and it handles the coats, coverage, and waste math instantly, in gallons and liters.