How to use this calculator
- Measure the house perimeter. Walk around the house and add up the wall lengths. Two-story homes — measure first story + second story separately.
- Multiply by wall height. Single story = perimeter × 9 ft (typical). Two stories = perimeter × 18 ft. Subtract gable triangles or measure them separately.
- Subtract openings. A typical window is 15 ft²; a door is 21 ft². Subtract these from the wall area.
- Pick coats. Two coats over existing paint is standard. New, primed siding may need three.
Formula
Gallons = (Wall ft² ÷ 250) × number of coats
Worked example
A 1,800 ft² two-story house has roughly 2,000 ft² of siding. Two coats = 4,000 ft² ÷ 250 = 16 gallons of exterior paint. Add 2 gallons for trim and 2 gallons of primer if any siding is bare or weathered → ~20 gallons total.
Tips for accurate results
- Buy 5-gallon buckets — exterior paint is significantly cheaper per gallon at that size.
- Always paint the soffits and fascia in the same color as the trim to keep the eye flowing up.
- Cool, overcast days are best for exterior painting — direct sun causes lap marks.
- Spray-and-back-roll on textured surfaces (stucco, rough cedar) for the best coverage.