The Ultimate Weather Barrier
Exterior house paint is more than just a cosmetic upgrade; it is the primary shield protecting your home's framing and siding from sun damage, wood rot, and water intrusion.
Because exterior painting requires massive amounts of prep work (pressure washing, scraping, caulking, and masking), you absolutely do not want to run out of paint on a Sunday afternoon while standing on a 20-foot extension ladder.
Accurately calculating the exterior surface area of your home ensures you buy enough premium paint from the same color batch to complete the job seamlessly.
Understanding Exterior Paint Coverage
A standard gallon of high-quality acrylic latex exterior house paint covers roughly 350 to 400 square feet on a smooth, primed surface.
However, unlike smooth interior drywall, exterior surfaces dramatically impact paint coverage:
- Smooth HardieBoard (Fiber Cement): Yields maximum coverage (approx. 400 sq ft/gallon).
- Rough Cedar Siding / T-111: The rough wood grain acts like a sponge, soaking up the first coat. Yields drastically lower coverage (approx. 250 to 300 sq ft/gallon).
- Stucco / Brick: Deeply textured masonry requires specialty elastomeric paint and heavy roller naps, often reducing coverage to just 150 to 200 sq ft/gallon.
How to Calculate Exterior Paint Gallons
To find your paint requirements, you must calculate the total square footage of your exterior walls, subtract the unpainted areas (doors and windows), and divide by the coverage rate.
The Formula
- Measure the total Perimeter of your house (add the length of all four sides together).
- Measure the Height from the foundation to the roofline.
- Multiply Perimeter × Height to find the Gross Wall Area.
- Subtract the area of standard windows (roughly 15 sq ft each) and doors (roughly 20 sq ft each) to find the Net Wall Area.
- Divide the Net Area by 350 (the standard coverage per gallon).
- Multiply the result by the Number of Coats you plan to apply.
Total Gallons = ((Perimeter × Height) ÷ 350) × Number of Coats
(Note: For homes with large triangular gables under the roofline, calculate the area of the gable [Base × Height ÷ 2] and add it to your Gross Wall Area before dividing).
Example Calculation
You are painting a single-story ranch house. The house is 60 feet long and 40 feet wide (Perimeter = 200 ft). The walls are 10 feet high. You are applying 2 coats of premium paint over smooth siding.
- Gross Wall Area:
200 ft × 10 ft = 2,000 sq ft - Subtract Windows/Doors: Assume 10 windows and 2 doors (roughly 190 sq ft).
2,000 - 190 = 1,810 sq ft - Divide by 350:
1,810 ÷ 350 = 5.17 gallons for one coat - Multiply by 2 coats:
5.17 × 2 = 10.34 gallons
You should purchase 11 gallons of exterior paint. Usually, this means buying two 5-gallon buckets and one single 1-gallon can.
The Importance of Two Coats
It is universally recommended to apply two coats of exterior paint, even if the label claims "One Coat Hide."
The first coat is often partially absorbed by the siding, leaving microscopic pinholes. The second coat fills these gaps, providing the actual weather-resistant seal and ensuring the UV-blocking pigments are thick enough to prevent fading for a decade.