Exterior Paint Calculator
Enter your home's perimeter, wall height, and details about doors, windows, and trim to calculate the gallons of exterior body and trim paint needed for a complete repaint.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is Exterior Paint?
An exterior paint calculator estimates the gallons of paint required to cover a house exterior, separately calculating body paint (for walls and siding) and trim paint (for fascia, window casings, door frames, and decorative trim). Exterior painting is one of the largest home maintenance investments, and accurate quantity estimation prevents both wasteful over-purchasing and frustrating mid-project runs to the store. The calculation starts with gross wall area: the house perimeter multiplied by wall height, times the number of stories, plus any gable areas above the eave line. From this total, subtract areas that will not receive body paint — doors (approximately 21 square feet each, or 3x7 feet) and windows (approximately 15 square feet each, or 3x5 feet). The result is the net paintable wall area. Exterior paint coverage is approximately 350 square feet per gallon on typical surfaces. This is lower than the 400 square feet per gallon often listed on cans because exterior surfaces — wood siding, stucco, brick — are rougher and more absorbent than interior drywall. Weathered wood may only get 250-300 square feet per gallon on the first coat. The calculator uses 350 as a reliable average for previously painted surfaces in reasonable condition. Two coats are standard for exterior painting. A single coat rarely provides uniform coverage and durability, especially when changing colors. The first coat seals and primes the surface, while the second provides the final color depth and UV protection. Some premium paints advertise one-coat coverage, but professionals still apply two coats for longevity — exterior paint must withstand years of sun, rain, wind, and temperature extremes. Trim paint is calculated separately because it is typically a different color and sheen (semi-gloss or gloss for durability and moisture resistance on horizontal surfaces). Trim width averages 4 inches for typical fascia, casings, and corner boards. The linear footage of trim multiplied by this average width gives the trim area, divided by coverage rate and multiplied by number of coats.
How to Calculate
- Measure the house perimeter (sum of all exterior wall lengths) in feet
- Measure wall height for one story (eave line to foundation, typically 8-10 ft)
- Enter the number of stories
- Estimate total gable area (triangular wall sections above eave line)
- Count all exterior doors and windows
- Measure or estimate total trim linear footage (fascia, casings, corner boards, rake boards)
- Set number of coats (2 is standard; 3 for dramatic color changes)
- Review gallons needed for body and trim paint separately
Formula
Gross Wall Area = (Perimeter x Wall Height x Stories) + Gable Area Door Area = Number of Doors x 21 sq ft Window Area = Number of Windows x 15 sq ft Net Paintable Area = Gross Wall Area - Door Area - Window Area (minimum 0) Body Paint Gallons = Net Paintable Area x Coats / 350 Trim Area = Trim Linear Feet x (4 inches / 12) Trim Paint Gallons = Trim Area x Coats / 350 Coverage rate of 350 sq ft/gallon is the industry standard for exterior surfaces. Standard door = 3 ft x 7 ft = 21 sq ft. Standard window = 3 ft x 5 ft = 15 sq ft. Average trim width = 4 inches.
Example Calculation
A 2-story house: 160 ft perimeter, 9 ft walls, 100 sq ft gable, 2 doors, 12 windows, 200 ft trim, 2 coats: Gross Wall Area = (160 x 9 x 2) + 100 = 2,880 + 100 = 2,980 sq ft Door Area = 2 x 21 = 42 sq ft Window Area = 12 x 15 = 180 sq ft Net Paintable Area = 2,980 - 42 - 180 = 2,758 sq ft Body Paint Gallons = ceiling(2,758 x 2 / 350 x 10) / 10 = ceiling(157.6) / 10 = 15.8 gallons Trim Area = 200 x (4 / 12) = 200 x 0.333 = 66.67 sq ft Trim Paint Gallons = ceiling(66.67 x 2 / 350 x 10) / 10 = ceiling(3.81) / 10 = 0.4 gallons
Frequently Asked Questions
How many square feet does a gallon of exterior paint cover?
Approximately 350 square feet on typical previously painted exterior surfaces. New or bare wood absorbs more paint and may yield only 250-300 sq ft per gallon. Smooth surfaces like primed hardboard siding approach 400 sq ft per gallon. Rough textures like stucco or rough-sawn cedar may only get 200-250 sq ft per gallon. When in doubt, buy based on 300 sq ft per gallon.
Do I really need two coats of exterior paint?
Yes, for durability and proper coverage. Two coats provide a thicker paint film that resists UV degradation, moisture intrusion, and chalking significantly longer than one coat. A properly applied two-coat system lasts 7-10 years, while a single coat may fail in 3-5 years. The labor cost of painting is far greater than the paint cost — skimping on the second coat saves little and shortens the repaint cycle dramatically.
Should I use the same paint for trim and body?
Use the same brand but different sheens. Body paint is typically flat or low-luster (satin) for aesthetic reasons and to hide surface imperfections. Trim paint should be semi-gloss or high-gloss for maximum durability, moisture resistance, and easy cleaning. Horizontal trim surfaces (window sills, door thresholds) take the most weather abuse and benefit from the hardest finish available.
How do I estimate gable area?
A gable is a triangle: area equals one-half times base times height. Measure the width of the gable at the eave line (base) and the height from the eave to the peak. A typical gable on a 30-foot wide house with a 6:12 pitch is about 0.5 x 30 x 7.5 = 112 sq ft. If your home has two identical gables, double the result. Hip roofs have smaller or no gable areas.
How long does exterior paint last?
Quality exterior latex paint lasts 7-10 years on properly prepared surfaces with two coats. South and west-facing walls fade faster due to UV exposure. Factors that shorten paint life include: poor surface prep, single coat application, painting in direct sun or extreme temperatures, painting over chalked or peeling surfaces, and excess moisture from lack of ventilation or flashing issues.
Should I prime before painting exterior?
Prime bare wood, bare metal, stained areas (water, rust, tannin bleed), and patches or repairs. Previously painted surfaces in good condition that are being repainted with the same type of paint (latex over latex) generally do not need primer. When changing from oil-based to latex paint, use a bonding primer. Premium self-priming exterior paints work for color changes over existing sound paint.