How this exterior estimate works
The calculator multiplies the rectangular footprint perimeter—2 × (length + width)—by average wall height. It subtracts doors and windows, then applies coats, an explicit allowance, and the selected paint's coverage.
Example: small one-storey house
A 30 × 24 ft footprint has a 108 ft perimeter. At 9 ft high, gross wall area is 972 sq ft. After 150 sq ft of openings, 822 sq ft remains. Two coats plus 10% allowance create 1,808 coverage-square-feet. At 300 sq ft/gal, 1,808 ÷ 300 = 6.03 gallons. Round that theoretical result up to containers actually sold.
Example: typical two-storey house
A 40 × 30 ft footprint has a 140 ft perimeter. At an 18 ft average wall height, gross wall area is 2,520 sq ft. Subtract 220 sq ft of openings to get 2,300 sq ft. Two coats plus 10% allowance create 5,060 coverage-square-feet. At 300 sq ft/gal, 5,060 ÷ 300 = 16.87 gallons, rounded to the available container combination.
Exterior paint price per square foot
The calculated paint cost per square foot is a material-planning figure: rounded gallons × price per gallon ÷ paintable siding area. In the 2,300 sq ft example, 17 gallons at $55 cost $935, or about $0.41 per sq ft for paint alone. It does not represent the installed market price.
For a project budget, enter an optional labor rate per square foot based on an actual local quote. The calculator adds that amount to paint material cost. Preparation, scraping, repairs, primer, trim, access equipment, tax, and minimum charges remain separate unless your entered rate includes them.
Choose a realistic exterior coverage rate
| Surface condition | Planning approach |
|---|---|
| Smooth, sound, previously painted siding | Start with the selected product's published range |
| Rough wood, stucco, or masonry | Use the lower end and confirm substrate-specific instructions |
| Bare or repaired substrate | Calculate a compatible primer separately |
| Spray application | Add an explicit allowance for overspray and transfer loss |
A label rate of 10 m²/L means one litre covers about 10 square metres for one coat, before application loss. Calculate litres as area in m² × coats × (1 + allowance%) ÷ coverage in m²/L. To compare units, 350 sq ft/gal is about 8.6 m²/L, while 10 m²/L is about 407 sq ft/gal. Always use the selected product's label rate.
What changes the estimate?
- Siding: Texture and porosity can move field coverage toward the lower end of a label range.
- Preparation: Cleaning, sound repairs, and compatible primer affect adhesion and coverage.
- Colour and coats: Do not assume one or two coats universally; follow the product and substrate instructions.
- Weather: Apply only within the product's stated air, surface, humidity, and rain-free conditions.
- Geometry: Gables, dormers, and multiple storeys must be measured rather than inferred from floor area.
Frequently asked questions
How do I measure doors and windows?
Multiply width by height for each substantial opening and add the areas. Treat painted trim as a separate calculation rather than assuming it requires no coating.
Should I use exterior primer?
Use the primer specified for bare, repaired, stained, chalky, glossy, or difficult substrates. Compatibility depends on both the substrate and finish system; use the primer calculator only after choosing the product.
One coat or two?
Follow the product instructions. Substrate condition, colour change, application rate, and performance requirements determine whether another coat is needed.
Before ordering exterior paint
- Confirm coating compatibility with the siding and climate.
- Separate wall, trim, door, and masonry colours.
- Measure gables, dormers, garages, and irregular elevations.
- Keep coverage and application allowance as separate inputs.
- Round the theoretical result to containers actually sold.
Sources and review
Behr's current exterior range shows that product coverage may span 250–400 sq ft per gallon, while its technical material explains that texture, porosity, and application affect coverage. This page therefore uses editable planning assumptions rather than a universal promise. Check the chosen coating's label and data sheet before buying.
