Exterior Paint Square Footage Calculator

This calculator estimates the total paintable exterior surface area of a house or building in square feet, based on the dimensions you enter. You provide the building’s perimeter, wall height, and the total area of doors and windows, and it returns the net paintable area you need to cover. It also gives you the number of gallons for one coat, assuming a standard coverage rate of 350 square feet per gallon.

Formula

The core formula is straightforward: Net Paintable Area = (Perimeter × Wall Height) − Door/Window Area. Then, to find gallons: Gallons = (Net Area × Number of Coats) ÷ Coverage per Gallon, rounded up to the nearest whole gallon.

Variables:

• Perimeter (P) – total length around the outside of the house, in feet (or meters if you convert).
• Wall height (H) – average height from the foundation to the eaves, in feet. For gables, use the average height.
• Door & window area (D) – sum of all exterior door and window openings, in square feet.
• Coats (C) – number of paint coats you plan to apply (usually 2).
• Coverage – typically 350 sq ft per gallon for exterior latex paint on smooth siding; rough surfaces may cover less.

Worked Example

Using the default inputs: Perimeter = 150 ft, Wall height = 10 ft, Door/window area = 100 sq ft, Coats = 2.

1. Gross wall area = 150 × 10 = 1,500 sq ft.
2. Subtract openings: 1,500 − 100 = 1,400 sq ft (net area for one coat).
3. For two coats: 1,400 × 2 = 2,800 sq ft.
4. Gallons = 2,800 ÷ 350 = 8 gallons. Rounded up, you need 8 gallons.

The calculator shows net area = 1,400 sq ft and gallons = 8, matching the example.

How to Measure Your House

You don’t need a laser or blueprints – a tape measure and a calculator are enough. Walk around the house and measure the length of each exterior wall. Add those lengths to get the perimeter. For a two-story home, measure the height from the foundation to the roofline (about 10 ft per story is typical, but measure to be sure). If you have gables (triangular sections under the roof), you can approximate by adding half the gable height to the wall height, or measure the gable area separately and add it.

For doors and windows, measure the width and height of each opening and multiply to get square footage. Sum all of them. A standard exterior door is about 20 sq ft, and a typical window is 10–15 sq ft. If you skip this step, you’ll overestimate paint by 10–15%.

Choosing the Right Coverage Rate

The 350 sq ft per gallon figure is a good average for smooth, primed wood or vinyl siding. However, coverage varies:

• Smooth surfaces (vinyl, aluminum, smooth stucco): 350–400 sq ft/gal.
• Lightly textured (wood siding, brick): 250–300 sq ft/gal.
• Rough or porous (unpainted stucco, old concrete): 150–250 sq ft/gal.

If your surface is rough, use a lower coverage rate in the formula (e.g., replace 350 with 250). The calculator uses 350 as a default, but you can adjust your estimate accordingly.

Common Mistakes and How to Avoid Them

• Forgetting the second coat – Most exterior paints need two coats for durability and color depth. The calculator multiplies by coats, but many DIYers forget to double.
• Ignoring trim and soffits – This calculator covers only the main wall area. Trim, fascia, soffits, and porches add 10–20% more area. Measure them separately or add a buffer.
• Mixing units – If you measure in meters, convert to feet (1 m = 3.28084 ft) before using the formula. Or convert the result: 1 sq m = 10.7639 sq ft.
• Rounding too early – Always round up the final gallon count. Paint coverage is optimistic, and you’ll want extra for touch-ups.

Cost Considerations

Paint prices vary widely, but a gallon of quality exterior latex typically costs between $30 and $80. For an average 1,500 sq ft house, you might need 10–15 gallons, so paint alone could cost $300–$1,200. Professional labor can double or triple that. To estimate cost, multiply the gallons from the calculator by the price per gallon you plan to buy, then add 10% for waste.

If you’re hiring a contractor, they will often provide a quote based on square footage, but knowing the area yourself helps you compare bids fairly.

Safety and Prep

Exterior painting involves ladders and potentially lead paint in older homes. Always use a sturdy ladder on level ground, have someone spot you, and never overreach. If your house was built before 1978, test for lead before sanding or scraping. Wear a dust mask and safety glasses when scraping or power washing. For two-story work, consider renting scaffolding or using a paint sprayer with proper PPE – sprayers produce fine mist that can be inhaled.

Prep is as important as the paint itself. Clean the siding, scrape loose paint, sand rough edges, and apply primer to bare wood. The calculator assumes a primed or previously painted surface; if you’re painting bare wood, you’ll need an extra coat of primer, which adds about 1 gallon per 400 sq ft.

Alternatives to This Method

If you have architectural blueprints, you can compute wall area directly from the elevation drawings. Some paint manufacturers offer online calculators that ask for more detail (e.g., number of windows, door sizes, siding type). You can also use a laser distance measurer to get precise perimeter and height in minutes. For complex homes with many gables, dormers, or multiple stories, breaking the house into rectangles and triangles is more accurate than a single perimeter × height.

Another approach is the “square footage of floor plan� method: for a single-story home, exterior wall area is roughly equal to the floor area; for two stories, double the floor area. This is a quick estimate but less accurate than measuring.

Frequently Asked Questions

How do I measure the perimeter of an irregularly shaped house?

Break the house into simple rectangles and measure each side. Add all the outside lengths. For an L-shaped house, measure each leg separately and add. If you can’t reach a corner, measure from inside and add wall thickness (usually 6–8 inches).

Do I need to subtract the area of gutters and overhangs?

No, you’re painting the wall surface, not the roof. Gutters and soffits are separate – if you’re painting them, measure their width × length and add to your total.

What if my walls are brick or stucco?

Brick and stucco are porous and absorb more paint. Use a coverage rate of 200–250 sq ft per gallon instead of 350. Also, brick may need a special masonry paint, which has different coverage.

Should I buy all the paint at once?

Yes, because paint batches can vary slightly in color. Calculate the total gallons you need, then buy all of it from the same store and same batch number if possible. This ensures color consistency across the whole house.

How much extra paint should I keep for touch-ups?

Keep at least 1 quart for every 10 gallons used. Store it in a cool, dry place, and write the date and room on the can. Touch-up paint is especially important for exterior surfaces that face harsh weather.

Can I use this calculator for interior rooms?

It works similarly, but interior walls are usually measured per room (length × height × number of walls). For interiors, subtract door and window openings as well, but you’ll also need to account for baseboards and ceiling trim separately.

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in p | Perimeter (feet) | ft | 150 | 0
in h | Wall height (feet) | ft | 10 | 0
in d | Door & window area (sq ft) | sq ft | 100 | 0
in c | Coats | coats | 2 | 1
out area | Net paintable area | sq ft | (p * h) - d | 0
out gal | Gallons needed (rounded up) | gal | ceil((((p * h) - d) * c) / 350) | 0