Exterior Paint Calculator: How Many Gallons Do You Need?

An exterior paint calculator estimates how many gallons you need from the area to be painted, the paint’s coverage rate, and the number of coats. Enter the wall area in square feet and the coverage in square feet per gallon; the calculator returns gallons to buy, with a practical whole-gallon estimate.

Formula

Gallons needed = paintable area × number of coats ÷ coverage per gallon.

• A is the paintable surface area, in square feet (sq ft).
• C is the number of coats, as a count.
• R is the paint’s stated coverage, in square feet per US gallon (sq ft/gal).
• G is the calculated paint volume, in US gallons.
• L is the same volume converted to liters; one US gallon is approximately 3.78541 liters.

The formula assumes the stated coverage applies to your surface. It gives a theoretical quantity, not a guarantee: roughness, porosity, application method, and paint loss can change actual use. The calculator shows a decimal result so you can plan; paint is commonly sold in fixed container sizes, so round up to a purchasable quantity rather than rounding down.

Worked Example

Using the defaults above: paintable area is 1,500 sq ft, coverage is 350 sq ft per gallon, and the plan is two coats.

1. Calculate total coat area: 1,500 × 2 = 3,000 sq ft.
2. Divide by coverage: 3,000 ÷ 350 = 8.5714 US gallons.
3. Rounded to two decimal places, the result is 8.57 gallons.
4. Convert to liters: 8.5714 × 3.78541 = 32.44 liters, rounded to two decimals.

For purchasing, this result means you will need at least enough product to cover about 8.57 gallons at the assumed rate. If the available package sizes do not total that amount, buy the next suitable quantity above it. A modest reserve can help with touch-ups, but the right allowance depends on the surface and project.

Measure the Paintable Exterior

Measure each wall’s width and height, then multiply them to find its gross area. Add the wall areas together. For a rectangular wall, area is width × height; for a triangular gable, use base × height ÷ 2. Include surfaces that will receive the paint, such as fascia or trim, as separate measurements if their area is not already included.

Subtract large areas that will not be painted, such as broad windows or doors, when you can measure them accurately. Small openings often make little difference, and subtracting every one can create false precision. If trim or siding uses a different color or product, calculate those areas separately because coverage and coat requirements may differ.

• Keep measurements in one system. If you measure in feet, convert inches to a fraction of a foot before calculating.
• For a wall 24 ft wide and 10 ft high, the gross area is 240 sq ft.
• For several elevations, calculate each one separately before adding them; this makes omissions easier to spot.

Choose a Realistic Coverage Rate and Coat Count

Use the coverage rate printed on the paint container or product information for the specific coating. Coverage is usually stated as a range because one gallon does not cover every surface equally. The calculator’s default of 350 sq ft per gallon is only a planning example, not a universal rate. If the manufacturer gives a range, estimate conservatively for a rough, porous, or previously unpainted surface.

Count coats based on the project requirements and the product instructions. Two coats are a common planning assumption for exterior repainting, but the right number can vary with the existing color, the new color, surface condition, and whether a separate primer is required. Primer is a different coating: estimate its amount separately using its own coverage rate and the area it will cover.

Coverage figures are generally estimates under suitable application conditions. Textured siding, masonry, weathered wood, and surfaces that absorb material may use more paint than smooth, sealed siding. Spraying can also involve overspray and transfer loss; follow the product’s instructions for the selected application method.

Common Calculation Errors and Limits

The most frequent mistake is confusing total wall area with the area for one coat. The formula multiplies area by the coat count before dividing by coverage. For 1,500 sq ft and two coats, the coating must cover 3,000 sq ft in total. Another common error is using a coverage number stated per liter as if it were per gallon, or mixing square meters with square feet.

• Do not enter the perimeter of a building as its wall area; height is also needed.
• Do not round the area or gallon result down early. Keep the calculation precise, then round up to an amount you can buy.
• Do not assume a calculator includes primer, trim, shutters, soffits, or other separate surfaces unless you entered those areas.
• Do not treat the result as a precise prediction of material consumed; the formula cannot account for every surface or application condition.

For metric measurements, calculate area in square meters and use coverage in square meters per liter, or convert consistently. One square meter is about 10.764 square feet. The calculator above takes square feet and US gallons as its inputs, then provides a liter conversion for comparison.

Frequently Asked Questions

How many gallons of exterior paint do I need for a 2,000-square-foot house?
A house’s listed floor area is not its paintable wall area. Measure the exterior walls, subtract substantial unpainted openings if appropriate, then multiply by the coat count and divide by the product’s coverage per gallon. The result can differ greatly with building shape and siding.

How much area does one gallon of exterior paint cover?
There is no single rate for all exterior paints and surfaces. Check the product label or technical information; coverage is often given as a range and can be lower on rough or absorbent material.

Should I subtract windows and doors?
You can subtract large openings that will not be painted, especially when estimating a sizeable project. Small openings may not justify the extra measurement, and you should include any frames or trim that will be painted as separate areas.

Should I add extra paint for waste and touch-ups?
Consider a reserve when surfaces are rough, application may involve overspray, or future touch-ups matter. There is no dependable universal percentage for every project. Use the product coverage guidance and round up to a practical package amount.

Does this calculation include primer?
No. Primer should be calculated separately when required, using the area to be primed and the primer’s stated coverage. Some systems may call for primer on particular surfaces or conditions; follow the coating instructions.

How do I calculate paint for siding and trim separately?
Measure each surface type separately, then use the coverage rate and coat count that apply to its product. Add the quantities only after calculating each coating independently; different colors or products should not be treated as one shared coverage rate.

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in a | Paintable area | sq ft | 1500 | 0
in r | Coverage per gallon | sq ft/gal | 350 | 0
in c | Number of coats | coats | 2 | 0
out g | Paint needed | gallons | a * c / r | 2
out l | Paint needed | liters | a * c / r * 3.78541 | 2