Exterior Paint Calculator: How Much Paint You Need

This exterior paint calculator works out how many gallons or liters of paint you need for the outside walls of a house or building. You enter the total wall area (or each wall’s height and width), the number of coats you plan to apply, and the paint’s coverage rate per gallon or liter, and it returns the exact amount of paint to buy, plus a recommended purchase amount with a 10% buffer for waste and touch-ups.

Formula

The calculation is straightforward: multiply the total wall area by the number of coats, then divide by the coverage rate of the paint.

Paint needed (gallons) = (Total wall area × Number of coats) ÷ Coverage per gallon

• Total wall area – the sum of all exterior wall surfaces, measured in square feet (or square meters).
• Number of coats – how many layers of paint you will apply. Two is standard for most exterior repaints; one may suffice for a same-color touch-up.
• Coverage per gallon – the area one gallon covers at the recommended film thickness, usually printed on the paint can. Typical values are 250–400 sq ft per gallon, depending on the surface texture.

For metric, replace gallons with liters and square feet with square meters. Most paint labels list coverage in both systems (e.g., 10–12 m² per liter).

Worked Example

Using the default values from the calculator: a wall area of 1,500 sq ft, 2 coats, and a coverage of 350 sq ft per gallon.

1. Multiply area by coats: 1,500 × 2 = 3,000 sq ft of total surface to cover.
2. Divide by coverage: 3,000 ÷ 350 = 8.57 gallons.
3. Add 10% waste: 8.57 × 1.1 = 9.43 gallons.

The calculator shows 8.6 gallons as the exact paint needed and 9.4 gallons as the recommended purchase. You would round up to 10 gallons when buying, since paint is sold in whole gallons.

How to Measure Your Exterior Wall Area

Getting the wall area right is the most important step. Here is a reliable method for a typical house:

• Measure the perimeter of the house (length of all exterior walls at ground level) in feet.
• Multiply the perimeter by the average wall height (usually 8–10 ft for a single story, more for two stories). This gives the gross wall area.
• Subtract the area of windows and doors. A standard exterior door is about 21 sq ft; a typical window is 12–15 sq ft. Count them and subtract the total.
• For gable ends, calculate the triangular area (base × height ÷ 2) and add it.

If you are painting siding with deep profiles (e.g., board-and-batten or rough stucco), the actual surface area can be 10–20% larger than the flat measurement. In that case, increase the coverage divisor or add extra to the area.

Factors That Change Coverage

The same gallon of paint will cover different areas depending on the surface and conditions. These are the main variables:

• Surface texture – Smooth, primed wood or metal covers at the high end (400 sq ft/gal). Rough stucco or brick can drop coverage to 200–250 sq ft/gal because the paint fills pores.
• Porosity – New, bare wood or drywall soaks up more paint than a previously painted surface. A primer coat is often needed, which counts as an extra coat.
• Application method – Spraying uses more paint than rolling or brushing because of overspray. Rollers with a thick nap hold more paint and deposit a thicker film.
• Weather – Painting in direct sun or high wind causes the paint to dry faster, reducing coverage and potentially causing lap marks. Ideal conditions are 50–80°F, low humidity, and no rain for 24 hours.
• Color change – Going from a dark to a light color (or vice versa) often requires an extra coat for full hiding. A high-quality primer tinted toward the final color can reduce the number of topcoats.

Always check the paint can label for the manufacturer’s stated coverage. That number assumes a smooth, primed surface and a single coat at the recommended dry film thickness.

Common Mistakes and How to Avoid Them

Even with a calculator, people often end up with too little or too much paint. Here are the typical errors:

• Forgetting to subtract windows and doors – This can overestimate paint by 15–20%. Measure the actual openings.
• Ignoring the number of coats – Many calculators assume one coat. If you are covering a dark color with a light one, you need two or three coats. Always multiply by the number of coats you will actually apply.
• Using the wrong coverage number – The label’s coverage is for ideal conditions. On rough surfaces or in hot weather, reduce the coverage by 10–20%.
• Rounding down when buying – Paint is sold in whole gallons. If the calculator says 9.4 gallons, you must buy 10. Buying 9 gallons leaves you short, and a second trip to the store is costly.
• Not accounting for waste – Some paint is lost in the roller tray, on the brush, or as overspray. The 10% buffer in this calculator covers that.

A good rule of thumb: if you are between two gallon sizes, buy the larger one. Unopened paint can usually be returned, but running out mid-project is far more inconvenient.

Primer vs. Topcoat: Do You Need Both?

Primer is a separate product with its own coverage rate, usually lower than topcoat (around 300 sq ft/gal). You should include primer in the calculation if you are painting:

• New, bare wood, metal, or masonry
• Stained or chalky surfaces
• Over a much darker or lighter color
• Repairing patches of bare material after scraping old paint

For a repaint over sound existing paint, a primer is often unnecessary. Many modern exterior paints are self-priming, but they still require a clean, dry surface. If you do use primer, calculate it separately with the same formula: (wall area × 1 coat) ÷ primer coverage.

Cost Considerations and Buying Tips

While this calculator gives you the volume of paint, the total cost depends on the paint quality and your location. Exterior paints range from economy to premium, with premium lines offering better UV resistance, flexibility, and longevity (often 10+ years versus 3–5 for budget paints).

To estimate cost, multiply the recommended purchase amount (in gallons) by the price per gallon. For a typical 1,500 sq ft house needing 10 gallons, that could be $300–$500 for mid-grade paint, plus primer if needed. Labor, if you hire a professional, usually doubles or triples the total project cost.

Buying tips:

• Buy all paint at once from the same batch to ensure color consistency.
• Check if the store offers a discount for full cases (usually 4 gallons per case).
• Keep the receipt and unopened cans for returns – many stores accept them within 30 days.
• Store leftover paint in a cool, dry place with the lid sealed; it can last 5–10 years if unopened.

Frequently Asked Questions

How many square feet does a gallon of exterior paint cover?
Most exterior paints cover 250–400 square feet per gallon on smooth, primed surfaces. Rough surfaces like stucco or brick will cover less, often 200–250 square feet per gallon. Always check the label for the specific product’s coverage rate.

Do I need to prime before painting exterior walls?
Primer is recommended for bare wood, metal, masonry, or when making a drastic color change. It seals the surface, improves adhesion, and can reduce the number of topcoats needed. For repainting over sound existing paint, primer is usually not required.

How do I measure the exterior wall area of a two-story house?
Measure the perimeter at ground level, then multiply by the height from the foundation to the roofline. For a two-story house, that height is typically 16–20 feet. Add gable areas separately using the triangle formula (base × height ÷ 2). Subtract windows and doors as you would for a single story.

What happens if I buy too little paint?
You will need to make a second trip to the store, and the new can may be from a different batch, causing a slight color mismatch. To avoid this, always buy 10% more than the calculated amount and keep the receipt for unopened cans.

Can I use the same calculator for interior walls?
Yes, the formula is identical. The main difference is that interior walls are usually smooth and primed, so coverage tends to be higher (400 sq ft/gal). Also, interior projects often involve more trim and doors, which you should subtract from the wall area.

Does paint color affect how much I need?
Yes. Light colors over dark surfaces (or vice versa) often require an extra coat for full hiding. If you are changing color dramatically, add one more coat to the calculation. A tinted primer can also reduce the number of topcoats needed.

Related

• Build It Brick Calculator: How It Works
• Board and Batten Vinyl Siding Calculator
• Board Foot Calculator for Spray Foam

in area | Total wall area | sq ft | 1500 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per gallon | sq ft/gal | 350 | 50
out paint | Paint needed | gallons | (area * coats) / coverage | 1
out buy | Buy (with 10% waste) | gallons | ((area * coats) / coverage) * 1.1 | 1