Paint Coverage Calculator: How Much Paint Do I Need

A paint coverage calculator works out how many gallons or liters of paint you need for a room by taking the wall area, number of coats, and the paint’s coverage rate, then returning the total paint volume required. You enter the room’s dimensions (or total wall area), the number of coats, and the coverage rate found on the paint can label, and the calculator gives you the amount to buy. This helps you avoid buying too little (and making an extra trip) or too much (and wasting money).

Formula

The calculator uses the standard paint coverage formula:

Paint Needed (gallons) = (Total Wall Area × Number of Coats) ÷ Coverage per Gallon

Where:

• Total Wall Area (sq ft) = 2 × Height × (Width + Length) − (Door Area × Number of Doors) − (Window Area × Number of Windows)
• Door Area = 21 sq ft (standard interior door, about 3 ft × 7 ft)
• Window Area = 15 sq ft (average window, about 3 ft × 5 ft)
• Number of Coats = how many layers you plan to apply
• Coverage per Gallon = the manufacturer’s stated spread rate, usually 250–400 sq ft per gallon

All dimensions are in feet and the result is in US gallons. For metric, use meters and liters: coverage is often stated as 10–12 m² per liter. One US gallon ≈ 3.785 liters; one square foot ≈ 0.0929 m².

Worked Example

Using the default values from the calculator:

• Wall width = 12 ft, room length = 14 ft, wall height = 8 ft
• Doors = 2, windows = 1
• Coats = 2
• Coverage per gallon = 350 sq ft/gal

Step 1: Calculate the perimeter of the room: 2 × (12 + 14) = 52 ft

Step 2: Multiply by wall height: 52 × 8 = 416 sq ft (gross wall area)

Step 3: Subtract openings: doors = 2 × 21 = 42 sq ft; windows = 1 × 15 = 15 sq ft; total subtractions = 57 sq ft

Step 4: Net wall area = 416 − 57 = 359 sq ft

Step 5: Multiply by coats: 359 × 2 = 718 sq ft of surface to cover

Step 6: Divide by coverage: 718 ÷ 350 = 2.05 gallons

The calculator rounds to 2.1 gallons. Since paint is sold in whole gallons (or 5-gallon pails), you would need to buy 3 gallons for this project.

Why Coverage Rates Vary

The coverage rate on a paint can is a theoretical maximum under ideal conditions. Real-world factors reduce it:

• Surface porosity: Raw drywall, unpainted plaster, and bare wood absorb more paint, cutting coverage by 20–30%.
• Texture: Stucco, brick, and textured wallpaper have far more surface area than smooth drywall.
• Color change: Painting a dark color over light (or vice versa) often requires an extra coat.
• Application method: Spraying wastes more paint than rolling; a thick nap roller holds more paint but may leave a heavier film.
• Temperature and humidity: Very hot, dry conditions cause paint to dry faster, sometimes reducing spread.

For most interior walls, a good rule of thumb is 350 sq ft per gallon for smooth, primed surfaces. For unprimed drywall, use 250–300 sq ft per gallon.

How to Measure a Room Correctly

Accurate measurements are the foundation of a good estimate. Here’s the process:

1. Measure the length and width of the room in feet (or meters). Use a tape measure and record to the nearest 0.1 ft.
2. Measure the height from floor to ceiling. Standard is 8 ft, but older homes may have 9–10 ft ceilings.
3. Count the number of doors and windows. For each door, subtract 21 sq ft; for each window, subtract 15 sq ft. If you have unusually large windows (e.g., a bay window), measure them individually.
4. Decide on the number of coats. One coat for repainting the same color; two coats for a color change or new drywall; three coats for drastic changes or if the primer is skipped.
5. Multiply the net wall area by the number of coats, then divide by the coverage rate.

Do not subtract areas for baseboards, trim, or outlets – they are small and the extra paint is negligible. Also, if you plan to paint the ceiling, measure its area separately (length × width) because ceiling paint is usually a different product.

Common Mistakes and How to Avoid Them

Even with a calculator, people often make these errors:

• Forgetting to subtract doors and windows: A typical room with two doors and one window has about 57 sq ft of openings, which can reduce paint needs by 10–15%.
• Using the wrong coverage rate: Always check the can. Some premium paints cover 400 sq ft/gal, while economy paints may only cover 250 sq ft/gal.
• Rounding down: If the calculator says 2.05 gallons, buying 2 gallons will leave you short. Always round up to the nearest whole gallon.
• Ignoring the primer: If you’re painting bare drywall or a stained surface, you need a separate primer coat. Primer has its own coverage rate (often 300 sq ft/gal) and should be calculated separately.
• Measuring in different units: Mixing feet and inches, or meters and centimeters, leads to errors. Convert everything to the same unit first.

What the Result Really Means

The calculator gives a theoretical minimum. In practice, you should add about 10% extra for touch-ups, spills, and uneven application. For example, if the calculator says 2.1 gallons, you’d buy 3 gallons – that’s about 43% extra, which covers the rounding and waste. For a large project (over 10 gallons), buying an extra gallon is usually sufficient.

Paint is sold in quarts (0.25 gal), gallons, and 5-gallon pails. If your result is between sizes, buy the next size up. Leftover paint can be stored for touch-ups if kept sealed in a cool, dry place.

Comparing Paint Types

Coverage rates differ by finish and quality:

• Flat/matte: Typically covers 350–400 sq ft/gal, hides imperfections well but is less washable.
• Eggshell/satin: Around 300–350 sq ft/gal, good for living areas and hallways.
• Semi-gloss/gloss: About 250–300 sq ft/gal, more durable and moisture-resistant, used in kitchens and bathrooms.
• Primer: Often 300 sq ft/gal, designed to seal and provide a uniform base.

Higher-quality paints often have higher solids content, meaning better coverage and fewer coats needed. Cheaper paints may require an extra coat, negating the initial savings.

Frequently Asked Questions

How do I calculate paint for a room with a cathedral ceiling?

For angled or sloped ceilings, measure the wall height at the tallest point and at the shortest point, then average them for the height in the formula. Or, measure the actual surface area of each wall using a tape measure and add them up. The calculator works with total wall area, so you can bypass the room dimensions and enter the measured area directly.

Should I include the ceiling in the wall area?

No. Ceilings are painted separately with ceiling paint, which is flatter and more spatter-resistant. Measure the ceiling area (length × width) and calculate its paint separately using the same coverage formula.

How much paint do I need for a 12×12 room?

Using the calculator with a 12×12 room, 8 ft walls, one door, one window, and two coats of paint at 350 sq ft/gal, you get: wall area = 2 × 8 × (12+12) − 21 − 15 = 348 sq ft; paint = (348 × 2) / 350 = 1.99 gallons, so you’d need 2 gallons. If you’re painting the ceiling too, add about 1 gallon more.

Why does the paint can say 400 sq ft/gal but my painter says I need more?

The 400 sq ft/gal rating is measured on a smooth, non-porous surface under ideal conditions. In real homes, walls are often slightly textured, unprimed, or the paint is applied too thickly. Contractors typically assume 300–350 sq ft/gal to be safe. Also, if you’re using a brush instead of a roller, you’ll use about 20% more paint.

Can I use this calculator for exterior paint?

Yes, but exterior surfaces are more porous and textured, so reduce the coverage rate to 200–250 sq ft/gal for stucco or wood siding. Also, exterior paint is often applied at a thicker film, so you may need more coats. Measure the total square footage of the exterior walls, subtracting large openings like garage doors (about 180 sq ft each).

What’s the difference between a paint calculator and a paint estimator?

They’re the same thing. A paint estimator typically asks for room dimensions, number of doors/windows, and coats, then gives you the gallons needed. Some advanced estimators also ask for the type of surface and paint quality. This calculator uses the standard formula that most paint stores use.

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in w | Wall width | ft | 12 | 0
in h | Wall height | ft | 8 | 0
in l | Room length | ft | 14 | 0
in d | Number of doors | count | 2 | 0
in wd | Number of windows | count | 1 | 0
in c | Coats | count | 2 | 1
in cr | Coverage per gallon | sq ft/gal | 350 | 50
out area | Total wall area | sq ft | 2 * h * (w + l) - (d * 21 + wd * 15) | 0
out paint | Paint needed | gallons | (area * c) / cr | 1