Gallon of Paint Calculator: How Much Paint You Need

This calculator works out how many gallons of paint you need for a room or surface. You enter the total area to be painted, the number of coats, and the paint’s coverage rate, and it returns the required gallons (plus liters). It also accounts for doors and windows if you subtract their area first.

Formula

The basic formula is:

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

• Total area – the surface area you plan to paint, in square feet (or square meters). For walls, this is the sum of each wall’s height × width, minus doors and windows.
• Number of coats – usually 2 for a color change, 1 for a similar color or a high-quality primer + topcoat system.
• Coverage per gallon – the manufacturer’s stated spread rate, typically 250–400 sq ft per gallon for most interior paints. Check the can; it varies by paint type and surface.

The result is in gallons. To convert to liters, multiply gallons by 3.78541.

Worked Example

Using the default values: area = 400 sq ft, coats = 2, coverage = 350 sq ft/gal.

1. Multiply area by coats: 400 × 2 = 800 sq ft of total painted surface.
2. Divide by coverage: 800 ÷ 350 = 2.2857 gallons.
3. Round to two decimals: 2.29 gallons.
4. In liters: 2.29 × 3.78541 = 8.67 liters.

So you would need to buy 3 gallons (or one gallon + one quart, depending on store sizes) to have enough for the job.

How to Measure the Area Yourself

To use the calculator accurately, you need the actual paintable surface area, not the floor area. Here’s the standard method:

• For each wall, measure the height from baseboard to ceiling and the width from corner to corner. Multiply them to get that wall’s area.
• Add all wall areas together.
• Subtract the area of each door (about 21 sq ft for a standard door) and each window (about 12–15 sq ft for a typical window).
• For ceilings, measure length × width – no need to subtract anything.
• For trim, baseboards, and crown molding, measure the linear footage and multiply by the trim’s width (usually 3–6 inches).

If you’re painting a ceiling, add its area to the wall area before entering it. If you’re painting only walls, just enter the wall total.

Common Mistakes and How to Avoid Them

The biggest mistake is forgetting to subtract doors and windows. A 400 sq ft room with a large picture window might only have 350 sq ft of actual paint surface – that’s a 12% overestimate. Another common error is using the wrong coverage number. Coverage is lower on rough surfaces (drywall, stucco, brick) and higher on smooth, primed surfaces. Always check the can’s label for the specific paint you’re buying.

Another mistake is rounding down. Paint is sold in whole gallons and quarts. If the calculator says 2.29 gallons, you cannot buy 2.29 gallons – you need 3 gallons (or 2 gallons + 1 quart). Always round up to the nearest available container size. It’s better to have a little extra for touch-ups than to run out mid-project.

Also, remember that coverage assumes a properly prepared surface. If you’re painting over a dark color with a light one, or over bare drywall, you may need an extra coat. The calculator lets you adjust the number of coats – use 3 for dramatic color changes.

Paint Types and Coverage Rates

Different paints have different spread rates. Here are typical ranges (always verify on the can):

• Flat interior latex – 350–400 sq ft/gal
• Eggshell or satin – 300–350 sq ft/gal
• Semi-gloss or gloss – 250–300 sq ft/gal
• Primer – 300–400 sq ft/gal
• Exterior paint – 250–350 sq ft/gal (rough siding uses more)

These rates are for a single coat. If you’re using a sprayer instead of a roller, you’ll use about 20–30% more paint because of overspray and thinner coats. For spray application, reduce the coverage number in the calculator to 250–300 sq ft/gal.

Metric Users: Square Meters and Liters

If you measure in meters, the same formula works, but coverage is usually given in square meters per liter. Typical latex paint covers about 10–12 sq m per liter per coat. The formula becomes:

Paint needed (liters) = (Area in sq m × Coats) ÷ Coverage per liter

For example, a 40 sq m room with 2 coats and 11 sq m/L coverage: (40 × 2) ÷ 11 = 7.27 liters. You’d buy an 8-liter can (or two 4-liter cans). The calculator above uses gallons, but you can convert your area to sq ft (1 sq m = 10.764 sq ft) and then convert the result to liters.

Why the Result Is an Estimate

This calculator gives a theoretical minimum. Real-world paint usage varies due to:

• Surface porosity – new drywall or unpainted wood absorbs more paint.
• Application method – brushes and rollers leave more paint than sprayers.
• Temperature and humidity – hot, dry conditions cause paint to dry faster and may require more coats.
• Paint quality – cheaper paints often have lower solids content and cover less.

As a rule, add 10–15% to the calculated amount for waste, spills, and touch-ups. This calculator does not include that buffer, so consider adjusting the area up by 10% or just rounding up to the next gallon.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior latex paints cover 350–400 square feet per gallon per coat on smooth, primed surfaces. Rough surfaces like stucco or unpainted drywall can drop that to 250–300 sq ft. Always check the label on the specific can you buy – it’s the most reliable number.

Do I need to subtract doors and windows from the wall area?

Yes. A standard interior door is about 21 square feet, and a typical window is 12–15 square feet. Subtracting them prevents overbuying. For a 10×12 room with one door and one window, you’d subtract about 36 sq ft from the total wall area, saving roughly 10% of your paint.

How many coats of paint do I need?

Two coats are standard for most interior jobs, especially when changing colors. One coat is enough if you’re painting the same color over a well-primed surface, or when using a paint-and-primer product over a similar shade. For dark-to-light changes or bare drywall, plan on three coats.

Should I buy extra paint?

Yes, buy about 10–15% more than the calculated amount. This covers spills, uneven application, and future touch-ups. Store leftover paint in a cool, dry place with the lid sealed; it can last several years if kept from freezing.

Does primer count as a coat?

Primer is a separate product and should be counted separately. If you’re priming bare drywall or stained surfaces, apply one coat of primer, then two coats of paint. The calculator’s “number of coats” should include only the paint coats, not the primer.

Can I use this calculator for exterior painting?

Yes, but exterior surfaces are rougher and more porous, so reduce the coverage per gallon to 250–300 sq ft. Also, measure the actual wall area, not the footprint. For siding, measure the width and height of each wall and subtract windows and doors.

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in area | Total area to paint | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in cover | Coverage per gallon | sq ft/gal | 350 | 1
out gallons | Paint needed | gal | (area * coats) / cover | 2
out liters | Paint needed | L | ((area * coats) / cover) * 3.78541 | 1