Gallons per Square Foot Calculator (gal/ft², ft²/gal, L/m² and how much to buy)

Understanding the gallons per square foot calculator — concept and principle

This calculator converts between coverage (square feet per gallon) and application rate (gallons per square foot, or litres per square metre), and works out how much paint, stain, sealer or other liquid you need for an area and how many containers to buy.

Coverage to paint needed: with the default inputs the result is 2.67 gallons (≈ 10.1 L · buy 3 cans).

How it is calculated

Coverage and rate are reciprocals: gal/ft² = 1 ÷ (ft²/gal).
Metric: 1 gal/ft² = 3.785411784 L ÷ 0.09290304 m² = 40.746 L/m².
Product needed = rate × area × number of coats.
Containers = product ÷ container size, rounded up.

The principle

A product label usually says how many square feet one gallon covers. That is the same information as the thickness of liquid spread on the surface: covering 300 ft² with a gallon means each square foot gets 1/300 of a gallon. Converting to metric only needs the exact definitions of the US gallon and the foot. Each coat applies the same rate again, so the total scales with the number of coats. Containers are rounded up because you cannot buy part of a can.

Worked example

Default inputs: coverage 300 ft² per US gallon, area 400 ft², 2 coats, 1-gallon containers.

Rate = 1 ÷ 300 = 0.003333 gal/ft² = 40.746 ÷ 300 = 0.136 L/m², or 7.36 m²/L.
Product = 400 × 2 ÷ 300 = 2.67 US gallons.
In litres = 2.667 × 3.785 = 10.1 L.
Containers = 2.67 ÷ 1, rounded up to 3.

Each point is the calculator re-run with only “Application rate or coverage” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Application rate or coverage	Product needed (US gal)
75	10.67
150	5.33
225	3.56
300	2.67
375	2.13
450	1.78
600	1.33

How to read the result

The label coverage is measured on a smooth, sealed surface. Rough, porous or textured surfaces such as bare wood, stucco or new plaster can use much more, and the first coat often uses more than the second. Buying an extra container from the same batch avoids colour differences if you run short, and many stores accept unopened returns.

Common mistakes

Forgetting to multiply by the number of coats.
Including doors and windows in the wall area.
Mixing up coverage per gallon with coverage per litre.
Thinning the product and expecting the same coverage.

Keep in mind: Follow the product's data sheet for coverage, number of coats and safety. Ventilate when using solvent-based products.

Frequently asked questions

How much paint do I need for 400 square feet?

At 300 ft² per gallon and two coats, about 2.7 gallons, so three 1-gallon cans.

How do I convert ft² per gallon to m² per litre?

Multiply by 0.0929 and divide by 3.785; 300 ft²/gal is about 7.36 m²/L.

Why did I use more paint than the label said?

Porous or rough surfaces absorb more, and rollers and brushes waste some.

Sources: NIST Handbook 44, Appendix C, General Tables of Units of Measurement (1 US gallon = 231 in³ = 3.785411784 L; 1 ft = 0.3048 m)

Checked 9 Nov 2026. Exact unit definitions: NIST Handbook 44, Appendix C (General Tables of Units of Measurement) — 1 US gallon = 231 in³ = 3.785411784 L; 1 ft = 0.3048 m.

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