Methodology

Last reviewed: November 2, 2026.

Formulas

Rectangular wall area = 2 × (length + width) × height
Flat ceiling area = length × width
Net area = area − deductions
Paint = net area × coats ÷ coverage per coat
With an allowance: paint × (1 + allowance ÷ 100). It is only added if you select it.

Units

Everything is calculated in meters, square meters, and liters, and converted for display. The default display is feet, square feet, and US gallons.

1 foot = 0.3048 meters (NIST Handbook 44)
1 US gallon = 3.785411784 liters (NIST Handbook 44)
1 imperial gallon = 4.54609 liters (Weights and Measures Act 1985)

Coverage and defaults

Coverage varies by product and surface, so the calculators don't assume a figure. You enter the one for your product. Example values and the starting value of 2 coats are there for illustration, and are not manufacturer or professional recommendations.

Container plans

From the sizes you enter, "fewest containers" finds the smallest number of containers that meets your need, then the smallest volume. "Least paint" finds the smallest total volume, then the fewest containers. Neither is chosen by price, so neither is necessarily the cheapest. If you enter a price for each size in a plan, the cost is the count × price for each size, added up, before tax.

What isn't accounted for

Texture, primer, absorption, color change, application method, sloped or irregular surfaces, and trim. Actual use can differ from any estimate.

Testing

The calculations are tested automatically against worked examples calculated separately from the code, including conversions between units and invalid entries.

How content is made

Pages and code on this site are drafted with AI assistance. Facts about safety and units are checked against official and manufacturer sources, which are linked where they're used. Worked examples in guides are checked by the same automated tests as the calculators. Guides show the date they were last reviewed. Illustrations are drawn for this site and are not to scale.