Metric and imperial for building materials, without conversion errors

Why this goes wrong more often than it should

Converting a length is easy and everyone gets it right. Converting a rate — square metres per litre, square feet per gallon — is where it breaks, because two units change at once and in opposite directions. Invert the operation and you do not get an absurd number that alerts you; you get a plausible one that is wrong by a factor of about two and a half.

The second trap is that materials are increasingly sold in mixed units even within one country: paint in litres, room in feet, tiles in centimetres, timber in nominal inches. Nobody converts consistently under those conditions, which is why the safest method is not to convert at all — see the last section.

Length

One point that catches people: 8 feet 6 inches is 8.5 feet, not 8.6. Convert the inches to a decimal fraction of a foot before doing anything else — inches ÷ 12 — or every subsequent step inherits the error.

8 ft 6 in = 8 + 6⁄12 = 8.5 ft = 2.591 m

Area and volume

Area factors are the length factor squared, which is why they look unfamiliar and why guessing them fails.

A common error worth naming: converting the dimensions and the area both. If you convert 12 ft × 10 ft into 3.658 m × 3.048 m and then multiply, you get 11.15 m² and you are correct. If you multiply first to get 120 sq ft and then apply 0.3048 instead of 0.092903, you get 36.6 — three times too much. Convert dimensions, then multiply. Never the other way.

The gallon trap

There are two gallons still in daily use and they differ by about 20%. This is the single most expensive conversion error in decorating.

A "gallon of paint" from an American manufacturer is 3.785 L. British paint is sold in litres and has been for decades, so an imperial gallon of paint is essentially a historical unit — but it survives in old coverage tables and in conversation, and using 4.546 where 3.785 was meant will have you buying a fifth more paint than you need.

Rule: if the source is American, the gallon is 3.785 L. If you are unsure, work in litres and refuse the gallon entirely.

Converting coverage rates

The one that actually matters. Coverage is area per volume, so both units convert and the second one inverts.

m²/L → sq ft/US gal: × 10.7639 × 3.78541 = × 40.746

sq ft/US gal → m²/L: ÷ 40.746

So 12 m²/L is about 489 sq ft per US gallon, and a US tin claiming 400 sq ft per gallon is delivering about 9.8 m²/L. Both of those are believable figures for interior emulsion, which is precisely the danger: a mistaken conversion here lands inside the plausible range and nothing warns you.

The sanity check takes two seconds. Interior wall paint is always somewhere between 6 and 14 m²/L, equivalently 240 to 570 sq ft per US gallon. Anything outside that band is a conversion error, not a remarkable product.

Nominal sizes are not measurements

• A "2 × 4" is about 38 × 89 mm, not 50 × 100. Nominal timber sizes describe the rough sawn dimension before planing.
• A "12 inch" tile is often 300 mm, which is 11.8 inches. Over ten tiles that is a 5 mm discrepancy — enough to move a cut.
• Plasterboard sold as 4 × 8 ft may be 1200 × 2400 mm, which is slightly narrower and slightly longer.
• Pipe sizes are nominal almost everywhere and correspond to no measurable dimension of the pipe.

For anything where pieces must fit together, measure the actual product rather than converting its name.

The method that avoids all of this

• Pick the unit system your product is sold in, and do the whole calculation in it. Mixed input is the root cause.
• If you must convert, convert once, at the start, on the raw dimensions — never at the end and never on a rate you derived.
• Keep the exact factors: 25.4, 0.3048, 3.78541. Rounded factors compound.
• Sanity check the final answer against the band above before you buy.
• Do not round intermediate values. Round only the final quantity, and only upwards, to whole packs.

This is exactly how the calculators on this site work internally: imperial input is converted to metres and litres once, the calculation runs in metric, and the answer is converted back for display. Switching the unit toggle therefore never changes the physical result — only how it is written. If a tool you are using does not behave that way, it is doing something wrong.

Conversion factors given here are the exact international definitions where one exists (inch, foot, yard, US gallon) and rounded to six significant figures otherwise.

Switch units safely in the calculator

Switch units safely in the calculator ↗