Precision unit conversion
UK trade and DIY still runs on a mix of metric and imperial — here's how to convert confidently between the two without a mistake costing you materials.
Britain officially uses metric measurements, but walk onto any building site or into any hardware shop and you'll still hear timber described in feet and inches, sockets sold in imperial sizes, and older technical drawings dimensioned entirely in inches. Working across both systems accurately matters — a small rounding error on a cut list adds up fast.
| Convert | Result |
|---|---|
| 1 inch | 25.4mm (exact) |
| 1 foot | 304.8mm (0.3048m) |
| ½ inch | 12.7mm |
| ¼ inch | 6.35mm |
| 1/8 inch | 3.175mm |
Trade measurements are almost always given in fractions rather than decimals — 43¾ inches, not 43.75. Converting a fraction to a decimal in your head before converting to millimetres is where mistakes creep in, especially with awkward denominators like sixteenths. Typing the fraction directly into a converter that accepts mixed numbers (like "43 3/4") removes that step and the rounding error that comes with it.
Older vehicles, plumbing fittings and some imported hardware still use imperial sizes (AF — "across flats" — sizes in inches), while most modern fixings use metric (mm). A socket set with both scales is genuinely useful, since forcing a close-but-wrong size onto a bolt head is one of the most common ways to round one off.
UK timber is sold in nominal metric sizes (e.g. "2x4" is now labelled roughly 47mm x 100mm) that are the sawn, rough-cut dimensions — the timber is then planed down to a smaller finished (PAR/PSE) size, so the piece you actually get is a few millimetres smaller than the nominal label in every direction. This isn't a metric/imperial conversion issue exactly, but it trips people up in the same category of measurement mistake.
Multiply inches by 25.4. For fractional measurements like 43¾ inches, convert the fraction first (43.75) then multiply, or use a converter that accepts fractions directly to avoid the extra step.
UK timber is usually labelled by its nominal sawn size before planing — the finished (PAR) piece you buy is planed down and ends up a few millimetres smaller in each dimension than the label suggests.