Fraction to decimal inches

A lot of pipe, fitting and fastener sizes in the UK are still marked up in imperial fractions — a ½" BSP tail, a ¾" compression fitting, a 5/16" bolt — even though everything else on the job is metric. This tool takes the fraction as it's stamped or written and turns it into a decimal inch figure and an mm equivalent, so you can compare it straight against a metric spec sheet or a digital calliper reading without doing the division in your head.

Why fittings still come in fractions

Plumbing threads (BSP, NPT), a good chunk of imported fixings, and older mechanical drawings all still use imperial fractions as their native unit, and they're not going away — BSP thread sizes in particular are named after a nominal fraction that doesn't even match the actual thread diameter. When you're stood at a merchant's counter trying to match a fitting off an old system, or checking a fastener size against a metric drill bit chart, the fraction on the packet needs converting before it means anything you can measure against. Denominators of 4, 8, 16 and 32 cover the vast majority of what turns up on site — finer than 32nds and you're generally into machining tolerances rather than plumbing and fixing sizes.

The mm figure is what you'll want most often day to day, since tape measures, callipers and most modern spec sheets are metric — but the decimal inch figure matters too, since it's the form some US-made tools and some older UK catalogues quote sizes in.

A worked example

A fitting marked ¾" (3 over a denominator of 4) converts to 0.750in, or 19.05mm. Move to a finer-threaded fixing marked 5/16" and you get 0.3125in, or 7.94mm — close enough to an 8mm bolt that it's easy to grab the wrong one off a mixed tray without checking. That's the kind of mix-up this tool is meant to catch before you're back in the van for the third time.

What this isn't

This converts a stated fraction to decimal inches and mm — it doesn't verify that the fraction printed on a fitting is accurate, and it's not a substitute for measuring the actual part with a calliper if the fit is tight or the stakes are high. Threaded sizes especially (BSP, NPT) are nominal names rather than literal diameters, so use this to read and compare sizes, not to set cutting or machining tolerances.