Why is everything still in inches, and how do I convert without getting it wrong?
Where imperial survives in British building — timber, pipe, fixings, plumbing threads and old houses — which numbers are real conversions and which are just names, and the rounding discipline that stops a conversion introducing an error.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
Work in millimetres for everything you cut or order, and convert once at the start rather than repeatedly as you go. Remember that many imperial-sounding sizes are names, not measurements: a 4x2 is not four inches by two, and half-inch pipe is not half an inch across anything.
Britain builds in millimetres and talks in inches, and the gap between the two catches people out constantly. Some imperial numbers are genuine dimensions that need converting. Others are trade names that survived the conversion and no longer describe any real measurement at all. Knowing which is which prevents most of the mistakes.
Step by step
- Learn the one conversion that matters.One inch is exactly 25.4mm. One foot is 304.8mm. Those two numbers cover almost everything, and because 25.4 is exact there is never a reason to use a rounded 25.
- Work in millimetres, not centimetres.Every drawing, merchant and manufacturer in UK construction uses millimetres. Millimetres avoid decimal points entirely, and a decimal point in the wrong place is a tenfold error. Centimetres belong in dressmaking, not building.
- Convert once, at the start.Turn the whole job into one unit system on paper before you begin, and never mix. Converting on the fly, mark by mark, is where rounding errors creep in and where a number gets converted twice.
- Recognise the sizes that are names rather than measurements.A 4x2 stud is sold as 47x100mm or thereabouts after planing — it has never been four inches by two in your lifetime. A 2x1 batten is similar. The imperial figure is the nominal sawn size before drying and planing.
- Understand that pipe sizes are naming conventions too.Half-inch and three-quarter-inch plumbing threads describe a historic bore, not any dimension you can measure on the fitting. A half-inch BSP thread measures about 21mm across the outside. Copper pipe, by contrast, is named by its genuine outside diameter in millimetres — 15mm and 22mm are real measurements.
- Expect old houses to be genuinely imperial.A pre-metric house really was built to feet and inches: door openings, joist spacings at 16 inch centres, brick coursing, stair rises. Those are real dimensions and converting them gives untidy millimetre figures like 406mm, which are correct even though they look wrong.
- Round only at the end, and round the right way.Carry full precision through any calculation and round once, at the point of marking. Round in the direction that leaves material on: oversize for something you will trim, undersize for a hole.
- Write the unit next to every number.On every sketch, every note and every order. A bare number is the ancestor of most ordering disasters, and 8 could plausibly be feet, inches, millimetres or metres.
Tips
- Fixings are a genuine mix: metric bolts are M6, M8, M10 and so on, but many plumbing, gas and older machine threads are still BSP or BSW and are not interchangeable with metric despite fitting for a turn or two.
- Sheet materials are metric — 2440 x 1220mm is the metric expression of the old 8ft by 4ft sheet, and it is what merchants actually stock.
- Spanner and socket sizes are metric on almost anything modern and imperial on older British machinery, cycles and cars. A metric socket that nearly fits an imperial nut will round it off.
- When someone quotes a wall as 'nine inch solid', that is a real description of a real brick wall one brick thick — about 215mm. It is not a name.
Common mistakes
- Ordering timber as if 4x2 measured four inches by two inches — Planed timber loses several millimetres on every face. The nominal name and the finished size are different numbers, and only the finished size fits the space.
- Using 25mm as an inch — It is 1.6% short. Over a metre that is 16mm, and over a full sheet it is close to 40mm. Use 25.4.
- Mixing metres and millimetres on one sketch — It produces errors of a factor of a thousand, which are so large they usually get caught — and errors of a factor of ten, which do not.
- Assuming a half-inch fitting will screw into a 15mm one — They are different systems. Some combinations start and then bind or leak, which is worse than a straightforward refusal to fit.
Questions people ask
Is 15mm copper pipe the same as half-inch?
It is the modern replacement for half-inch imperial pipe in most domestic situations, but the two are not identical. Imperial half-inch copper is closer to 13.6mm outside diameter, and 22mm replaced three-quarter-inch. In an old house you may need an adaptor coupling rather than a straight fitting.
Why are joists sometimes at 400mm centres and sometimes at 406?
400mm is the modern metric standard. 406mm is 16 inches, which is what the house was built to if it predates metrication. Both exist, and measuring rather than assuming is the only way to know which you have.
Should I set my tape measure to metric or imperial?
Metric, and preferably use a tape marked only in millimetres and metres. A dual-scale tape is a standing invitation to read the wrong edge, which produces an error of a factor of about 2.5.