Inches to millimetres
Fractions work: type 3/4 or 1 1/2. Editing either field updates the other.
Nominal size is a name, not a measurement. These are the diameters behind it.
Why 1/2 inch is not 12.7 mm
An inch is 25.4 mm, so half of it is 12.7 mm. That arithmetic is correct and almost never what you need at the counter.
The inch in a pipe size is a name inherited from old steel pipe, where it described the rough bore — the hole — not the outside of anything. The wall sat around that hole, and the thread was cut into the wall. So a “1/2 inch” fitting carries a thread measuring 21 mm across, has a bore near 15 mm, and nothing about it is 12.7 mm.
Rule of thumb: measure the thread across the crests, then find that number in the chart below — don't try to convert the label.
Inch, thread and pipe sizes side by side
One row per trade size. The thread column is what a caliper reads across an external thread; the pipe columns are the plastic and copper sizes that normally meet that thread.
| Size | Thread Ø, mm | Nominal bore | PP-R | Multilayer | Copper |
|---|---|---|---|---|---|
| 1/4" | 13,2 | DN8 | — | — | — |
| 3/8" | 16,7 | DN10 | — | — | — |
| 1/2" | 21,0 | DN15 | Ø20 | Ø16 | Ø15 |
| 3/4" | 26,4 | DN20 | Ø25 | Ø20 | Ø22 |
| 1" | 33,2 | DN25 | Ø32 | Ø26 | Ø28 |
| 1 1/4" | 41,9 | DN32 | Ø40 | Ø32 | Ø35 |
| 1 1/2" | 47,8 | DN40 | Ø50 | — | Ø42 |
| 2" | 59,6 | DN50 | Ø63 | — | Ø54 |
A dash means the system isn't made in that size, not that the number is unknown. Plastic and copper pipes are named by outside diameter, steel by its bore — that difference is behind most size confusion.
Pure conversion, for reference
When the number really is a length — a drill bit, a wall thickness, a gap — this is the plain arithmetic.
| Size | Exactly, mm |
|---|---|
| 1/4" | 6,35 |
| 3/8" | 9,53 |
| 1/2" | 12,7 |
| 3/4" | 19,05 |
| 1" | 25,4 |
| 1 1/4" | 31,75 |
| 1 1/2" | 38,1 |
| 2" | 50,8 |
How to measure what you already have
Caliper on the threaded part itself, across the crests, close to the end of the fitting; jaws on the hex or a shoulder read high. Round to the nearest row in the chart above. Measuring without a caliper, counting crests and not misreading a worn thread — that's on the threads page.
Common questions
How do I convert inches to mm?
Multiply by 25.4 — an inch is defined as exactly 25.4 mm. To go the other way, divide the millimetres by 25.4. The calculator above does this for fractions too, so 3/4″ or 1 1/2″ work without converting to decimal first.
How many mm is 1/2 inch?
As pure arithmetic, 12.7 mm. As a pipe size, a 1/2″ thread measures about 21 mm across the outside and the bore is near 15 mm. If you are buying a fitting, 21 mm is the number you'll see on a caliper. The rest of the range is in the thread size table.
25 mm — how many inches is that?
Just under 1 inch (25.4 mm) as a length. But a pipe marked 25 is usually PP-R with a 25 mm outside diameter, and that one mates with a 3/4″ thread, not a 1″ one.
Is PP-R 25 a 3/4 inch or a 1 inch?
Three-quarters. PP-R 25 is a 25 mm outside diameter pipe, and the fittings that carry it come with 3/4″ threads. PP-R 32 is the one that goes with 1″.
What is the difference between DN and outside diameter?
DN is the nominal bore — the old inch name written in metric, so DN15 is 1/2″. Outside diameter is a real measurement of the pipe. They are close only for steel; for plastic they are different numbers entirely. DN15 is 1/2″, DN20 is 3/4″, DN25 is 1″ — see the table above for the rest.