Thread conversion table for Plumbing and Heating
Technical sheet · ESPINOSA
Thread conversions
Inches · millimetres · BSP threads — plumbing conversion guide
Conversion
Inches → millimetres
16 values · fractions
1/16"
1.5875 mm
1/8"
3.1750 mm
3/16"
4.7625 mm
1/4"
6.3500 mm
5/16"
7.9375 mm
3/8"
9.5250 mm
7/16"
11.1125 mm
1/2"
12.7000 mm
9/16"
14.2875 mm
5/8"
15.8750 mm
11/16"
17.4625 mm
3/4"
19.0500 mm
13/16"
20.6375 mm
7/8"
22.2250 mm
15/16"
23.8125 mm
1"
25.4000 mm
Pipes
Nominal diameters
Outer / inner Ø · mm
| Inch | Outer Ø (mm) | Inner Ø (mm) |
|---|---|---|
| 1/8" | 10 | 5 |
| 1/4" | 13 | 8 |
| 3/8" | 17 | 12 |
| 1/2" | 21 | 15 |
| 3/4" | 27 | 20 |
| 1" | 34 | 26 |
| 1" 1/4 | 42 | 33 |
| 1" 1/2 | 49 | 40 |
| 2" | 60 | 50 |
| 2" 1/4 | 70 | 60 |
| 2" 1/2 | 76 | 66 |
| 2" 3/4 | 82 | 72 |
| 3" | 90 | 80 |
| 3" 1/2 | 102 | 90 |
| 4" | 114 | 102 |
| 4" 1/2 | 127 | 114 |
| 5" | 140 | 127 |
| 5" 1/2 | 152 | 140 |
| 6" | 165 | 152 |
BSP standard
Gas thread designation
British Standard Pipe
| Inch | Designation | TPI | Outer Ø (mm) |
|---|---|---|---|
| 1/8" | 5 x 10 | 28 | 9.72 |
| 1/4" | 8 x 13 | 19 | 13.15 |
| 3/8" | 12 x 17 | 19 | 16.66 |
| 1/2" | 15 x 21 | 14 | 20.95 |
| 5/8" | 17 x 23 | 14 | 22.91 |
| 3/4" | 20 x 27 | 14 | 26.44 |
| 7/8" | 24 x 31 | 14 | 30.20 |
| 1" | 26 x 34 | 11 | 33.24 |
| 1" 1/4 | 33 x 42 | 11 | 41.91 |
| 1" 1/2 | 40 x 49 | 11 | 47.80 |
| 1" 3/4 | 46 x 55 | 11 | 53.74 |
| 2" | 50 x 60 | 11 | 59.61 |
| 2" 1/4 | 60 x 70 · ant. | 11 | 69.30 |
| 2" 1/2 | 66 x 76 | 11 | 75.18 |
| 2" 3/4 | 72 x 82 | 11 | 81.53 |
| 3" | 80 x 90 | 11 | 87.88 |
| 3" 1/2 | 90 x 102 | 11 | 100.33 |
| 4" | 102 x 114 | 11 | 113.03 |
| 5" | 127 x 140 | 11 | 141.30 |
| 6" | 152 x 165 | 11 | 168.30 |
Understand
Understanding & identifying a thread
Three clues are enough to identify a thread: its diameter, its pitch (threads per inch) and its form — parallel or tapered.
The outer Ø of the male thread is the reference value — the one in the «Outer Ø» column of the gas table. The core is the inner Ø, the pitch the distance between two threads, and the 55° angle (Whitworth profile) is specific to BSP.
Parallel — G
The Ø stays constant. Sealing is done with a gasket (flat fibre, O-ring) against a seat. Common on fittings, valves and nuts.
Tapered — R (male) / Rc (female)
The Ø decreases toward the tip: the threads wedge together and seal with PTFE tape or paste. Common on steel and gas.
Measuring a male thread
With a caliper, measure the outer Ø of the threads (crest to crest) and read it in «Outer Ø».
Measuring a female thread
Measure the inner Ø at the root of the threads; if unsure between two sizes, confirm with the pitch.
Warning — BSP ≠ NPT
BSP (British, 55° thread) and NPT (American, 60°) look alike but are not compatible: screwing them together causes leaks and damage. Check the origin of the fitting (EU/UK → BSP; USA → NPT).
BSP threads (British Standard Pipe) are the plumbing standard. Indicative values, may vary slightly by manufacturer.
Tip The inch diameter refers to the nominal inner Ø of the pipe; the metric designation matches the outer Ø of the thread.
Technical reference ESPINOSA · BSP standards (ISO 228 / ISO 7) — indicative values.