BSP (G) and NPT pipe threads: dimension chart and identification
Published October 7, 2026 — dimensions of G threads from 1/8 to 2 and NPT from 1/8 to 1, tap drills, the G / R / NPT differences and a tool that identifies a thread from the measured diameter.
In short: a G pipe thread (ISO 228) is parallel, 55°, and does not seal: a seal does. R / Rp / Rc threads (ISO 7) have the same profile but seal on the thread. American NPT is tapered, 60°: it is not compatible with G, even at 14 threads per inch like the 1/2. The size (1/2, 3/4…) is not the diameter: a G 1/2 measures 20.955 mm. To identify a thread, measure its diameter and count its threads.
1. G pipe thread (BSPP, ISO 228)
Male thread major diameter, pitch and tap drill for the female thread, in millimetres. The same basic dimensions apply to ISO 7 R threads, at the gauge plane.
2. NPT thread (American taper)
Pipe outside diameter (the thread is tapered: this is the size reference), threads per inch and usual drill per ASME B1.20.1.
ISO metric threads (M6, M8…) follow other rules: see the metric thread and tap drill chart.
3. G, R and NPT: the differences that matter
The classic trap: a 1/2 NPT fitting screws a few turns into a G 1/2 tapped hole (same thread count, close diameters), then leaks or galls. The different angle (60° versus 55°) and the taper prevent proper contact: never mix the two systems.
On a drawing, the full designation avoids ambiguity: G 1/2 (ISO 228-1), Rc 1/2 or R 1/2 (ISO 7-1), 1/2-14 NPT. In SOLIDWORKS, check in the Hole Wizard that the pipe tap type you pick matches the intended standard before dimensioning.
4. Identify a measured pipe thread
Measure the male thread major diameter and the threads per inch (or the pitch in mm): the tool finds the closest G and NPT sizes and shows which one matches best.
Pitch in mm to threads per inch: 25.4 ÷ pitch (a 1.814 mm pitch gives 14 threads per inch). If the diameter changes from one end of the thread to the other, it is tapered: R or NPT; the angle measured with a thread gauge decides.
5. FAQ
What is the difference between G and NPT?
G thread (ISO 228) is parallel, with a 55° thread angle; sealing is done by a flat or O-ring seal, not by the thread. NPT (American standard ASME B1.20.1) is tapered (1:16) with a 60° angle, and seals on the thread with paste or tape. They are not compatible, even when the threads per inch match.
Why does a G 1/2 thread measure 21 mm and not 12.7 mm?
A pipe thread size is not a thread diameter: it comes historically from the nominal bore of the pipe. A G 1/2 has a 20.955 mm major diameter; that is why a pipe thread is identified by measuring the diameter and the threads per inch, never from its name.
What do R, Rp and Rc mean?
They are the ISO 7-1 pipe threads that seal on the thread, with the same 55° profile as G: R is a taper external thread, Rc a taper internal thread, Rp a parallel internal thread designed to receive an R. ISO 228 G does not seal on the thread.
Which drill for tapping a G 1/4?
11.8 mm. For G 1/8: 8.8 mm; G 3/8: 15.25 mm; G 1/2: 19 mm. For a 1/4 NPT, the usual drill is 7/16 in (11.11 mm), with a taper reamer if the full thread must be formed.
How do you identify a pipe thread?
Measure the male thread major diameter with calipers and count the threads over one inch (25.4 mm), or use a thread gauge. Diameter and thread count give the size; a tapered thread (diameter changes along the part) means R or NPT, a parallel thread G. The angle (55° or 60°) separates G/R from NPT.