Inch / pipe thread: ID by Ø, pitch and TPI
How to use the calculator
Enter the measured outside diameter and pitch (in mm) or TPI (threads per inch) - the tool will show a list of the closest threads with deviations ΔØ and ΔP. The best match is highlighted. The filter by standard narrows the search: UNC, UNF, UNEF or pipe (NPT, BSPP, BSPT). TPI values and pitch are related: just enter one - the second is recalculated automatically.
How to measure inch threads
Diameter: micrometer along the ridges of the external thread (on a loose bolt or stud). For pipe threads - Ø is measured along the outer surface of the thread, not along the pipe.
Pitch / TPI: comb pattern or counting: attach a ruler, count the number of turns per 1 inch (25.4 mm). For example, 20 turns = 1/4-20 UNC.
The profile angle is more difficult to determine without a template - remember: UNC/UNF - 60°, BSP (BSPP/BSPT) - 55°. If the Ø and pitch match the BSP, but the thread does not work with metric fittings, it is most likely 55°.
What is the difference between UNC, UNF and pipe threads?
UNC (Unified Coarse) - standard fastening thread. 1/4-20 means 1/4" diameter = 6.35 mm and 20 TPI. Used in most American fasteners.
UNF (Unified Fine) - the same system, small step. 1/4-28 - the same diameter, but 28 TPI. For thin-walled connections and high shear loads.
NPT(National Pipe Taper, USA): conical, 60° angle, 1:16 taper. The seal is on the thread itself. Typically on American hydraulic and pneumatic equipment.
BSPP / G(ISO 228): cylindrical, 55° angle. Seal with a gasket. Widely used on European and Asian equipment.
BSPT / R(ISO 7): conical, 55° angle. Sealing - along a cone or with FUM tape.
Frequently Asked Questions
NPT and BSPT - can they be confused? No. Both are conical, but the profile angle is different (60° vs 55°) and the taper is different. When you try to assemble it, the thread will bite or leak. The comb pattern helps determine.
Pitch and TPI - which is easier to measure? Pitch in mm is easier for most - one turn under the micrometer. TPI is the standard for American threads: the comb pattern immediately shows the number.
Why is G 1/4 ~13.16 mm, and not 1/4"? In pipe threads, the nominal size (1/4, 1/2) is the nominal diameter of the pipe, and not the Ø of the thread. G 1/4 has a real Ø 13.16 mm.
Taps, dies and gauges for inch threads - RUKO, VÖLKEL, TRUTHREAD - in our catalog.