BSPT Thread Dimensions Calculator was developed to show British Standard Pipe Taper Thread form where pressure-tight joints are made on the threads. The calculator shows dimensions, tolerances and designation for jointing pipe threads for thread sizes from 1/16 to 6 according to ISO 7-1 standard. The thread form follows the British Standard Whitworth thread.

Some important notes from the ISO 7-1 standard:

- Threads are taper external, parallel internal or taper internal and are intended for use with pipes suitable for threading and for valves, fittings or other pipeline equipment interconnected by threaded joints.

-An appropriate jointing medium should be used on the thread to ensure pressure-tight joints.

-ISO 7-2 gives details of methods of verification of jointing thread dimensions and form and recommended gauging systems.

- The symbols for pipe thread type: Rp - Parallel internal threads, Rc - taper (conical) internal threads, R - external threads

- Designation: Internal thread - Parallel: Pipe thread ISO 7-Rp 1/2, Internal Thread - Taper:  Pipe thread ISO 7- Rc 1/2, External thread - Always taper: Pipe thread ISO 7 - R 1/2

- The ISO 7-1 thread shall be a right-hand thread unless otherwise specified.

- The basic form of the parallel and taper pipe thread shall be as shown in following figures. The taper is 1 to 16, measured on the diameter.

British Standard Pipe Taper Thread (BSPT)
British Standard Pipe Taper Thread (BSPT) per ISO 7-1


Parameter Value
Pipe Thread Size


Parameter Symbol Value Unit
Pipe Thread Size - --- ---
Threads per inch (25.4 mm) n ---
Pitch of Thread P --- mm
Height of Thread h ---
Diameters at gauge plane Major (gauge diameter) d ---
Pitch d2 ---
Minor d1 ---
Gauge length (external thread) Nominal --- ---
Tolerance in mm (+/- T1/2) ---
Tolerance in turns
of thread
--- ---
Max --- --- mm
Min --- ---
Tolerance on position of gauge plane
on internal thread
Tolerance in mm (+/- T2/2) ---
Tolerance in turns
of thread
--- ---
Length of useful external
 thread not less than
For nominal gauge length --- --- mm
For maximum gauge length ---
For minimum gauge length ---
Fitting allowance in mm --- ---
turns of thread --- ---
Diameter tolerance on
parallel internal threads
--- --- --- mm

British Standard Pipe Taper External Thread (BSPT)
British Standard Pipe Taper External Thread (BSPT) Terms per ISO 7-1

BSPP Thread: British Standard Pipe Parallel Thread

BSPT Thread: British Standard Pipe Taper Thread

Complete thread: The fully formed thread at both crest and root.

Gauge diameter: Major diameter of the external or internal thread at gauge plane.

Gauge plane: Plane, perpendicular to the axis of the taper thread, at which the major cone has the gauge diameter.

Gauge length: The distance from the gauge plane to the smaller end of the thread on an external thread.

Major cone: The imaginary cone which just touches all crests of a taper external thread or all roots of a taper internal thread.

Useful thread: All thread except washout thread.

Washout thread; Vanish thread: The thread which is not fully formed at the root.

Fitting Allowance: The external thread beyond the gauge plane which is required to provide an assembly with an internal thread at the upper limit of tolerance. It includes the wrenching allowance.

Wrenching Allowance: Useful thread length which is provided to accommodate for wrenching beyond the position of handtight engagement.


  • ISO 7-1 :2000 Pipe Threads Where Pressure-Tight Joints are Made on the Threads - Dimensions, Tolerances and Designation