Thread reference
Designation examples and notes for each thread family.
General purpose
Unified inch
Specify an inch thread by diameter, threads per inch, profile, and the separate classes for the screw and nut.
ISO metric
M and MJ threads use millimetre dimensions, with tolerance grades and positions that can differ between pitch and crest diameters.
Unified miniature
UNM covers miniature threads through a fixed size catalog. Its diameter and pitch are expressed in millimetres despite the Unified name.
Whitworth
Whitworth combines a 55° profile with rounded crests and roots. Diameter is entered in inches and pitch as threads per inch.
British Association
BA sizes are numbered, with a 47.5° rounded profile. The size number identifies a catalog entry rather than a diameter.
Power
Acme
Acme uses a 29° power-thread profile. General, stub, and centralizing forms have different depths and fit selections.
Inch buttress
The inch buttress profile has unequal flanks: a 7° load flank and a 45° clearance flank.
ISO trapezoidal
Tr threads are metric power threads with a 30° included angle. Pitch grades and tolerance positions control the mating screw and nut.
Metric buttress
Metric buttress uses a 3° load flank and a 30° clearance flank, with metric pitch and separate external and internal tolerance settings.
Pipe & conduit
NPT tapered pipe
NPT is a 60° tapered pipe thread. A diameter only has meaning together with its location along the thread.
NPTF dryseal pipe
NPTF dryseal threads include specific crest and root limits as well as the tapered pipe diameters and engagement lengths.
NPSM straight pipe
NPSM provides straight mechanical pipe-thread limits. Its nominal pipe size remains distinct from the actual thread diameter.
NPSL locknut pipe
NPSL is a straight locknut pipe-thread family with pitch-diameter ranges and some diameter limits specified on only one side.
BSPT tapered pipe
BSPT uses a 55° tapered pipe profile. The catalog shows nominal pipe sizes and TPI while dimensional results can be read in millimetres.
BSPP parallel pipe
BSPP uses a 55° parallel pipe profile. Choose the full catalog size and check the stated reference against the drawing callout.
Pg conduit
Pg conduit threads use an 80° included angle and numbered catalog sizes. A Pg number is not a millimetre diameter.
Inserts
Inch screw-thread insert
Inch STI dimensions describe the internal thread tapped to receive an insert, before the insert is installed.
Metric screw-thread insert
Metric STI covers the internal tapping dimensions for an insert housing, selected by the nominal metric screw size.
For units, diameter labels and saving results, see calculator help.