Dimension tables for ISO metric, NPT, Unified and BSP threads — pitch diameters, minor diameters and tap drills, with the gauging practice behind each. Maintained by DSN Enterprises, Coimbatore.
A thread is fully defined by three numbers: the nominal diameter, the pitch, and the basic profile it is cut on. Everything else — pitch diameter, minor diameter, tap drill, gauge limits — follows from those. These tables give the basic dimensions for each system so you can read a callout off a drawing and know what it means on the shop floor.
Basic dimensions are geometry, and every table here is derived the same way from the profile constants of the relevant standard. Where a value is a published preferred number rather than a computed one, the note under the table says so — a reference that quietly rounds is worse than no reference at all.
Coarse and fine pitch series with pitch, pitch diameter and minor diameters from M1 to M64.
ISO 68-1 / ISO 261 / ISO 965Taper pipe thread dimensions with gauging diameters at hand-tight and wrench-tight engagement.
ASME B1.20.1Unified National Coarse dimensions in inch and millimetre, covering #0 through 1-1/2".
ASME B1.1M12, 1/2" UNC and 1/2" NPT are all roughly 12-13 mm across the crest, and all three are 60° threads, so they are frequently assumed to be the same thing. They are not: the pitches differ, and a gauge for one will not pass a component cut for another.
| System | Designation | Major Ømm | Pitchmm | Pitch Ømm |
|---|---|---|---|---|
| Metric (M12) | M12 x 1.75 | 12.000 | 1.75 | 10.863 |
| Unified (1/2" UNC) | 1/2-13 UNC | 12.700 | 1.954 | 11.430 |
| Pipe (1/2" NPT) | 1/2 NPT | 21.336 | 1.814 | 19.772 |
The coarse pitch for each system, which is the value assumed when a drawing gives a designation with no pitch or thread series.
Per-size pages with the coarse and fine series, tap drill and gauging detail — the sizes specified most often on Indian drawings.
DSN Enterprises manufactures thread plug gauges, ring gauges and setting plugs at the Coimbatore works, cut and lapped as matched pairs and supplied with traceable calibration certificates. Send a drawing or a thread designation and we will quote to it.