About ThreadSpec.org
ThreadSpec.org is an independent reference for screw thread dimensions and tap drills, covering 214 threads in seven series. It is free to use and is funded by advertising.
What the site covers
| Series | Sizes | Range | Standard |
|---|---|---|---|
| ISO metric coarse | 36 | M1 to M68 | ISO 261, ISO 724 |
| ISO metric fine | 56 | M3×0.35 to M48×3 | ISO 261, ISO 724 |
| Unified coarse (UNC) | 33 | #1-64 to 4-4 | ASME B1.1 |
| Unified fine (UNF) | 24 | #0-80 to 1-1/2-12 | ASME B1.1 |
| Unified extra fine (UNEF) | 25 | #12-32 to 1-11/16-18 | ASME B1.1 |
| NPT | 19 | 1/16 to 12 | ASME B1.20.1 |
| BSP parallel (G) | 21 | G1/16 to G4 | ISO 228-1 |
How the data is produced and checked
All pages are generated from one data set, so a chart and the page for a single thread cannot disagree. Three kinds of value are handled differently.
Computed from the standard formulas
The pitch, minor and root diameters, thread heights, lead angles and stress areas of metric and Unified threads are calculated from the major diameter and pitch with the formulas of ISO 724 and ASME B1.1. The results are then compared automatically with values printed in the standards: the stress areas of ISO 898-1 and ASME B1.1, the basic diameters in ASME B1.1 Table 2, and the 6g and 6H limits of ISO 965-2. A page is not published if a computed value disagrees with the printed one.
Taken from tables in the standards
NPT dimensions are those of ASME B1.20.1 Table 2 and BSP dimensions those of ISO 228-1 Table 1. Class 2A and 2B limits are from ASME B1.1 Table 2, clearance holes from ISO 273 and ASME B18.2.8, and strength values from ISO 898-1 and SAE J429, each shown only for the sizes the standard covers.
Conventions
A tap drill is a recommendation, not a dimension of the thread, and published charts do not always agree. Metric drills follow the diameter-minus-pitch rule of ISO 2306 as rounded in standard tables. Inch drills are those of Machinery’s Handbook and the usual shop charts. NPT drills are the sizes printed on standard shop charts, with the drill suggested in the appendix of ASME B1.20.1 shown beside them where it differs. BSP drills are from tap manufacturers’ charts. Where a size appears in no chart, the page says how the drill was chosen.
Standards referred to
- ISO 68-1, ISO 261, ISO 724: metric thread profile, series and basic dimensions
- ISO 965-1 and ISO 965-2: metric thread tolerances and limits of size
- ISO 273: clearance holes for bolts and screws
- ISO 898-1: mechanical properties of steel fasteners
- ISO 2306: drills for tapping
- ISO 228-1 and ISO 7-1: pipe threads, parallel and taper
- ASME B1.1: Unified inch screw threads
- ASME B1.20.1: pipe threads, general purpose (inch)
- ASME B18.2.8: clearance holes for inch fasteners
- SAE J429: mechanical requirements for inch bolts
Limits of this reference
The pages give basic sizes and the general-purpose tolerance classes. They are a working reference and do not replace the standards themselves, which contain further classes, gauging rules and conditions. For safety-critical or contractual work, confirm the values against the current edition of the governing standard.
Corrections
If you find a value that disagrees with a standard, please write to info@threadspec.org with the thread, the value and the source. Confirmed errors are corrected and the page date is updated.
Contact
Email: info@threadspec.org
Last reviewed 29 September 2026.