Thread Specifications & Tap Drill Reference
Complete reference for 207 thread specifications — ISO metric, UNC, UNF, UNEF, NPT, and BSP. All dimensions calculated from ISO 724 and ASME B1.1 formulas.
ISO Metric Threads
36 coarse + 56 fine pitch sizes. M1 to M68 with complete dimensions and tap drills.
Unified Inch Threads
33 UNC + 24 UNF + 22 UNEF sizes. #0 to 4" with inch and metric dimensions.
Pipe Threads
19 NPT + 17 BSP sizes. Tapered and parallel pipe thread specifications.
Tap Drill Chart
Complete tap drill sizes for all thread standards in mm and inches.
Thread Calculator
Calculate thread dimensions, tap drills, and stress areas for any pitch and diameter.
NPT vs BSP
Side-by-side comparison of NPT and BSP pipe thread standards.
Popular Metric Thread Sizes
| Thread | Pitch (mm) | Major Ø (mm) | Pitch Ø (mm) | Tap Drill (mm) | Stress Area (mm²) |
|---|---|---|---|---|---|
| M3 | 0.5 | 3 | 2.675 | 2.46 | 5.0 |
| M4 | 0.7 | 4 | 3.545 | 3.24 | 8.8 |
| M5 | 0.8 | 5 | 4.48 | 4.13 | 14.2 |
| M6 | 1.0 | 6 | 5.351 | 4.92 | 20.1 |
| M8 | 1.25 | 8 | 7.188 | 6.65 | 36.6 |
| M10 | 1.5 | 10 | 9.026 | 8.38 | 58.0 |
| M12 | 1.75 | 12 | 10.863 | 10.11 | 84.3 |
| M16 | 2.0 | 16 | 14.701 | 13.84 | 156.7 |
| M20 | 2.5 | 20 | 18.376 | 17.29 | 244.8 |
| M24 | 3.0 | 24 | 22.052 | 20.75 | 352.5 |
Popular UNC Thread Sizes
| Thread | Major Ø (in) | Pitch Ø (in) | Minor Ø (in) | Tap Drill (mm) |
|---|---|---|---|---|
| #6-32 UNC | 0.138 | 0.1177 | 0.0997 | 2.6 |
| #8-32 UNC | 0.164 | 0.1437 | 0.1257 | 3.3 |
| #10-24 UNC | 0.19 | 0.1629 | 0.1389 | 3.7 |
| 1/4-20 UNC | 0.25 | 0.2175 | 0.1887 | 5 |
| 5/16-18 UNC | 0.3125 | 0.2764 | 0.2443 | 6.4 |
| 3/8-16 UNC | 0.375 | 0.3344 | 0.2983 | 7.8 |
| 1/2-13 UNC | 0.5 | 0.45 | 0.4056 | 10.6 |
| 5/8-11 UNC | 0.625 | 0.566 | 0.5135 | 13.4 |
| 3/4-10 UNC | 0.75 | 0.6851 | 0.6273 | 16.3 |
| 1-8 UNC | 1 | 0.9188 | 0.8466 | 22 |
About ThreadSpec.org
ThreadSpec.org is a comprehensive thread specification reference built for engineers, machinists, and makers. Every dimension on this site is calculated directly from published engineering formulas — ISO 724 for metric threads and ASME B1.1 for unified inch threads — not copied from other sources.
Our database covers 207 thread specifications across six major standards: ISO Metric (coarse and fine), UNC, UNF, UNEF, NPT, and BSP. Each thread page includes major, pitch, and minor diameters, tap drill sizes, tensile stress areas, and proof loads for common material grades.
Frequently Asked Questions
What is a thread pitch diameter?
The pitch diameter is the theoretical diameter where the thread tooth and space are equal in width. For ISO metric threads, it is calculated as d2 = d - 0.6495 × P, where d is the nominal diameter and P is the pitch.
How do I find the right tap drill size?
The tap drill size equals the internal minor diameter (D1) of the thread. For metric threads: D1 = d - 1.0825 × P. This gives approximately 75% thread engagement, which is the industry standard recommendation.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) has fewer threads per inch and is the default for general-purpose fastening. UNF (Unified National Fine) has more threads per inch, providing higher strength, better vibration resistance, and finer adjustment, but is more susceptible to cross-threading.
What is the difference between NPT and BSP pipe threads?
NPT uses a 60-degree thread angle and is standard in North America. BSP uses a 55-degree Whitworth thread angle and is standard in Europe, Asia, and Australia. They are not interchangeable despite similar nominal sizes.