How to identify a thread
- Measure the outside diameter of the bolt (or the bore of the nut and add about one pitch). Used threads measure a little under nominal, typically 0.1 – 0.2 mm.
- Measure the pitch. Use a thread gauge, or measure across 10 crests and divide by 10 (mm pitch). For inch threads, count the crests in 1 inch (TPI).
- Check for taper. If the diameter gets smaller along the thread it is a taper pipe thread (BSPT/R or NPT, 1 in 16 on diameter).
- Look at the form. Rounded crests and roots = 55° Whitworth or BSP. Sharp V with small flats = 60° metric, unified or NPT. Square-shouldered flat teeth = trapezoidal or Acme lead screw.
- Match diameter and pitch in the tables below. Remember that pipe threads are named after the pipe bore: a 1/2" BSP thread is about 21 mm across.
Common thread forms
| Thread | Angle | Pitch given as | Where you'll find it |
|---|---|---|---|
| ISO metric (M) | 60° | Pitch in mm | General engineering, worldwide |
| Unified UNC / UNF | 60° | TPI | USA, Canada; American machinery |
| Whitworth BSW / BSF | 55° | TPI | Older British machinery and vehicles |
| BSP parallel (G) / taper (R, Rc) | 55° | TPI | Pipes, fittings, hydraulics |
| NPT (taper) / NPS (parallel) | 60° | TPI | American pipe fittings |
| BA | 47.5° | Pitch in mm | Instruments, electrical, models |
| Trapezoidal (Tr) | 30° | Pitch in mm | Lead screws, vices, jacks (metric) |
| Acme | 29° | TPI | Lead screws (inch) |
Pitch and TPI
Metric pitch to TPI
| Pitch mm | = TPI |
|---|---|
| 0.35 | 72.6 |
| 0.4 | 63.5 |
| 0.45 | 56.4 |
| 0.5 | 50.8 |
| 0.6 | 42.3 |
| 0.7 | 36.3 |
| 0.75 | 33.9 |
| 0.8 | 31.7 |
| 1 | 25.4 |
| 1.25 | 20.3 |
| 1.5 | 16.9 |
| 1.75 | 14.5 |
| 2 | 12.7 |
| 2.5 | 10.2 |
| 3 | 8.5 |
| 3.5 | 7.3 |
| 4 | 6.3 |
TPI to pitch
| TPI | = pitch mm |
|---|---|
| 80 | 0.318 |
| 64 | 0.397 |
| 56 | 0.454 |
| 48 | 0.529 |
| 40 | 0.635 |
| 36 | 0.706 |
| 32 | 0.794 |
| 28 | 0.907 |
| 26 | 0.977 |
| 24 | 1.058 |
| 22 | 1.155 |
| 20 | 1.27 |
| 19 | 1.337 |
| 18 | 1.411 |
| 16 | 1.587 |
| 14 | 1.814 |
| 13 | 1.954 |
| 12 | 2.117 |
| 11 | 2.309 |
| 10 | 2.54 |
| 9 | 2.822 |
| 8 | 3.175 |
Easily confused threads
| Metric | Similar inch thread | OD mm | Pitch mm |
|---|---|---|---|
| M5 × 0.8 | #10-32 UNF | 5 vs 4.83 | 0.8 vs 0.79 |
| M6 × 1 | 1/4" BSF | 6 vs 6.35 | 1 vs 0.98 |
| M6 × 1 | 1/4" UNC | 6 vs 6.35 | 1 vs 1.27 |
| M8 × 1.25 | 5/16" UNC | 8 vs 7.94 | 1.25 vs 1.41 |
| M10 × 1.25 | 3/8" BSF | 10 vs 9.52 | 1.25 vs 1.27 |
| M10 × 1.5 | 3/8" UNC | 10 vs 9.52 | 1.5 vs 1.59 |
| M12 × 1.75 | 1/2" UNC | 12 vs 12.70 | 1.75 vs 1.95 |
Pitches that are within a few percent of each other will start by hand and then jam. If a thread binds after two or three turns, stop and check the pitch: forcing it ruins both parts.
Bolt threads by measured diameter (up to 1")
| Thread | Pitch | OD mm | Family |
|---|---|---|---|
| M1.6 | 0.35 mm | 1.6 | Metric coarse |
| M2 | 0.4 mm | 2 | Metric coarse |
| #2 UNC | 56 TPI | 2.18 | Unified coarse |
| #2 UNF | 64 TPI | 2.18 | Unified fine |
| M2.5 | 0.45 mm | 2.5 | Metric coarse |
| #4 UNC | 40 TPI | 2.84 | Unified coarse |
| #4 UNF | 48 TPI | 2.84 | Unified fine |
| M3 | 0.5 mm | 3 | Metric coarse |
| #5 UNC | 40 TPI | 3.17 | Unified coarse |
| 1/8" BSW | 40 TPI | 3.17 | Whitworth |
| #6 UNC | 32 TPI | 3.51 | Unified coarse |
| #6 UNF | 40 TPI | 3.51 | Unified fine |
| M4 | 0.7 mm | 4 | Metric coarse |
| #8 UNC | 32 TPI | 4.17 | Unified coarse |
| #8 UNF | 36 TPI | 4.17 | Unified fine |
| 3/16" BSF | 32 TPI | 4.76 | British fine |
| 3/16" BSW | 24 TPI | 4.76 | Whitworth |
| #10 UNC | 24 TPI | 4.83 | Unified coarse |
| #10 UNF | 32 TPI | 4.83 | Unified fine |
| M5 | 0.8 mm | 5 | Metric coarse |
| #12 UNC | 24 TPI | 5.49 | Unified coarse |
| #12 UNF | 28 TPI | 5.49 | Unified fine |
| 7/32" BSF | 28 TPI | 5.56 | British fine |
| M6 | 1 mm | 6 | Metric coarse |
| 1/4" BSF | 26 TPI | 6.35 | British fine |
| 1/4" UNC | 20 TPI | 6.35 | Unified coarse |
| 1/4" UNF | 28 TPI | 6.35 | Unified fine |
| 1/4" BSW | 20 TPI | 6.35 | Whitworth |
| 5/16" BSF | 22 TPI | 7.94 | British fine |
| 5/16" UNC | 18 TPI | 7.94 | Unified coarse |
| 5/16" UNF | 24 TPI | 7.94 | Unified fine |
| 5/16" BSW | 18 TPI | 7.94 | Whitworth |
| M8 | 1.25 mm | 8 | Metric coarse |
| 3/8" BSF | 20 TPI | 9.52 | British fine |
| 3/8" UNC | 16 TPI | 9.52 | Unified coarse |
| 3/8" UNF | 24 TPI | 9.52 | Unified fine |
| Thread | Pitch | OD mm | Family |
|---|---|---|---|
| 3/8" BSW | 16 TPI | 9.52 | Whitworth |
| M10 | 1.5 mm | 10 | Metric coarse |
| 7/16" BSF | 18 TPI | 11.11 | British fine |
| 7/16" UNC | 14 TPI | 11.11 | Unified coarse |
| 7/16" UNF | 20 TPI | 11.11 | Unified fine |
| 7/16" BSW | 14 TPI | 11.11 | Whitworth |
| M12 | 1.75 mm | 12 | Metric coarse |
| 1/2" BSF | 16 TPI | 12.7 | British fine |
| 1/2" UNC | 13 TPI | 12.7 | Unified coarse |
| 1/2" UNF | 20 TPI | 12.7 | Unified fine |
| 1/2" BSW | 12 TPI | 12.7 | Whitworth |
| M14 | 2 mm | 14 | Metric coarse |
| 9/16" BSF | 16 TPI | 14.29 | British fine |
| 9/16" UNC | 12 TPI | 14.29 | Unified coarse |
| 9/16" UNF | 18 TPI | 14.29 | Unified fine |
| 5/8" BSF | 14 TPI | 15.88 | British fine |
| 5/8" UNC | 11 TPI | 15.88 | Unified coarse |
| 5/8" UNF | 18 TPI | 15.88 | Unified fine |
| 5/8" BSW | 11 TPI | 15.88 | Whitworth |
| M16 | 2 mm | 16 | Metric coarse |
| M18 | 2.5 mm | 18 | Metric coarse |
| 3/4" BSF | 12 TPI | 19.05 | British fine |
| 3/4" UNC | 10 TPI | 19.05 | Unified coarse |
| 3/4" UNF | 16 TPI | 19.05 | Unified fine |
| 3/4" BSW | 10 TPI | 19.05 | Whitworth |
| M20 | 2.5 mm | 20 | Metric coarse |
| M22 | 2.5 mm | 22 | Metric coarse |
| 7/8" BSF | 11 TPI | 22.22 | British fine |
| 7/8" UNC | 9 TPI | 22.22 | Unified coarse |
| 7/8" UNF | 14 TPI | 22.22 | Unified fine |
| 7/8" BSW | 9 TPI | 22.22 | Whitworth |
| M24 | 3 mm | 24 | Metric coarse |
| 1" BSF | 10 TPI | 25.4 | British fine |
| 1" UNC | 8 TPI | 25.4 | Unified coarse |
| 1" UNF | 12 TPI | 25.4 | Unified fine |
| 1" BSW | 8 TPI | 25.4 | Whitworth |
Pipe, small and lead-screw threads
Pipe threads - BSP vs NPT
| Pipe size | BSP TPI | BSP thread OD mm | NPT TPI | NPT pipe OD mm |
|---|---|---|---|---|
| 1/8" | 28 | 9.73 | 27 | 10.3 |
| 1/4" | 19 | 13.16 | 18 | 13.7 |
| 3/8" | 19 | 16.66 | 18 | 17.1 |
| 1/2" | 14 | 20.95 | 14 | 21.3 |
| 3/4" | 14 | 26.44 | 14 | 26.7 |
| 1" | 11 | 33.25 | 11.5 | 33.4 |
| 1-1/4" | 11 | 41.91 | 11.5 | 42.2 |
| 1-1/2" | 11 | 47.8 | 11.5 | 48.3 |
| 2" | 11 | 59.61 | 11.5 | 60.3 |
BSP and NPT have different thread angles (55° vs 60°) and, for most sizes, different TPI. 1/2" and 3/4" share 14 TPI, so they may start, but they will not seal. Never mix them.
BA threads
| BA | OD mm | Pitch mm |
|---|---|---|
| 0 BA | 6 | 1 |
| 1 BA | 5.3 | 0.9 |
| 2 BA | 4.7 | 0.81 |
| 3 BA | 4.1 | 0.73 |
| 4 BA | 3.6 | 0.66 |
| 5 BA | 3.2 | 0.59 |
| 6 BA | 2.8 | 0.53 |
| 7 BA | 2.5 | 0.48 |
| 8 BA | 2.2 | 0.43 |
| 9 BA | 1.9 | 0.39 |
| 10 BA | 1.7 | 0.35 |
Trapezoidal (DIN 103 preferred)
| Trapezoidal | Pitch mm |
|---|---|
| Tr8 | 1.5 |
| Tr10 | 2 |
| Tr12 | 3 |
| Tr14 | 3 |
| Tr16 | 4 |
| Tr18 | 4 |
| Tr20 | 4 |
| Tr22 | 5 |
| Tr24 | 5 |
| Tr26 | 5 |
| Tr28 | 5 |
| Tr30 | 6 |
| Tr32 | 6 |
| Tr36 | 6 |
| Tr40 | 7 |
| Tr44 | 7 |
| Tr50 | 8 |
| Tr60 | 9 |
Acme (general purpose)
| Acme | TPI |
|---|---|
| 1/4" | 16 |
| 5/16" | 14 |
| 3/8" | 12 |
| 7/16" | 12 |
| 1/2" | 10 |
| 5/8" | 8 |
| 3/4" | 6 |
| 7/8" | 6 |
| 1" | 5 |
| 1-1/4" | 5 |
| 1-1/2" | 4 |
| 1-3/4" | 4 |
| 2" | 4 |