Identifying Unknown Threads
You found a bolt. You don’t know what it is. Here’s how to figure it out in 60 seconds with a thread pitch gauge, a caliper, and this chart.
Step 1: Measure the Major Diameter
Use a caliper to measure the outside diameter of the bolt threads. This tells you whether you’re dealing with US (fractional) or Metric threads:
| Measured OD (approx) | US Size | Metric Size |
|---|---|---|
| 0.112″ | #4 | — |
| 0.138″ | #6 | M3.5 (0.138″) |
| 0.164″ | #8 | M4 (0.157″) |
| 0.190″ | #10 | M5 (0.197″) |
| 0.250″ | 1/4″ | M6 (0.236″) |
| 0.312″ | 5/16″ | M8 (0.315″) |
| 0.375″ | 3/8″ | M10 (0.394″) |
| 0.500″ | 1/2″ | M12 (0.472″) |
| 0.625″ | 5/8″ | M16 (0.630″) |
| 0.750″ | 3/4″ | M20 (0.787″) |
| 1.000″ | 1″ | M24 (0.945″) or M27 (1.063″) |
Step 2: Count the Threads Per Inch (or Measure Pitch)
Place a thread pitch gauge on the threads. For US bolts, you’re counting threads per inch (TPI). For metric, you’re measuring the distance between threads in mm.
US Thread Identification
| Size | UNC (Coarse) TPI | UNF (Fine) TPI |
|---|---|---|
| #4 | 40 | 48 |
| #6 | 32 | 40 |
| #8 | 32 | 36 |
| #10 | 24 | 32 |
| 1/4″ | 20 | 28 |
| 5/16″ | 18 | 24 |
| 3/8″ | 16 | 24 |
| 7/16″ | 14 | 20 |
| 1/2″ | 13 | 20 |
| 9/16″ | 12 | 18 |
| 5/8″ | 11 | 18 |
| 3/4″ | 10 | 16 |
| 7/8″ | 9 | 14 |
| 1″ | 8 | 12 (or 14) |
Metric Thread Identification
| Size | Standard Pitch (mm) | Fine Pitch (mm) |
|---|---|---|
| M3 | 0.50 | 0.35 |
| M4 | 0.70 | 0.50 |
| M5 | 0.80 | 0.50 |
| M6 | 1.00 | 0.75 |
| M8 | 1.25 | 1.00 |
| M10 | 1.50 | 1.25 or 1.00 |
| M12 | 1.75 | 1.50 or 1.25 |
| M14 | 2.00 | 1.50 |
| M16 | 2.00 | 1.50 |
| M20 | 2.50 | 2.00 or 1.50 |
| M24 | 3.00 | 2.00 |
Step 3: Confirm with a Nut or Thread Checker
The fastest confirmation: try threading a known nut onto the bolt. If it goes on smoothly by hand with no resistance or wobble, you’ve got your match.
Common Gotchas
- M12 vs 1/2″: Very close in diameter (0.472″ vs 0.500″) — the pitch is the tell. M12 is 1.75mm pitch; 1/2″-13 UNC is 1.95mm pitch (13 TPI). Feel the difference with a gauge.
- Pipe threads (NPT): If the threads are tapered and the diameter doesn’t match any standard bolt size, it’s probably NPT. See NPT Dimensions.
- Left-hand threads: If the nut won’t start at all, try turning it the other way. Left-hand threads exist on bicycle pedals, gas fittings, some lathe spindles, and turnbuckles.
- BSP vs NPT: British Standard Pipe and NPT look almost identical but have different thread angles (55° vs 60°). They’ll start threading but won’t seal properly if mismatched.
Related: UNC Tap Drill Sizes | UNF Tap Drill Sizes | UNC Thread Dimensions