Reference Chart
Thread Pitch Charts.
Thread pitch is a critical measurement when identifying screws and bolts. Using the wrong thread pitch can strip pre-tapped holes, damage fasteners, or result in structurally weak connections. Use these standard tables to match metric and imperial threads.
Metric Threads (ISO)
In the metric system, **thread pitch** is defined as the direct distance (in millimeters) between the crests of adjacent threads.
- Coarse (Standard): The default, widely-used pitch for general fastening.
- Fine: Tighter thread spacing for higher vibration resistance or thinner walls.
- Example: An M6 × 1.0 screw has a 6mm diameter and a 1.0mm pitch.
Imperial Threads (UTS)
In the Unified Thread Standard (UTS), pitch is measured as **TPI** (Threads Per Inch) — the number of thread crests along one inch of shank.
- UNC (Coarse): Standard general-purpose threads.
- UNF (Fine): Tighter spacing for high-strength mechanical assemblies.
- Example: A 1/4"-20 bolt has a 1/4" diameter and 20 threads per inch.
Metric Thread Pitch Chart (ISO)
Standard Coarse vs. Fine| Thread Size | Major Dia. (mm) | Major Dia. (in) | Coarse Pitch | Fine Pitch |
|---|---|---|---|---|
| M2 | 2.00 mm | 0.079" | 0.40 mm | 0.25 mm |
| M2.5 | 2.50 mm | 0.098" | 0.45 mm | 0.35 mm |
| M3 | 3.00 mm | 0.118" | 0.50 mm | 0.35 mm |
| M4 | 4.00 mm | 0.157" | 0.70 mm | 0.50 mm |
| M5 | 5.00 mm | 0.197" | 0.80 mm | 0.50 mm |
| M6 | 6.00 mm | 0.236" | 1.00 mm | 0.75 mm |
| M8 | 8.00 mm | 0.315" | 1.25 mm | 1.00 mm |
| M10 | 10.00 mm | 0.394" | 1.50 mm | 1.25 mm |
| M12 | 12.00 mm | 0.472" | 1.75 mm | 1.50 mm (1.25 mm) |
| M14 | 14.00 mm | 0.551" | 2.00 mm | 1.50 mm |
| M16 | 16.00 mm | 0.630" | 2.00 mm | 1.50 mm |
Imperial Thread TPI Chart (UTS)
Unified Coarse (UNC) vs. Fine (UNF)| Thread Size | Major Dia. (in) | Major Dia. (mm) | UNC Coarse TPI | UNF Fine TPI |
|---|---|---|---|---|
| #2 | 0.086" | 2.18 mm | 56 TPI (0.45mm) | 64 TPI (0.40mm) |
| #3 | 0.099" | 2.51 mm | 48 TPI (0.53mm) | 56 TPI (0.45mm) |
| #4 | 0.112" | 2.84 mm | 40 TPI (0.64mm) | 48 TPI (0.53mm) |
| #5 | 0.125" | 3.18 mm | 40 TPI (0.64mm) | 44 TPI (0.58mm) |
| #6 | 0.138" | 3.51 mm | 32 TPI (0.79mm) | 40 TPI (0.64mm) |
| #8 | 0.164" | 4.17 mm | 32 TPI (0.79mm) | 36 TPI (0.71mm) |
| #10 | 0.190" | 4.83 mm | 24 TPI (1.06mm) | 32 TPI (0.79mm) |
| #12 | 0.216" | 5.49 mm | 24 TPI (1.06mm) | 28 TPI (0.91mm) |
| 1/4" | 0.250" | 6.35 mm | 20 TPI (1.27mm) | 28 TPI (0.91mm) |
| 5/16" | 0.312" | 7.94 mm | 18 TPI (1.41mm) | 24 TPI (1.06mm) |
| 3/8" | 0.375" | 9.53 mm | 16 TPI (1.59mm) | 24 TPI (1.06mm) |
| 7/16" | 0.438" | 11.11 mm | 14 TPI (1.81mm) | 20 TPI (1.27mm) |
| 1/2" | 0.500" | 12.70 mm | 13 TPI (1.95mm) | 20 TPI (1.27mm) |
How to Verify Thread Pitch
If you need to replace a bolt and don't know the thread pitch, there are three common ways to measure it:
- Use a Thread Pitch Gauge: Place the teeth of a pitch comb against the screw threads. When the teeth fit perfectly in the valleys without gaps, read the pitch stamped on the blade.
- Interactive Screen Matching: Visit our homepage and switch to the Thread Pitch tab. Lay your screw directly on the screen (or use the camera overlay mode) to compare its crests with the digital pitch templates.
- Print a Scaled Reference Sheet: Use our Printable Cheat Sheet. It has standard calibrated thread patterns that you can lay your screw against in the workshop.