A 1/4-20 × 1 bolt and an M6 × 1.0 × 25 bolt use different systems but both separate thread identity from length.
Common bolt thread callouts
This chart decodes nominal diameter and pitch only. Head dimensions and wrench size depend on the fastener product standard and head style.
| Callout | System | Nominal diameter | Pitch / TPI |
|---|---|---|---|
| 1/4-20 | UNC | 0.2500 in | 20 tpi |
| 1/4-28 | UNF | 0.2500 in | 28 tpi |
| 5/16-18 | UNC | 0.3125 in | 18 tpi |
| 3/8-16 | UNC | 0.3750 in | 16 tpi |
| 3/8-24 | UNF | 0.3750 in | 24 tpi |
| 1/2-13 | UNC | 0.5000 in | 13 tpi |
| M3 × 0.5 | Metric | 3 mm | 0.5 mm |
| M4 × 0.7 | Metric | 4 mm | 0.7 mm |
| M5 × 0.8 | Metric | 5 mm | 0.8 mm |
| M6 × 1.0 | Metric | 6 mm | 1 mm |
| M8 × 1.25 | Metric | 8 mm | 1.25 mm |
| M10 × 1.5 | Metric | 10 mm | 1.5 mm |
| M12 × 1.75 | Metric | 12 mm | 1.75 mm |
| 5/16-24 | UNF | 0.3125 in | 24 tpi |
| 7/16-14 | UNC | 0.4375 in | 14 tpi |
| 7/16-20 | UNF | 0.4375 in | 20 tpi |
| 1/2-20 | UNF | 0.5000 in | 20 tpi |
| 9/16-12 | UNC | 0.5625 in | 12 tpi |
| 9/16-18 | UNF | 0.5625 in | 18 tpi |
| 5/8-11 | UNC | 0.6250 in | 11 tpi |
| 5/8-18 | UNF | 0.6250 in | 18 tpi |
| 3/4-10 | UNC | 0.7500 in | 10 tpi |
| 3/4-16 | UNF | 0.7500 in | 16 tpi |
| 7/8-9 | UNC | 0.8750 in | 9 tpi |
| 7/8-14 | UNF | 0.8750 in | 14 tpi |
| 1-8 | UNC | 1.0000 in | 8 tpi |
| 1-12 | UNF | 1.0000 in | 12 tpi |
| M1.6 × 0.35 | Metric | 1.6 mm | 0.35 mm |
| M2 × 0.4 | Metric | 2 mm | 0.4 mm |
| M2.5 × 0.45 | Metric | 2.5 mm | 0.45 mm |
| M3.5 × 0.6 | Metric | 3.5 mm | 0.6 mm |
| M4.5 × 0.75 | Metric | 4.5 mm | 0.75 mm |
| M7 × 1.0 | Metric | 7 mm | 1.0 mm |
| M8 × 1.0 | Metric | 8 mm | 1.0 mm |
| M10 × 1.25 | Metric | 10 mm | 1.25 mm |
| M12 × 1.25 | Metric | 12 mm | 1.25 mm |
| M14 × 2.0 | Metric | 14 mm | 2.0 mm |
| M16 × 2.0 | Metric | 16 mm | 2.0 mm |
| M16 × 1.5 | Metric | 16 mm | 1.5 mm |
| M18 × 2.5 | Metric | 18 mm | 2.5 mm |
| M18 × 1.5 | Metric | 18 mm | 1.5 mm |
| M20 × 2.5 | Metric | 20 mm | 2.5 mm |
| M20 × 1.5 | Metric | 20 mm | 1.5 mm |
| M22 × 2.5 | Metric | 22 mm | 2.5 mm |
| M22 × 1.5 | Metric | 22 mm | 1.5 mm |
| M24 × 3.0 | Metric | 24 mm | 3.0 mm |
| M24 × 2.0 | Metric | 24 mm | 2.0 mm |
Nominal size is not every dimension
The thread diameter does not directly state the head width, clearance hole, tap drill or tensile strength. Those values depend on fastener style and governing specification.
Measure without forcing
Use a diameter measurement and pitch gauge, then compare with a known specification. A near-matching inch and metric thread can damage both parts if forced.
Important limitation
This identification guide does not assign bolt grades, torque values or structural capacity.
This identification guide does not assign bolt grades, torque values or structural capacity.