Reference & Charts
Metric Fastener Reference
The metric fastener numbers you look up at the machine: coarse-thread pitch, tap drill (~75% thread), normal clearance hole, and socket cap-screw hex key for M2 through M20 — one spec sheet per size.
11 entries · Last verified: 2026-07-10
Metric screw threads are specified by diameter and pitch — M6 × 1.0 means a 6 mm major diameter with a 1.0 mm coarse pitch. To cut an internal thread, drill the tap drill (roughly the major diameter minus the pitch for a ~75% thread, which balances strength against tapping torque), then tap. To pass a bolt through, drill the clearance hole (ISO 273 — normal fit shown here; close and loose fits exist). The hex key column is for socket head cap screws (ISO 4762). All values below are for the standard coarse pitch.
| Metric Size | Pitch(mm) | Tap drill(mm) | Clearance (normal)(mm) | Hex key(mm) |
|---|---|---|---|---|
| M2 | 0.4 mm | 1.6 mm | 2.4 mm | 1.5 mm |
| M2.5 | 0.45 mm | 2.05 mm | 2.9 mm | 2 mm |
| M3 | 0.5 mm | 2.5 mm | 3.4 mm | 2.5 mm |
| M4 | 0.7 mm | 3.3 mm | 4.5 mm | 3 mm |
| M5 | 0.8 mm | 4.2 mm | 5.5 mm | 4 mm |
| M6 | 1 mm | 5 mm | 6.6 mm | 5 mm |
| M8 | 1.25 mm | 6.8 mm | 9 mm | 6 mm |
| M10 | 1.5 mm | 8.5 mm | 11 mm | 8 mm |
| M12 | 1.75 mm | 10.2 mm | 13.5 mm | 10 mm |
| M16 | 2 mm | 14 mm | 17.5 mm | 14 mm |
| M20 | 2.5 mm | 17.5 mm | 22 mm | 17 mm |
Select a metric size for its full spec sheet.
Frequently asked
- How do I find the tap drill for a metric thread?
- The shop rule for a ~75% thread is major diameter minus pitch: an M6 × 1.0 taps with a 5.0 mm drill (6 − 1.0). It's an approximation that balances thread strength against tapping torque — this chart lists the standard drill for each coarse size. Use a lower thread percentage (larger drill) in hard or gummy material to ease tapping.
- What's the difference between the tap drill and the clearance hole?
- The tap drill is the smaller hole you cut a thread into (the female side). The clearance hole is the larger hole a bolt passes through (the male side) — for M6, tap with 5.0 mm but clear with 6.6 mm (normal fit). Don't confuse them: tapping a clearance-sized hole leaves almost no thread.
- Are these fine-thread values too?
- No — these are the standard coarse-thread (ISO 261) values. Fine-pitch metric threads (e.g. M8 × 1.0 instead of 1.25) have a different pitch, and therefore a different tap drill. The clearance hole and hex key don't change with pitch.
Sources
- ISO 261 — ISO general-purpose metric screw threads (coarse pitch)
- ISO 273 — clearance holes for bolts and screws (normal fit)
- ISO 4762 — hexagon socket head cap screws (hex key sizes)