Metric Fasteners – Socket Screw Hex Key & Nut Wrench Size
We recommend measuring by thread diameter, but when you only know the tool you used, these quick charts map common hex key (Allen) and wrench sizes to likely metric fastener sizes. Use them to choose replacements with confidence.
Need a refresher? See How to Measure Fastener Length & Diameter.
Hex Key (Allen) Size by Socket Screw Type
| Screw Size | Socket Cap (DIN 912 / ISO 4762) | Low Head (DIN 7984) | Button Head (ISO 7380) | Flat Head (DIN 7991 / ISO 10642) | Set Screw (DIN 916 / ISO 4029) |
|---|---|---|---|---|---|
| M2 | 1.5 mm | — | 1.3 mm | 1.3 mm | 0.9 mm |
| M2.5 | 2 mm | — | 1.5 mm | 1.5 mm | 1.3 mm |
| M3 | 2.5 mm | 2.5 mm | 2 mm | 2 mm | 1.5 mm |
| M4 | 3 mm | 2.5 mm | 2.5 mm | 2.5 mm | 2 mm |
| M5 | 4 mm | 3 mm | 3 mm | 3 mm | 2.5 mm |
| M6 | 5 mm | 4 mm | 4 mm | 4 mm | 3 mm |
| M8 | 6 mm | 5 mm | 5 mm | 5 mm | 4 mm |
| M10 | 8 mm | 7 mm | 6 mm | 6 mm | 5 mm |
| M12 | 10 mm | 8 mm | 8 mm | 8 mm | 6 mm |
| M16 | 14 mm | 12 mm | 10 mm | 10 mm | — |
Wrench Size for Common Metric Nuts
| Screw Size | Standard Hex Nut (DIN 934) |
Alt Size – Flange Nuts (DIN 6923 / 6926 / 6927) |
Alt Size – Acorn / Cap (DIN 1587 / DIN 917) |
|---|---|---|---|
| M2 | 4 mm | — | — |
| M2.5 | 5 mm | — | — |
| M3 | 5.5 mm | 5.5 mm | — |
| M4 | 7 mm | 7 mm | 7 mm |
| M5 | 8 mm | 8 mm | 8 mm |
| M6 | 10 mm | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm | 13 mm |
| M10 | 17 mm | 15 mm | 16 mm |
| M12 | 19 mm | 18 mm | 18 mm |
| M16 | 24 mm | 24 mm | — |
| M20 | 30 mm | 30 mm | — |
* Standard nut types: DIN 934 Hex, DIN 439B Jam, DIN 557 Square, DIN 985 Nylock (Regular), DIN 982 Nylock (High), DIN 980V All-Metal Prevailing Torque
** Flange / prevailing torque alternates: DIN 6926 Nylock Flange, DIN 6927 All-Metal Flange, DIN 6923 Flange
*** Acorn / cap alternates: DIN 1587 Acorn, DIN 917 Low Cap Acorn
Do-It-Right Tips
- Metric vs. US lookalikes: A driver that “fits” doesn’t guarantee thread compatibility. Always confirm the metric size and pitch (e.g., M6-1.0 vs M6-1.25) before ordering.
- Allow for alternates: Flange, acorn/cap, and prevailing-torque nuts can use different wrench sizes. Use the Alt Size columns above.
- Tighten by torque, not feel: For critical joints, use a torque spec. See our Metric Socket Screw Torque & Specs.
- Replace like-for-like: Match head style, material, and property class (e.g., 12.9 alloy) when strength matters.
Shop fasteners: All Metric · Socket Cap · Low Head · Button Head · Flat Head · Set Screws · Metric Nuts
FAQ
What size Allen key fits an M6 socket head cap screw?
M6 socket head cap screws typically use a 5 mm hex key. Low head and button head variants usually step down to 4–5 mm depending on style; see the table for exact pairings.
What wrench size do I need for an M10 metric nut?
Standard M10 hex nuts typically use a 17 mm wrench. Some flange or acorn styles may use 15–16 mm. Check the Alt Size columns above.
Can I identify a metric screw size from the hex key alone?
It gets you close, but verify the thread diameter and pitch (e.g., M6-1.0 vs M6-1.25). The charts map common hex key sizes to likely screw sizes, but measuring diameter and pitch is best practice.
Do different nut styles use different wrench sizes?
Yes. Flange, acorn/cap, or prevailing-torque nuts can use alternate wrench sizes compared to standard hex nuts. We list common alternates for quick reference.
Where can I find torque specs for metric socket screws?
See our Metric Socket Screw Torque & Specifications page for recommended seating torques by size and head style.