Metric Bolt Size Chart
This chart gives head and body dimensions for the most common metric bolt head styles — hexagon, socket (Allen) head, countersunk, and button head — so you can check across-flats size, head height, and body diameter without needing to measure a sample bolt by hand. Head style is usually chosen based on available clearance and the tooling available to drive it: hexagon heads are the most common general-purpose choice and can be driven with a spanner or socket; socket head bolts need only a compact hex key and work well in confined spaces; countersunk bolts sit flush with the surrounding surface; and button heads give a low profile with a rounded, snag-resistant top.
The charts below gives the dimensions for Hexagon head, Socket cap, Low head socket cap, Socket countersink, Socket button and Shoulder screw with Metric Threads.
Dimensions shown are in mm, if you wish to convert to Inches, please use our universal converter.
We also have information on Imperial and Unified Bolts.
HEXAGON HEAD DIMENSIONS - METRIC - BS 3692:2001
| Nominal Size | Body Diameter (D) | Head Thickness (H) | Across Flats (A/F) | Across Corners (A/C) | ||||
|---|---|---|---|---|---|---|---|---|
| Max | Min | Max | Min | Max | Min | Max | Min | |
| M1.6 | 1.60 | 1.47 | 1.225 | 0.975 | 3.20 | 3.02 | 3.41 | 3.41 |
| M2 | 2.00 | 1.87 | 1.525 | 1.275 | 4.00 | 3.82 | 4.32 | 4.32 |
| M2.5 | 2.50 | 2.38 | 1.825 | 1.575 | 5.00 | 4.82 | 5.45 | 5.45 |
| M3 | 3.00 | 2.87 | 2.125 | 1.875 | 5.50 | 5.32 | 6.01 | 6.01 |
| M4 | 4.00 | 3.84 | 2.925 | 2.675 | 7.00 | 6.78 | 7.66 | 7.66 |
| M5 | 5.00 | 4.82 | 3.650 | 3.350 | 8.00 | 7.78 | 8.79 | 8.79 |
| M6 | 6.00 | 5.82 | 4.150 | 3.850 | 10.00 | 9.78 | 11.05 | 11.05 |
| M8 | 8.00 | 7.78 | 5.450 | 5.150 | 13.00 | 12.73 | 14.38 | 14.38 |
| M10 | 10.00 | 9.78 | 6.580 | 6.220 | 17.00 | 16.73 | 18.90 | 18.90 |
| M12 | 12.00 | 11.73 | 7.680 | 7.320 | 19.00 | 18.67 | 21.10 | 21.10 |
| M14 | 14.00 | 13.73 | 8.980 | 8.620 | 22.00 | 21.67 | 24.49 | 24.49 |
| M16 | 16.00 | 15.73 | 10.180 | 9.820 | 24.00 | 23.67 | 26.75 | 26.75 |
| M20 | 20.00 | 19.67 | 13.150 | 12.650 | 30.00 | 29.16 | 33.53 | 33.53 |
SOCKET CAP SCREW DIMENSIONS - METRIC - BS EN ISO 4762: 1998
| Nominal Size | Body Dia (D) | Head Dia (Dk) | Head Height (K) | Hex Key Size (S) | Key Engagement (T) |
|---|---|---|---|---|---|
| M3 | 3.0 | 5.5 | 3.0 | 2.5 | 1.3 |
| M4 | 4.0 | 7.0 | 4.0 | 3.0 | 2.0 |
| M5 | 5.0 | 8.5 | 5.0 | 4.0 | 2.5 |
| M6 | 6.0 | 10.0 | 6.0 | 5.0 | 3.0 |
| M8 | 8.0 | 13.0 | 8.0 | 6.0 | 4.0 |
| M10 | 10.0 | 16.0 | 10.0 | 8.0 | 5.0 |
| M12 | 12.0 | 18.0 | 12.0 | 10.0 | 6.0 |
| M16 | 16.0 | 24.0 | 16.0 | 14.0 | 8.0 |
| M20 | 20.0 | 30.0 | 20.0 | 17.0 | 10.0 |
SOCKET CAP SCREW DIMENSIONS - LOW HEAD - DIN 7984
| Nominal Size | Body Dia (D) | Head Dia (Dk) | Head Height (K) | Hex Key Size (S) | Key Engagement (T) |
|---|---|---|---|---|---|
| M3 | 3.0 | 5.5 | 2.0 | 2.0 | 1.5 |
| M4 | 4.0 | 7.0 | 2.8 | 2.5 | 2.3 |
| M5 | 5.0 | 8.5 | 3.5 | 3.0 | 2.7 |
| M6 | 6.0 | 10.0 | 4.0 | 4.0 | 3.0 |
| M8 | 8.0 | 13.0 | 5.0 | 5.0 | 3.8 |
| M10 | 10.0 | 16.0 | 6.0 | 7.0 | 4.5 |
| M12 | 12.0 | 18.0 | 7.0 | 8.0 | 5.0 |
| M16 | 16.0 | 24.0 | 9.0 | 12.0 | 5.5 |
| M20 | 20.0 | 30.0 | 11.0 | 14.0 | 7.5 |
SOCKET COUNTERSINK SCREW DIMENSIONS - METRIC - BS EN ISO 10642:1998
| Nominal Size | Body Dia (D) | Head Dia (Dk) | Head Height (K) | Hex Key Size (S) |
|---|---|---|---|---|
| M3 | 3.0 | 6.0 | 1.7 | 2.0 |
| M4 | 4.0 | 8.0 | 2.3 | 2.5 |
| M5 | 5.0 | 10.0 | 2.8 | 3.0 |
| M6 | 6.0 | 12.0 | 3.3 | 4.0 |
| M8 | 8.0 | 16.0 | 4.4 | 5.0 |
| M10 | 10.0 | 20.0 | 5.5 | 6.0 |
| M12 | 12.0 | 24.0 | 6.5 | 8.0 |
| M16 | 16.0 | 30.0 | 7.5 | 10.0 |
| M20 | 20.0 | 37.0 | 8.5 | 12.0 |
SOCKET BUTTON SCREW DIMENSIONS - METRIC - BS EN ISO 7380-1 : 2011
| Nominal Size | Body Dia (D) | Head Dia (Dk) | Head Height (K) | Hex Key Size (S) |
|---|---|---|---|---|
| M3 | 3.0 | 5.7 | 1.65 | 2.0 |
| M4 | 4.0 | 7.6 | 2.20 | 2.5 |
| M5 | 5.0 | 9.5 | 2.75 | 3.0 |
| M6 | 6.0 | 10.5 | 3.30 | 4.0 |
| M8 | 8.0 | 14.0 | 4.40 | 5.0 |
| M10 | 10.0 | 18.0 | 5.50 | 6.0 |
| M12 | 12.0 | 21.0 | 6.60 | 8.0 |
| M16 | 16.0 | 28.0 | 8.80 | 10.0 |
SOCKET HEAD SHOULDER SCREW DIMENSIONS - METRIC
| Nominal Size | Thread Size | Shoulder Dia (Ds) | Head Dia (Dk) | Head Height (K) | Hex Key Size (S) |
|---|---|---|---|---|---|
| M5 | M4 | 6.0 | 10.0 | 4.5 | 3.0 |
| M6 | M5 | 8.0 | 13.0 | 5.5 | 4.0 |
| M8 | M6 | 10.0 | 16.0 | 7.0 | 5.0 |
| M10 | M8 | 12.0 | 18.0 | 9.0 | 6.0 |
| M12 | M10 | 16.0 | 24.0 | 11.0 | 8.0 |
| M16 | M12 | 20.0 | 30.0 | 14.0 | 10.0 |
| M20 | M16 | 24.0 | 36.0 | 16.0 | 12.0 |
RELATED PAGES
- Screw Head Types
- Screw Driving Types
- Bolt Grades and Markings
- Metric Nut Dimensions
- Metric Washer Dimensions
FREQUENTLY ASKED QUESTIONS
How is a metric bolt size specified?
As a thread designation followed by length, e.g. M10x50, meaning a 10mm nominal diameter thread (at the standard coarse pitch unless a fine pitch is stated) and 50mm length measured from under the head to the tip. See Thread Designation Charts for the full designation format.
What spanner or hex key size do I need for a given bolt?
See our Spanner Size Comparison Chart for across-flats sizes matched to bolt diameter.
Does head style affect a bolt's strength?
The thread and shank carry the load in most applications, so head style mainly affects how the bolt is driven and how it sits relative to the surrounding surface, rather than its tensile strength. However, very thin head styles (such as some button heads) can have a lower head shear or torque capacity than a full hex head of the same nominal size — check the manufacturer's rating for high-torque applications.
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