SHOP REFERENCE / HEX HEADS
Bolt Size to Socket Size Chart
The bolt size names the outside diameter of its threads. The socket size names the opening that fits across the bolt head’s flat faces. Use the chart for the head style and standard you actually have.
Open the Socket Finder toolA bolt marked M8 has an 8 mm nominal thread diameter. A standard M8 hex head takes a 13 mm socket.
Metric hex bolts
Modern ISO 4014/4017 and older DIN 931/933 standard hex heads. Socket and wrench sizes are nominal across-flats openings.
| Bolt thread | Modern ISO head | Older DIN head |
|---|---|---|
| M4 | 7 mm | 7 mm |
| M5 | 8 mm | 8 mm |
| M6 | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm |
| M10 | 16 mm | 17 mm |
| M12 | 18 mm | 19 mm |
| M14 | 21 mm | 22 mm |
| M16 | 24 mm | 24 mm |
| M18 | 27 mm | 27 mm |
| M20 | 30 mm | 30 mm |
| M22 | 34 mm | 32 mm |
| M24 | 36 mm | 36 mm |
| M27 | 41 mm | 41 mm |
| M30 | 46 mm | 46 mm |
SAE hex bolts
Typical US standard hex bolts. Values are for the outside hex head, measured across flats.
| Bolt thread | Hex head socket / wrench |
|---|---|
| 1/4 in. | 7/16 in. |
| 5/16 in. | 1/2 in. |
| 3/8 in. | 9/16 in. |
| 7/16 in. | 5/8 in. |
| 1/2 in. | 3/4 in. |
| 9/16 in. | 13/16 in. |
| 5/8 in. | 15/16 in. |
| 3/4 in. | 1-1/8 in. |
| 7/8 in. | 1-5/16 in. |
| 1 in. | 1-1/2 in. |
Some 1/4 in. hex bolts use a 3/8 in. head instead of 7/16 in. Measure the head. Heavy hex, flange, lag and socket-head fasteners can require different tools.
AT THE BENCH
How to read the chart
An M8 standard hex bolt has an 8 mm nominal thread diameter and a 13 mm hex head opening. Use a 13 mm socket on that head.
A modern ISO M10 standard hex head takes 16 mm; an older DIN version takes 17 mm. Measure across the flats before choosing.
A standard US 3/8 in. hex bolt has a nominal 3/8 in. thread and a 9/16 in. hex head opening.
A standard US 1/2 in. hex bolt typically takes a 3/4 in. socket; a heavy hex head is larger.
Measure across two opposite flat sides of the head, not across the corners. These charts describe standard external hex bolt heads, not nuts, internal Allen sockets, Torx, or bolt strength grades. Head marking and application requirements are separate from dimensional identification.
How to measure the hex head and socket opening → Identify a fastener with the Socket Finder tool →Sources and scope
Fastenal ISO 4017 product specification lists the modern ISO across-flats sizes. Bolt Depot’s metric bolt head chart distinguishes ISO and DIN sizes; its US bolt head chart lists standard and heavy hex sizes. These are nominal socket/wrench openings. Actual fastener styles and tolerances vary; if the socket is loose or will not seat, measure the head and use the correct tool.