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Quickly match metric and SAE fasteners with the correct tool size. Simple, accurate, and built for real-world use.
Socket, wrench and bolt size lookup tool
Recommended Socket/Wrench
ISO HEXClosest SAE: 5/8″
Older DIN M10 hardware may commonly use a 17 mm head.
I measured the bolt head
Measure straight across two opposite, flat sides.
13 mm matches the nominal head size. Confirm the threads.
An identification aid, not a guarantee. Standards, wear and measurement accuracy affect results.
Metric ↔ SAE
A familiar conversion, with the difference made clear.
3/4″ = 19.05 mm · Difference: 0.05 mm
Extremely close numerically. Always prefer the correct tool size and check for a snug, fully seated fit.
Common sizes chart
A quick reference for standard hex bolt heads.
Metric · ISO & legacy DIN
SAE · standard & heavy hex
| Bolt | Standard hex | Heavy hex |
|---|---|---|
| 1/4″ | 7/16″ * | — |
| 5/16″ | 1/2″ | — |
| 3/8″ | 9/16″ | — |
| 7/16″ | 5/8″ | — |
| 1/2″ | 3/4″ | 7/8″ |
| 9/16″ | 13/16″ | — |
| 5/8″ | 15/16″ | 1-1/16″ |
| 3/4″ | 1-1/8″ | 1-1/4″ |
| 7/8″ | 1-5/16″ | 1-7/16″ |
| 1″ | 1-1/2″ | 1-5/8″ |
* Some 1/4″ hex bolts use a 3/8″ head. A dash means not covered by this reference, not that the size never exists. Source: Bolt Depot.
No sizes match. Try M10, 13 mm or 1/2.
ISO values are nominal wrench sizes for ISO 4014 / 4017 hex bolts. Older DIN 931 / 933 differences are shown separately. M7 is a less-common size. Nuts and specialty heads may differ.
Frequently asked questions
Why does an M10 bolt sometimes use a 16 mm wrench and sometimes a 17 mm wrench?
Modern ISO 4014 / 4017 M10 standard hex bolts use a 16 mm wrench. Older DIN 931 / 933 dimensions use 17 mm, and that hardware is still common. M12 changes from 19 mm DIN to 18 mm ISO; M14 from 22 to 21 mm. M22 goes the other way: 32 mm DIN versus 34 mm ISO. Identify the standard or measure the actual head.
Can I use an SAE socket on a metric bolt?
Use the correct size whenever possible. Some nominal sizes are very close: 3/4 inch equals 19.05 mm, just 0.05 mm from 19 mm. But tolerance, wear and loading affect the fit. A smaller socket may not seat; a larger socket may round the corners. Never force a tool or use one that rocks on the fastener. The conversion labels describe numerical closeness, not a guarantee of interchangeability.
Are bolt diameter and wrench size the same thing?
No. M10 describes a nominal 10 mm thread diameter, not a 10 mm head. A modern ISO M10 standard hex bolt takes a 16 mm wrench; an M10 full-height socket head cap screw takes an 8 mm hex key. The head style matters as much as the thread size.
What does “across flats” mean?
It is the straight-line distance between two opposite flat faces of a hexagonal head or nut. Place the caliper jaws against the flats, not the corners. Clean off dirt first and avoid damaged edges. This distance corresponds to the nominal wrench or socket opening.
Do Allen keys use the same sizing system as normal sockets?
Both come in metric millimeters and SAE inches, but they engage different surfaces. An Allen key fits an internal hexagonal recess; a normal socket fits around the outside of a head. An M8 ISO hex bolt uses a 13 mm socket, while an M8 full-height socket head cap screw uses a 6 mm hex key. Button, flat and low-profile heads need their own charts. Use the exact key size.
How do I measure a bolt if I don’t know its size?
Use calipers to measure across the head flats for an initial tool choice. To identify the bolt itself, measure the outside diameter of the threads and use a thread-pitch gauge to find metric pitch or inch threads per inch. Measure length from under the head to the tip for a standard hex or socket cap screw. Compare the head style and measurements with a verified reference; a wrench size alone cannot identify the thread.
About SocketFinder & sources
Fast, free bolt and socket lookup. All calculations run in your browser, with no accounts, tracking, database or external API. Dimensions are nominal tool sizes; the cited references define the applicable fastener styles.
Reference tables checked September 25, 2026. Flange heads, button heads, countersunk screws and specialty fasteners are outside this database. Standard hex nuts are included separately: DIN 934 metric and US hex nuts. Other nut styles can differ.
Fastener dimensions can vary by standard, manufacturer, age, and head style. SocketFinder is a reference tool; confirm the fastener before applying high torque.