Bolt Head Size Chart (Bolt to Wrench Size)
Pick a bolt to get its wrench and hex key size, or enter a wrench size to see which bolts it fits and the nearest wrench in the other system.
How to use the Bolt Head Size Chart
- Pick a mode. Choose the bolt size to find the wrench or hex key for it, or choose the wrench size to find which bolts it fits.
- Choose the head type: Hex head bolt (wrench or socket), or Socket head cap screw (Allen / hex key).
- Enter the size. For inch sizes, type a fraction such as 1/2. For metric, type the size in mm, such as 13 mm, or the M-size, such as M8.
- Read the result. It updates as you type, so you do not need to press a button. The main result gives the matching size, and the nearest size in the other system appears with the difference in mm and inches. The tool marks a difference within 0.15 mm as a close fit; that cut-off is the tool’s own, not a value from a standard. Anything larger is marked as not interchangeable.
- Open “How it was calculated” to see the formula and each step worked out with your numbers.
Copy result copies the result lines with a link, so you can paste them into an email or a note. Copy link copies the page address, which stores every input in the address (?s=…). Opening that link later restores the same calculation. Print / save as PDF prints the inputs, results, calculation steps and the date, which is useful for shop records. Two tables further down the page list all the inch and metric sizes.
What size wrench for a 1/2 inch bolt?
A 1/2 inch hex head bolt takes a 3/4 inch wrench, which measures 19.05 mm across the flats. This pairing comes from the ASME B18.2.1 inch table, which gives the wrench size for each hex head bolt size. The wrench is wider than the thread because it must grip the outside of the head. For a 1/2 in socket head cap screw, the hex key is 3/8 in instead, per ASME B18.3.
What bolt does a 13 mm wrench fit?
A 13 mm wrench fits an M8 hex head bolt under the ISO 4014/4017 metric standard. The nearest inch wrench is 1/2 in (12.70 mm), which is 0.30 mm smaller. That gap is larger than the 0.15 mm close-fit limit, so the tool marks the two as not interchangeable. Check the fit on the actual bolt before you substitute one for the other.
What size Allen key for an M8 socket head?
An M8 socket head cap screw takes a 6 mm hex key under ISO 4762. The nearest inch key, 1/4 in (6.35 mm), is 0.35 mm larger, so it is not a close fit. A socket head cap screw has a round head with a hex recess in the top, so the key goes into the recess rather than fitting around the outside.
Is a 19 mm wrench the same as 3/4?
No, but it is a close fit. 3/4 in equals 19.05 mm, so a 19 mm wrench is 0.05 mm (0.0020 in) smaller. That is within the tool’s 0.15 mm close-fit limit, so the pair is marked as a close fit. A close fit is not an identical size, and the difference is too small to see by eye.
Why do M10 bolts use 16 or 17 mm wrenches?
Two metric standards give M10 different head sizes. ISO 4014/4017 specifies a 16 mm head, which is 0.6299 in. The older DIN 931/933 standard specifies 17 mm. A 16 mm wrench is the right size for ISO bolts, and a 17 mm wrench is the right size for DIN bolts. If you are unsure which standard applies, check your drawing or parts list, or measure across the flats.
What is the bolt head size for M12?
An M12 hex head bolt has an 18 mm head under ISO 4014/4017, and the older DIN 931/933 standard gives 19 mm. An M12 socket head cap screw takes a 10 mm hex key under ISO 4762. Use the ISO size for newer parts, and check your drawing or parts list if you are not sure which standard the bolts follow.
Sources
Checked October 2026:
- Hex cap screw dimensions to ASME B18.2.1 (AFT Fasteners) for inch widths across flats
- DIN EN ISO 4017 and DIN 933 tables (fasten.it) for metric widths across flats
- Eugene Fastener socket screw hex key chart (ASME B18.3 inch) and DIN EN ISO 4762 table (fasten.it) for socket head keys
For estimation; verify critical designs against the applicable standard.
Worked examples
| Input | Result |
|---|---|
| 1/2, Hex head bolt (wrench or socket) | 3/4 in (19.05 mm); Close fit (within 0.15 mm, this tool's own cut-off) |
| M10, Hex head bolt (wrench or socket) | 16 mm (0.6299 in); Close fit (within 0.15 mm, this tool's own cut-off) |
| M8, Socket head cap screw (Allen key) | 6 mm (0.2362 in); Not interchangeable (0.35 mm apart) |
| Wrench 3/4 in, Hex head bolt (wrench or socket) | fits 1/2 in; nearest other: 19 mm (0.7480 in), difference −0.050 mm, −0.0020 in |
| Wrench 13 mm, Hex head bolt (wrench or socket) | fits M8; nearest other: 1/2 in (12.70 mm), difference −0.300 mm, −0.0118 in |
Calculated by the tool above with these values.
Frequently asked questions
Does it work on a phone?
Yes. The tool runs in the browser, so it works on a phone. No sign-up is needed, and nothing you type is uploaded.
What is the difference between a hex head bolt and a socket head cap screw?
A hex head bolt has a six-sided head that you turn with a wrench or socket. The other type has a round head with a hex recess in the top, which you drive with an Allen key or hex key. Pick the head type that matches your fastener before you read the result.
Does the chart cover every bolt size?
Coverage follows the standards listed on the page: ASME B18.2.1 and B18.3 for inch sizes, and ISO 4014/4017, DIN 931/933 and ISO 4762 for metric sizes. Both tables on the page list every inch and metric size, so a quick check there settles it if needed.
Can I use the results in quotes or shop paperwork?
Yes. Results may be used in commercial work, such as quotes, customer calculations, shop paperwork and reports, without attribution.
Does a 1/2 inch wrench fit a metric bolt?
Across the flats, a 1/2 in wrench measures 12.70 mm. The closest metric hex, 13 mm (M8), is 0.30 mm larger, so the tool flags them as not interchangeable.
Related tools
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