Bolt Size to Wrench & Socket Size Chart — Metric & Inch

Quick bolt-to-wrench lookup

This chart starts with the fastener size and returns the nominal wrench or socket opening. It is not a list of socket-drive sizes. The table is limited to explicitly identified hex-cap-screw families, with the applicable standard family shown beside every row.

Important: A nominal wrench size is not a guarantee for every bolt with the same thread diameter. Metric M10, M12, and M14 fasteners are especially sensitive to the product standard and head-size family. The same nominal thread can appear with a different across-flats dimension in an older DIN or JIS design, a flange-head product, or a heavy-hex product.

How to use the chart

1. Read the thread designation on the fastener or drawing. Examples include M8 × 1.25, 1/4-20 UNC, and 5/16-24 UNF.
2. Identify the head or nut family. A standard hex cap screw, flange bolt, socket-head cap screw, and heavy-hex structural bolt do not share one universal wrench table.
3. Use the across-flats value as the nominal wrench or socket opening.
4. Check the standard, grade, and supplier drawing when fit, clearance, torque tooling, or production interchangeability matters.

Thread size, wrench size, and socket drive size are different

The thread size describes the fastener’s outside thread diameter in the selected system. The wrench size describes the distance across two opposite hex flats. The socket drive size describes the square drive used by a ratchet, such as 1/4 in, 3/8 in, or 1/2 in. A 1/2 in drive socket can have a 3/4 in hex opening, so “1/2 in socket” can mean two different things in a workshop conversation.

Why metric wrench sizes can differ

Current ISO product standards and older national standards are not interchangeable lookup columns. A public fastener reference may show, for example, 16 mm and 17 mm options for M10 depending on whether the row represents an ISO or older DIN family. Label the standard beside the row instead of silently merging them.

For the same reason, do not use a close SAE size as a substitute for a metric size. A tool that appears to fit can round the corners, slip under torque, or fail to seat because of plating, wear, or tolerance.

Reverse lookup: start with the tool size

The reverse lookup should return possible fasteners, not a single guaranteed answer. A 13 mm tool points to M8 in the ISO 4017 rows. A 1/2 in tool points to 5/16 in UNC and UNF ASME-style hex cap screws. The result must show the standard family and mark the match as “possible” when more than one product family uses the same opening.

Related Mechanitec references

• Metric Socket Sizes Chart — use this for socket inventory and metric socket conversion.
• SAE Socket Sizes Chart — use this for inch socket inventory and conversion.
• How to Use SOLIDWORKS Toolbox — related CAD hardware workflow.

Source note

The draft uses the public source records listed in the accompanying provenance file. Standards and manufacturer tables can change by product family, edition, and revision. Verify production-critical dimensions against the applicable current standard or supplier drawing.

Frequently asked questions

What wrench fits an M8 bolt?

A common ISO-style M8 hex bolt uses a 13 mm wrench or socket. Confirm the head standard before using that value for a special, flange, or legacy fastener.

What wrench fits a 1/2 inch bolt?

A common ASME-style 1/2-13 hex cap screw uses a 3/4 in wrench or socket. The thread diameter is 1/2 in; the across-flats size is 3/4 in.

Is a socket-drive size the same as the bolt size?

No. The drive size is the square connection to the ratchet. The socket opening is the hex size that fits the fastener.

Can I use the nearest metric or SAE wrench?

Do not treat a near match as an approved substitute. Use the correct nominal size and verify the fastener standard, especially when tightening torque or damage risk matters.