Wrench Size Chart: Bolt to Socket, Inch and Metric

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The wrench or socket that fits a bolt is set by the width of its hex head across the flats, not by the bolt’s thread size. A 1/2 in bolt takes a 3/4 in wrench; an M10 bolt takes a 16 mm wrench under ISO, but 17 mm under the older DIN standard still common on machinery. These charts give the across-flats size for standard inch hex bolts and heavy hex bolts and for metric hex bolts in both ISO and DIN sizes, with the metric-inch conversion so you can see which substitutes work.

Inch hex bolts: wrench sizes

Bolt diameterHex cap screw / bolt, across flatsIn mmHeavy hex, across flats
1/4″7/16″11.1 mm—
5/16″1/2″12.7 mm—
3/8″9/16″14.3 mm—
7/16″5/8″15.9 mm—
1/2″3/4″19.0 mm7/8″
9/16″13/16″20.6 mm—
5/8″15/16″23.8 mm1-1/16″
3/4″1-1/8″28.6 mm1-1/4″
7/8″1-5/16″33.3 mm1-7/16″
1″1-1/2″38.1 mm1-5/8″
1-1/8″1-11/16″42.9 mm1-13/16″
1-1/4″1-7/8″47.6 mm2″
1-1/2″2-1/4″57.1 mm2-3/8″
Across-flats widths of standard hex cap screws and heavy hex bolts in the ASME B18.2.1 series, as widely published. Standard hex nuts generally match the cap screw size; heavy hex nuts are 1/8 in larger.

Metric hex bolts: wrench sizes

BoltISO (ISO 4014/4017)In inchesDIN 933 if different
M35.5 mm0.217 insame
M47 mm0.276 insame
M58 mm0.315 insame
M610 mm0.394 insame
M813 mm0.512 insame
M1016 mm0.630 in17 mm
M1218 mm0.709 in19 mm
M1421 mm0.827 in22 mm
M1624 mm0.945 insame
M1827 mm1.063 insame
M2030 mm1.181 insame
M2234 mm1.339 in32 mm
M2436 mm1.417 insame
M2741 mm1.614 insame
M3046 mm1.811 insame
Across-flats widths of ISO and DIN hex bolts. ISO changed four sizes (M10, M12, M14, M22) in the 1990s; DIN sizes are still common on older and European equipment.

Metric and inch substitutes

Some metric and inch sockets are close enough to swap, and many are not. A 13 mm socket is 0.512 in across, a hair over 1/2 in, so it will turn a 1/2 in hex but slip on a worn one. 19 mm (0.748 in) and 3/4 in (0.750 in) are practically identical, as are 8 mm and 5/16 in (0.315 against 0.3125 in). By contrast 10 mm (0.394 in) falls between 3/8 and 13/32, and 14 mm (0.551 in) between 1/2 and 9/16, so neither has a good inch substitute. A loose socket rounds the corners of the hex; when in doubt use the exact size and a 6-point socket.

Identifying a bolt from its head

  • Measure across two parallel flats with calipers. If the reading is a round inch fraction (9/16, 3/4), it is an inch bolt; if it is a whole number of millimetres (13, 17), it is metric.
  • Read the head markings: radial lines (three for Grade 5, six for Grade 8) mean an inch bolt; numbers such as 8.8 or 10.9 mean metric.
  • Confirm with the thread: measure the diameter and pitch and look them up in the UNC/UNF or metric chart.

Frequently asked questions

What size wrench for a 1/2 inch bolt?

3/4 in for a standard hex bolt or cap screw; 7/8 in for a heavy hex bolt.

What size socket for an M10 bolt?

16 mm for ISO hex bolts, 17 mm for DIN 933 bolts, which are still common on older and European machinery.

What size wrench for a 3/8 bolt?

9/16 in.

What size socket for an M12 bolt?

18 mm (ISO) or 19 mm (DIN). A 3/4 in socket fits a 19 mm head.

Sources and method

Across-flats widths are listed from the standards as published by fastener suppliers; conversions are ours.

  1. ASME B18.2.1, Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws (Inch Series): nominal across-flats widths, as widely published in supplier tables; not reproduced.
  2. ISO 4014 and ISO 4017 (hexagon head bolts and screws) and DIN 933: across-flats widths, as widely published.
  3. Conversions: inch = mm ÷ 25.4 (our arithmetic).

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Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.