Bolt, Nut and Spanner Size Chart (Metric)

The spanner size of a bolt or nut is the width across flats (AF) of its hexagon, in mm. It is not the thread size: an M12 bolt takes an 18 mm or 19 mm spanner. This chart gives the spanner, socket and Allen key sizes for metric bolts, nuts and socket screws from M3 to M36.

Metric Spanner and Allen Key Size Chart

How to read the chart: all sizes in mm. ISO hex means ISO 4014 / ISO 4017 bolts and ISO 4032 nuts. DIN hex means DIN 931 / DIN 933 bolts and DIN 934 nuts. Allen key sizes are for socket head cap screws to ISO 4762 / DIN 912, countersunk socket screws to ISO 10642 / DIN 7991 and button head screws to ISO 7380. A dash means the standard does not cover that size. A value marked DIN 912 or DIN 7991 is only in that older DIN standard, not in the ISO one.

ISO and DIN Hex Sizes: Four Sizes Differ

ISO 4014, ISO 4017 and ISO 4032 have replaced the old DIN 931, DIN 933 and DIN 934, and four spanner sizes changed:

• M10: 16 mm (ISO) instead of 17 mm (DIN)
• M12: 18 mm instead of 19 mm
• M14: 21 mm instead of 22 mm
• M22: 34 mm instead of 32 mm

All other sizes are the same. Bolts and nuts to the DIN sizes are still widely sold, so one job can have both. Check the head before you choose the spanner, and keep both 16 / 17 mm and 18 / 19 mm spanners in the kit. ISO nuts are also a little taller than DIN nuts, for example 10.8 mm against 10 mm in M12.

High strength structural bolts with large heads are an exception to this chart. EN 14399-4 HV bolts, for example, take a 27 mm spanner in M16 and 32 mm in M20, so check the bolt standard on the drawing.

Why a Loose Spanner Rounds Off Nuts

A spanner of the right size grips the flats of the hexagon over a fair area. A spanner even 1 mm too big, such as a 17 mm spanner on a 16 mm ISO M10 head, turns a little before it bites and then touches only the corners. The whole force then goes through the corners, the steel there is crushed and pushed over, and after a few turns the corners are round and the spanner slips. Worn spanners with spread jaws do the same.

• Use the exact size: 16 mm on an ISO M10 head, 17 mm on a DIN M10 head. Never use the next size up.
• Use a ring spanner or a 6-point socket on tight or rusted nuts. A 6-point socket grips the flats away from the corners; open-end spanners and 12-point sockets grip closer to the corners.
• Do not use inch spanners on metric bolts. Some inch sizes are close but not equal: 11/16 inch is 17.46 mm, loose on a 17 mm head.
• Replace Allen keys with rounded corners, and push the key fully into the socket before turning.

Allen Key Sizes for Socket Screws

Socket head cap screws (Allen bolts) take a bigger key than countersunk and button head screws of the same thread. An M8 cap screw takes a 6 mm key, while M8 countersunk and button head screws take 5 mm. Because of the smaller key and the head shape, countersunk and button head screws cannot take as much torque as a cap screw, so use a cap screw for heavy joints.

Bolts, Nuts and Socket Screws from JMIKS Fasteners

JMIKS Fasteners supplies hex bolts, hex nuts, Allen cap screws, Allen CSK screws and button head screws in SS 304, SS 316 and SS 202 and in mild steel and high tensile grades from 4.6 to 12.9. Stainless steel bolts and nuts have the same spanner sizes as steel ones. See the bolt torque chart for tightening values, the hex bolt weight chart for weights and the bolt grade chart for grades.

Spanner Size FAQs

What spanner size fits an M10 bolt?

16 mm for ISO 4017 / ISO 4014 bolts and ISO 4032 nuts, and 17 mm for DIN 933 / DIN 931 bolts and DIN 934 nuts. Check which type you have before tightening.

What size Allen key fits an M8 Allen bolt?

A 6 mm key fits an M8 socket head cap screw to ISO 4762 / DIN 912. M8 countersunk and button head socket screws take a 5 mm key.

Is the spanner size the same as the bolt size?

No. M12 means a thread diameter of 12 mm. The spanner size is the width across flats of the hexagon: 18 mm for an ISO M12 bolt or nut and 19 mm for a DIN one.

What spanner fits M16, M20 and M24 bolts?

24 mm, 30 mm and 36 mm. These three sizes are the same in ISO and DIN.

Do stainless steel bolts take the same spanner as steel bolts?

Yes. Stainless steel bolts and nuts made to the same standard have the same across-flats size as steel ones, so the same spanner fits.

Get a Quote for Bolts, Nuts and Allen Screws

JMIKS Fasteners is a fastener wholesaler and supplier in Nava Odhav, Ahmedabad. Send the size, standard (ISO or DIN), grade, finish and quantity on WhatsApp (+91 90333 11425) or through the enquiry form, or download the catalogue (PDF). We reply within one working day.