Metric bolt size chart: heads, keys and lengths for every screw standard
Hex bolts, socket head cap screws, countersunk, button head, set screws, threaded rod and more, all with coarse metric threads. The charts below compare the head sizes and the lengths across the most used standards; each standard has its own page with the full table and a STEP, DXF and DWG file for every size and length.
Metric bolt standards: ISO and DIN screw types
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ISO 4762 / DIN 912 socket head cap screwHexagon socket head cap screw with coarse metric thread, from M3 to M24. -
ISO 4017 / DIN 933 hex screw, fully threadedHexagon head screw threaded up to the head, from M3 to M24. -
ISO 4014 / DIN 931 hex bolt, partially threadedHexagon head bolt with a plain shank and a thread length b that depends on the bolt length. -
ISO 10642 countersunk socket head screwHexagon socket countersunk head screw with a 90° head. -
DIN 7991 countersunk socket head screwHexagon socket countersunk head screw to the withdrawn DIN 7991, still the one most catalogues sell, from M3 to M24. -
ISO 7380-1 button head socket screwHexagon socket button head screw with a low domed head, fully threaded. -
ISO 7380-2 button head socket screw with collarButton head socket screw with an integral collar under the head, which spreads the load like a washer. -
DIN 7984 low head socket cap screwSocket head cap screw with a lower head and smaller socket than ISO 4762, for tight spaces. -
DIN 6912 low head socket cap screw with pilot recessLow head socket cap screw with a pilot recess below the hexagon socket. -
ISO 4026 / DIN 913 set screw, flat pointHexagon socket set screw with flat point, from M3 to M24. -
ISO 4027 / DIN 914 set screw, cone pointHexagon socket set screw with cone point, from M3 to M24. -
ISO 4029 / DIN 916 set screw, cup pointHexagon socket set screw with cup point, from M3 to M24. -
ISO 7045 / DIN 7985 pan head screw, cross recessPan head machine screw with cross recess, from M1.6 to M10, fully threaded. -
ISO 7046 / DIN 965 countersunk screw, cross recessCountersunk head machine screw with cross recess and a 90° head, from M1.6 to M10. -
DIN 603 carriage boltRound head bolt with a square neck that locks into wood or a square hole, from M5 to M16. -
DIN 6921 hex flange screwHexagon head screw with an integral flange under the head, from M5 to M16. -
DIN 571 hexagon head wood screw (coach screw)Hexagon head wood screw with a plain shank and a pointed wood thread. -
DIN 7981 pan head tapping screw, cross recessPan head tapping screw with ST thread and pointed end (type C), from ST2.2 to ST6.3. -
DIN 975 threaded rodFully threaded rod with coarse metric thread, sold in 1 m lengths, from M2 to M52. -
DIN 976-1 threaded studFully threaded stud bolt to DIN 976-1, in the stock lengths of each size. -
DIN 580 lifting eye boltForged eye bolt with collar for lifting, from M8 to M36.
Metric bolt head size chart in mm
Across flats (s) is the spanner size of a hex head, and for a socket head it is the Allen key. Head diameter (dk) and head height (k) are the maximum values of each standard. Select a standard in the header to see all its dimensions.
| Thread | Pitch Pcoarse, mm | Hex head sISO 4017, mm | Hex head kISO 4017, mm | Socket head dkISO 4762, mm | Socket head kISO 4762, mm | Allen key sISO 4762, mm | Countersunk dkISO 10642, mm | Button head dkISO 7380-1, mm |
|---|---|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 | 2 | 5.5 | 3 | 2.5 | 6.72 | 5.7 |
| M4 | 0.7 | 7 | 2.8 | 7 | 4 | 3 | 8.96 | 7.6 |
| M5 | 0.8 | 8 | 3.5 | 8.5 | 5 | 4 | 11.2 | 9.5 |
| M6 | 1 | 10 | 4 | 10 | 6 | 5 | 13.44 | 10.5 |
| M8 | 1.25 | 13 | 5.3 | 13 | 8 | 6 | 17.92 | 14 |
| M10 | 1.5 | 16 | 6.4 | 16 | 10 | 8 | 22.4 | 17.5 |
| M12 | 1.75 | 18 | 7.5 | 18 | 12 | 10 | 26.88 | 21 |
| M14 | 2 | 21 | 8.8 | 21 | 14 | 12 | – | – |
| M16 | 2 | 24 | 10 | 24 | 16 | 14 | 33.6 | 28 |
| M18 | 2.5 | 27 | 11.5 | – | – | – | – | – |
| M20 | 2.5 | 30 | 12.5 | 30 | 20 | 17 | 40.32 | – |
| M22 | 2.5 | 34 | 14 | – | – | – | – | – |
| M24 | 3 | 36 | 15 | 36 | 24 | 19 | – | – |
A dash means the standard has no such size. The older DIN 933 and DIN 931 hex heads are one millimetre larger at M10, M12 and M14 (17, 19 and 22 mm) and two smaller at M22 (32 mm).
Metric bolt length chart: shortest and longest lengths
Lengths available here with a CAD file. They are the lengths that the commercial series and the stock tables agree on, from the shortest to the longest for each thread size. The step between lengths is given on each standard's page.
| Thread | Hex screw, shortestISO 4017, mm | Hex screw, longestISO 4017, mm | Hex bolt, shortestISO 4014, mm | Hex bolt, longestISO 4014, mm | Socket head, shortestISO 4762, mm | Socket head, longestISO 4762, mm |
|---|---|---|---|---|---|---|
| M3 | 5 | 30 | 20 | 30 | 5 | 30 |
| M4 | 6 | 40 | 25 | 40 | 6 | 40 |
| M5 | 8 | 50 | 25 | 50 | 8 | 50 |
| M6 | 12 | 60 | 30 | 60 | 10 | 60 |
| M8 | 16 | 80 | 40 | 80 | 12 | 80 |
| M10 | 20 | 100 | 35 | 180 | 16 | 100 |
| M12 | 25 | 120 | 40 | 180 | 20 | 120 |
| M14 | 25 | 140 | 50 | 150 | 25 | 140 |
| M16 | 30 | 150 | 65 | 160 | 25 | 160 |
| M18 | 35 | 200 | 70 | 180 | – | – |
| M20 | 40 | 160 | 80 | 200 | 30 | 100 |
| M22 | 45 | 200 | 80 | 220 | – | – |
| M24 | 50 | 180 | 90 | 240 | 40 | 200 |
How to measure a metric bolt
Diameter. Measure the outside of the thread with a caliper. The result is usually a little under the nominal size, because the tolerance of an external thread sits below it: an M8 bolt reads somewhere under 8 mm, never over. The nominal diameter is the number after the M.
Pitch. Use a thread gauge, or measure the length of ten threads and divide by ten. Coarse pitch is the default and is left out of the designation, so M8 means M8 × 1.25. A fine thread is always written in full, as in M8 × 1.
Length. For hex, socket, pan and button heads, measure from the underside of the head to the end of the screw. For countersunk screws the length includes the head, because the head ends flush with the surface. Set screws and threaded rod are measured end to end.
Hex bolt and Allen bolt head sizes: DIN and ISO
When the German DIN standards for hex bolts and nuts were replaced by ISO, four spanner sizes changed: M10 went from 17 to 16 mm, M12 from 19 to 18 mm, M14 from 22 to 21 mm and M22 from 32 to 34 mm. The rest of the sizes kept the same head. Both versions are still sold, so a hex screw marked DIN 933 and one marked ISO 4017 can need different spanners. On the ISO 4017 / DIN 933 page you can switch between the two and download each one.
Socket heads did not change. An ISO 4762 screw takes a 5 mm key at M6, 6 mm at M8 and 8 mm at M10, the same as DIN 912. Low head and button head screws use a smaller key for the same thread: a DIN 7984 or ISO 7380-1 screw at M6 takes a 4 mm key.
Metric bolt sizes: common questions
- Is M12 the same as 12 mm?
- M12 means an ISO metric thread with a nominal outside diameter of 12 mm. A caliper will read slightly less on the bolt, because the thread tolerance sits below the nominal size. The head is not 12 mm: an ISO 4017 hex head at M12 measures 18 mm across flats.
- What is the pitch of an M24 thread?
- The coarse pitch of M24 is 3 mm, which is the pitch used when the designation says only M24. Fine-pitch M24 threads, such as M24 × 2, are always written with the pitch.
- What is the difference between a hex screw and a hex bolt?
- In the ISO standards a hex screw (ISO 4017, formerly DIN 933) is threaded up to the head, while a hex bolt (ISO 4014, formerly DIN 931) has a plain shank under the head and is threaded only for part of its length. The head dimensions are the same.
- What size Allen key does an M8 cap screw take?
- A 6 mm key for an ISO 4762 or DIN 912 socket head cap screw. Low head (DIN 7984) and button head (ISO 7380) screws at M8 take a 5 mm key.