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Button Head Socket Screw Dimensions (ISO 7380-1, M3 to M16)

Guide · Updated October 28, 2026

ISO 7380-1 covers hexagon socket button head screws, the low domed-head screws used on guards, covers and fixtures where a socket head cap screw would stick up or snag. The table gives coarse pitch, head diameter dk, head height k, nominal hex key size s and minimum socket depth t from M3 to M16. Values agree between the ISO 7380-1 table, the PartXT ISO 7380 table and the Aspen Fasteners ISO 7380 spec sheet. Check the current edition or your supplier for critical work.

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Step by step

Read the thread: Measure the screw diameter and check pitch with a thread gage. ISO 7380-1 covers coarse threads M3 to M16 only.
Pick the hex key: Button heads take one key size smaller than a socket head cap screw of the same thread: M6 is 4 mm (SHCS 5 mm), M8 is 5 mm (SHCS 6 mm).
Check head clearance: Head diameter dk is about 1.75 × d and height k about 0.55 × d. An M6 head is 10.5 mm across and 3.3 mm tall.
Size a counterbore if needed: For a flush fit, bore about 1 mm over dk and 0.3 to 0.5 mm deeper than k max.
Torque it lower: The shallow socket and low head carry about 80% of a full screw. Use roughly 60 to 80% of the SHCS torque for the same grade.
Inspect the socket: Shallow sockets cam out. Use a good ball-free key fully seated and replace rounded screws.

ISO 7380-1 button head dimensions (mm)

Columns are: thread size, coarse pitch P, dk max, dk min, k max, k min, hex key s (nominal) and socket depth t min. All sizes are product grade A. Fine threads and sizes above M16 are not in ISO 7380-1.

Size | P | dk max | dk min | k max | k min | s | t min
M3 | 0.5 | 5.70 | 5.40 | 1.65 | 1.40 | 2 | 1.04
M4 | 0.7 | 7.60 | 7.24 | 2.20 | 1.95 | 2.5 | 1.30
M5 | 0.8 | 9.50 | 9.14 | 2.75 | 2.50 | 3 | 1.56
M6 | 1 | 10.50 | 10.07 | 3.30 | 3.00 | 4 | 2.08
M8 | 1.25 | 14.00 | 13.57 | 4.40 | 4.10 | 5 | 2.60
M10 | 1.5 | 17.50 | 17.07 | 5.50 | 5.20 | 6 | 3.12
M12 | 1.75 | 21.00 | 20.48 | 6.60 | 6.24 | 8 | 4.16
M16 | 2 | 28.00 | 27.48 | 8.80 | 8.44 | 10 | 5.20

Button head vs socket head cap screw

Compare an ISO 4762 socket head cap screw of the same thread. M6: button head 10.5 mm across and 3.3 mm tall with a 4 mm key; SHCS 10.0 mm across and 6.0 mm tall with a 5 mm key. M10: button 17.5 × 5.5 mm, 6 mm key; SHCS 16.0 × 10.0 mm, 8 mm key. The button head is wider and about half as tall, and its socket is shallow, so it takes less torque and less tensile load.

Hex key size quick list

M3 2 mm, M4 2.5 mm, M5 3 mm, M6 4 mm, M8 5 mm, M10 6 mm, M12 8 mm, M16 10 mm. Keep a separate short-arm set for button heads so nobody grabs the cap screw key and rounds a socket.

Worked example: cover plate on a mill table guard

A 3 mm aluminum guard bolts to a tapped bracket with M5 screws. A socket head would stand 5 mm proud and catch chips and gloves. An M5 button head sits only 2.75 mm tall and spreads the load over a 9.5 mm head, so no washer is needed on the thin sheet. Use a 3 mm key and snug it, roughly 4 N·m for a class 010.9 screw into steel threads, less into aluminum.

Worked example: flush counterbore for an M8 button head

dk max is 14.00 mm and k max is 4.40 mm. Bore a 15 mm counterbore 4.8 mm deep over a 9 mm clearance hole. The head then sits about 0.4 mm below the surface with 0.5 mm side clearance. If depth is not limited, a standard SHCS counterbore is usually the stronger choice.

Common problems

Socket rounds out: wrong key (cap screw size), worn key, or overtorque. Use the correct key fully seated and lower the torque.
Head pops off: button heads used in a high-load joint. Switch to ISO 4762 socket head cap screws.
Head too wide for the slot: dk is about 1.75 × d. Check a narrower low-head screw such as DIN 7984.
Won't sit flat: no radius clearance at the hole edge. Break the edge or chamfer the clearance hole.

FAQ

What size Allen key fits an M6 button head?

4 mm. An M6 socket head cap screw takes 5 mm, so the keys are not interchangeable.

How tall is an M8 button head?

ISO 7380-1 head height k is 4.10 to 4.40 mm, and the head is 13.57 to 14.00 mm across.

Are button head screws as strong as socket head cap screws?

No. ISO 7380-1 rates them as reduced loadability; the head is assumed to carry 80% of the thread load, and they are marked with a leading zero such as 010.9.

Is ISO 7380 the same as the old flanged button head?

No. ISO 7380-1 is the plain button head. The flanged version is ISO 7380-2.

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