ENGINEERING TOOL
Hex Key Size Chart (Allen Wrench) — Which Key Fits Which Socket Head Cap Screw
Family index of the hex key size charts: for every socket head cap screw size from M3–M24 the chart lists the head diameter, head height and hexagon socket width across flats s — and s is the hex key (Allen wrench) size you need, with its long-arm L1 and short-arm L2 lengths on the ISO 2936 / GB/T 5356-2021 standard series. Dimensions come from the same source as the per-size pages; each size opens its own page.
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Size Index (M3–M24)
Click a size for its dedicated chart (head dimensions, matching key arms, minimum test torque and unit weight by length). d = nominal thread diameter, dk = head diameter, k = head height, s = hexagon socket width across flats = key size. Two screw sizes can share one key size: M16/M18 both take s=14, M20/M22 both take s=17.
Basis & Disclosed Assumptions
Where the numbers come from
- Head dimensions (dk / k) and the hexagon socket width across flats s are taken from the ISO 4762 / GB/T 70.1-2008 series (or, where the series has no such size, the DIN 912 basis) — the same tables the product catalog and the per-size pages use, so values can be cross-checked.
- The key size you need is s itself: the socket width across flats is what the key has to fit. Arm lengths L1 / L2 come from the ISO 2936 / GB/T 5356-2021 standard series (Table 1), and hex keys are sold by that s value rather than by the screw size M — which is why M16/M18 share one key and M20/M22 another.
- Sizes and arm lengths are nominal (maximum) values from the standards; the standards state them as a tolerance band, so a real key may be shorter than the value listed.
Scope & limits
- This index answers one question: which hex key size fits a given cap screw. It is a selection aid, not a tolerance or acceptance specification.
- Fits also depend on socket tolerance and on how the key was made — an undersized key in a wide socket rounds off more easily. Where torque matters, work from the screw's property class and the tightening torque, not from the key size alone.
- Hex keys are also specified in long (L) and extra-long (XL) variants: only the long arm grows, and the standard defines no long or extra-long value at s=19. This table lists the standard series, which covers every size.
References
Need the weight, or a length outside the chart?
The weight calculator covers socket head cap screws with size, length and material inputs — unit weight, batch weight and pieces per tonne in one pass, on the same engine as the product catalog.
Open the weight calculatorCite this chart
YAXIIO. Hex Key Size Chart (Allen Wrench) — Which Key Fits Which Socket Head Cap Screw. https://www.yaxiio.com/en/tools/hex-key-size-chart (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
M16 and M18 take the same hex key — how do I tell which key size I need?
You select the key by the socket, not by the screw's nominal diameter. M16 and M18 both have a 14 mm socket, and M20 and M22 both have 17 mm — so one key covers both sizes in each pair. Measure the socket across flats or read the drawing when the screw size alone leaves two candidates.
Why do M18 and M22 appear here when they are not standard ISO 4762 / GB/T 70.1 sizes?
The ISO 4762 / GB/T 70.1-2008 series runs M1.6–M64 and does not include M18 / M22; those two sizes are listed on the DIN 912 basis, where they appear bracketed as non-preferred. They are kept here because they are still drawn and ordered, and the pages say so on the size itself.
Related References
Bolt Weight Calculator
Use it for lengths and materials outside the chart, and for whole-order totals; the same engine and basis as the per-size pages.
Wrench & Socket Size Chart
The external counterpart: which spanner or socket opening fits the hex head, the nut, the flange nut and the carriage bolt neck at each size.
Tap Drill Size Chart
The thread underneath the same screw: tapping drill sizes and the nearest available drill, metric and inch series.
Socket Head Cap Screw Product Catalog
ISO 4762 / DIN 912 / GB/T 70.1 socket head cap screws — request a quote by size, length and property class.
Disclaimer: this chart provides engineering reference data for selection and quotation only; it is not a design basis or an acceptance standard.