Engineering Tools · Hex Key Reference
M6 Socket Head Cap Screw: Which Hex Key Size Fits?
M6 Socket Head Cap Screw Head Dimensions (unit mm)
Socket head cap screw form: a cylindrical head with a hexagon socket in the top face, driven with a hex key from inside the socket. It tightens in counterbores and confined spaces and is common in machine tools, moulds, fixtures, automation equipment and structural joints.
| Nominal diameter d | 6 mm |
| Coarse pitch P (ISO 724 / GB/T 196 series) | 1 mm |
| Head diameter dk (max) | 10 mm |
| Head height k (max) | 6 mm |
| Hexagon socket width across flats s (key size) | 5 mm |
| Thread length b (partly threaded basis) | 2d+6 for L<=125, 2d+12 for 125<L<=200, 2d+25 for L>200 (b <= L) |
- Dimensions follow the ISO 4762 / GB/T 70.1-2008 socket head cap screw series (GB/T 70.1-2008 is the edition used, max nominal values; head diameter is the plain-head max and head height k is taken from that series) as the primary source; ISO 4762 is the equivalent international standard, cited for series reference only.
- The ISO 4762 / GB/T 70.1-2008 size series runs from M1.6 to M64 and does not include M18 or M22 — the M18 and M22 pages here are given on the DIN 912 basis (that German series includes M18 and M22, bracketed there as non-preferred sizes).
- The other eleven sizes are on the ISO 4762 / GB/T 70.1 basis (including M14, also bracketed as non-preferred there).
- The socket width across flats s is a nominal size; actual fit also depends on the socket tolerance and the key’s manufacturing tolerance. Tolerances, property classes and surface finish are agreed separately against the buyer’s drawing or contract.
References
- GB/T 70.1 Socket Head Cap Screws (National Public Service Platform for Standards Information)
- GB/T 5356-2021 Hexagon Socket Screw Keys (National Public Service Platform for Standards Information; issued 2021-11-26, effective 2022-06-01, replacing GB/T 5356-2008)
- GB/T 5356-2021 Table 1 (arm lengths) and Table 2 (minimum hardness, minimum test torque) — table values transcribed row by row with page references (PDF p.6–7 / p.8–9)
- GB/T 196 General Purpose Metric Screw Threads - Basic Dimensions, coarse pitches (same table as fastenerCalc.pitchOf)
- DIN 912 hexagon socket head cap screw dimension table (the basis for the M18 and M22 pages, which are outside the GB/T 70.1-2008 / ISO 4762 size series; three-way cross-check across kovafasteners / fullerfasteners / fasten.it, second-level sources)
- Unit weights cross-checked row by row against socket head cap screw SKU spec_json weightCalc in the product catalog (same engine and basis as the product pages)
Full PDF
Download the full PDF (M3–M24, all sizes)
Printable chart covering every size in this family — enter your email to get it.
Other Sizes
Matching Hex Key Dimensions (s=5 mm)
- The socket width across flats s of the screw is the key size you need: M6 takes a 5 mm key.
- The table below gives the two arm lengths of the 5 mm key (long arm L1 / short arm L2) on the ISO 2936 / GB/T 5356-2021 standard length series.
- That standard also defines a long (L) and an extra long (XL) variant, where only the long arm L1 grows while the short arm L2 is shared across all three, and it specifies no long or extra long value at s=19 — so this chart tabulates the standard series, which covers every size.
- The standard states the arm lengths as a maximum/minimum tolerance band; the values here are the maximum (nominal), and a real key may measure down to the lower limit.
| Across flats s (mm) | Long arm l₁ (mm) | Short arm l₂ (mm) | Min. test torque M_d (N·m) | Min. hardness (HRC) |
|---|---|---|---|---|
| 5 | 85 | 33 | 30 | 52 |
- ISO 2936 / GB/T 5356-2021 also specifies for each key size a minimum hardness and a minimum test torque M_d — an acceptance test value the key itself must withstand without permanent deformation or damage.
- M_d is not a tightening torque for the screw, and it is not a usable working limit for the key: the torque a key actually transmits depends on wear, material batch and how the load is applied, and is usually lower than the test value.
- Both values are indexed by the key’s width across flats s, which is not the same dimension as the screw size M (M16/M18 share s=14, M20/M22 share s=17).
Why screw size is not key size
The nominal diameter d of a socket head cap screw is not the same number as the key size it needs: a M6 screw has d=6 mm but a socket width across flats of s=5 mm, a difference of 1 mm. Buying a key by nominal screw diameter is the most common way to get the wrong one. On top of that, a hex key drives on six faces from inside a closed socket, so the usable clearance is much smaller than for a wrench gripping two faces from outside - which is why an imperial key pushed into a metric socket often enters but then rounds the corners. Always pick the metric size that matches s by name rather than an imperial key that is merely close.
M6 Length Series: Plain Shank, Thread and Unit Weight
- Unit weights are computed from the socket head cap screw configuration: full-diameter plain shank plus a pitch-diameter threaded section plus the cylindrical head (with the hexagon socket reduction) — identical basis to the spec_json weightCalc of the socket head cap screw SKUs in our product catalog (same parameters, same formula, fully reproducible).
- Thread length b follows the same partly threaded rule as the catalog in three bands (2d+6 for L≤125, 2d+12 for 125<L≤200, 2d+25 for L>200, capped at the shank length — fully threaded b = L when the shank is shorter).
| Nominal length L (mm) | Thread b (mm) | Plain shank (mm) | Unit weight (g/pc) | Bulk (kg/1,000 pcs) |
|---|---|---|---|---|
| M6×10 | 10 | 0 | 4.91 | 4.91 |
| M6×12 | 12 | 0 | 5.26 | 5.26 |
| M6×16 | 16 | 0 | 5.97 | 5.97 |
| M6×20 | 18 | 2 | 6.77 | 6.77 |
| M6×25 | 18 | 7 | 7.88 | 7.88 |
| M6×30 | 18 | 12 | 8.98 | 8.98 |
| M6×35 | 18 | 17 | 10.1 | 10.1 |
| M6×40 | 18 | 22 | 11.2 | 11.2 |
| M6×45 | 18 | 27 | 12.3 | 12.3 |
| M6×50 | 18 | 32 | 13.4 | 13.4 |
| M6×60 | 18 | 42 | 15.6 | 15.6 |
| M6×70 | 18 | 52 | 17.9 | 17.9 |
- Configuration basis (same source as the product catalog): shank = plain section π/4·d²·(L−b) plus threaded section π/4·d₂²·b (d₂ = d − 0.6495P, ISO basic profile pitch diameter); head = π/4·dk²·k × 0.85 (0.85 is the volume reduction factor for the hexagon socket cavity and top chamfer, the same value as the catalog headFactor); multiplied by 7.85 g/cm³ for carbon steel.
- Rounded to avoid false precision (display-layer tiered decimals: at least 2 decimals below 1 kg, 1 decimal from 1 to 10 kg, whole kg at 10 kg and above).
- Lengths listed are the common range; other lengths are made to order.
Material Density Conversion
Table values are for carbon steel (7.85 g/cm3); other materials convert by density ratio: weight x factor (geometry unchanged). Socket head cap screws are commonly supplied in A2-70 (304) / A4-80 (316) stainless and in alloy steel.
| Material | Density (g/cm3) | Conversion Factor |
|---|---|---|
| Carbon Steel | 7.85 | ×1.000 |
| 304 Stainless (A2) | 7.93 | ×1.010 |
| 316 Stainless (A4) | 7.98 | ×1.017 |
| Brass | 8.5 | ×1.083 |
Calculation Basis and Assumptions Disclosed
Calculation basis
- Dimensions come from fastenerCalc.SHCS_DIMS (ISO 4762 / GB/T 70.1 socket head cap screws, max nominal values) and fastenerCalc.HEX_KEY_DIMS (ISO 2936 / GB/T 5356-2021 hex key standard series arm lengths, max nominal values).
- Unit weights are computed by fastenerCalc.socketHeadWeightG with the same parameters and formula as the spec_json weightCalc (method=shcs_formula) of the socket head cap screw SKUs in our product catalog: full-diameter plain shank plus pitch-diameter threaded section (d₂ = d − 0.6495P) plus head π/4·dk²·k×0.85, at 7.85 g/cm³ for carbon steel, base metal only, coating excluded.
- Thread length b follows the same rule as the catalog (2d+6 for L≤125, 2d+12 for 125<L≤200, 2d+25 for L>200, capped at the shank length).
Scope & limits
- Unit weights are theoretical geometry estimates (excluding coatings and any part-to-part variation in the socket cavity and chamfer) for quotation, freight and packing budgets; trade settlement is against actual measurement, theoretical estimate tolerance ±5%.
- Head dimensions are max nominal values and actual parts vary within tolerance — this chart is not an acceptance basis.
- The lengths listed are the common range, and unlisted lengths are made to order.
- The socket width across flats s is nominal, and the fit varies with the socket tolerance and the key’s tolerance band — where rounding risk is high, use a higher-grade key or replace a worn one.
Cite this chart
YAXIIO. M6 Socket Head Cap Screw: Which Hex Key Size Fits?. https://www.yaxiio.com/en/tools/hex-key-size-chart/m6 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
How do I determine the hex key size for a socket head cap screw?
Measure the socket, not the key: use a caliper across two opposite flats of the hexagon socket (the width across flats, s) and the millimetre reading is the key size you need. Trial fitting also works - a key that engages all the way round, drives the screw and has no noticeable wobble is the right size. Looking it up is faster: a M6 socket head cap screw (ISO 4762 / GB/T 70.1) has a socket width across flats of s=5 mm, so it takes a 5 mm hex key. Do not mix this up with the width across flats of a hex head bolt: that is the wrench or socket opening size, a different series from the hexagon socket s.
Are an Allen key and a hex key the same thing?
Yes - the same tool under different names. Hex key names the shape (hexagon); Allen key and Allen wrench carry the early brand name into generic use. All of them mean the L-shaped (or T-handle, or ball-end) driver that goes into the hexagon socket. This site uses hex key throughout, and the size selection rule does not change with the name.
What standard covers socket head cap screws, and how do ISO 4762, DIN 912 and GB/T 70.1 differ?
All three — ISO 4762, DIN 912 and GB/T 70.1 — describe the same form (socket head cap screw): ISO 4762 is the ISO standard, DIN 912 the German standard and GB/T 70.1 the Chinese national standard, with substantially the same head dimensions and hexagon socket series. The differences sit in tolerance bands, property classes and how surface finish is specified. One size-series difference is worth knowing: GB/T 70.1-2008 and ISO 4762 cover M1.6 to M64 and do not include M18 or M22 (M14 is a bracketed, non-preferred size), so the M18 and M22 pages here are given on the DIN 912 basis. Apart from that, this page labels dimensions row by row on the ISO 4762 / GB/T 70.1 basis, citing ISO 4762 and DIN 912 for series equivalence only, reproducing no table values from them. Order against a drawing or the standard the buyer names.
Can I use an imperial hex key in a metric socket?
A few sizes will go in, but it should not be a routine practice. Only a handful of metric and imperial nominal sizes are close (for example 3/16 in is about 4.76 mm, undersize against 5 mm; 1/4 in is about 6.35 mm, oversize against 6 mm), and the sizes that do enter usually carry noticeable play. A hex key drives on six faces inside a closed socket, so the usable clearance is far smaller than for a wrench gripping from outside; once the play is significant it rounds the socket corners and burrs the key. The correct approach is to match s by name using the metric size. Reserve a near-equivalent imperial key for emergencies at low torque, and check the socket corners for damage as you go.
Related References
Bolt Weight Calculator
Use it for lengths and materials outside this table, and for whole-order totals; this page uses the same engine and basis as the product catalog, so the two can be cross-checked.
M6 Bolt Torque Chart
How hard to tighten it: torque range and target preload by property class and friction state, to use alongside hex key selection.
M6 Hex Bolt Weight Chart
The same diameter in a hex head bolt: unit weight and kg per 1,000 pcs across the length series, useful for comparing steel use between head forms.
M6 Hex Nut Weight Chart
Matching nut: unit weight and kg per 1,000 pcs for the same size on the GB/T 6170 basis, so it can go straight into costing.
Socket Head Cap Screw Category (GB/T 70.1-2008)
M4-M24 product catalog with specification and material class options, including made-to-drawing lengths and finishes.
Tap Drill Chart for Socket Head Screws
The thread under the same socket head cap screw: tap drill and nearest drill bit, alongside the hex key that turns it.
Disclaimer: This table is engineering estimate data for selection, quotation and freight estimation reference only, not a design basis or acceptance standard.