Engineering Tools · Hex Key Reference
M4 Socket Head Cap Screw: Which Hex Key Size Fits?
M4 socket head cap screw (GB/T 70.1 primary source, ISO 4762 / DIN 912 for series reference): head diameter dk=7 mm, head height k=4 mm, hexagon socket width across flats s=3 mm - so it takes a 3 mm hex key (GB/T 5356-2021 standard series, ISO 2936 equivalent, long arm L1=66 mm, short arm L2=23 mm). Head dimensions, matching key arm lengths and unit weight by length on one page, with the formula and basis disclosed.
A M4 socket head cap screw takes a 3 mm hex key (Allen wrench): head diameter dk=7 mm, head height k=4 mm, hexagon socket width across flats s=3 mm, coarse pitch 0.7 mm; the 3 mm key has a long arm L1=66 mm and a short arm L2=23 mm. M4x16 in carbon steel weighs about 2 g/pc (2 kg per 1,000 pcs) with 14 mm of thread.
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M4 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 | 4 mm |
| Coarse pitch P (GB/T 196 series) | 0.7 mm |
| Head diameter dk (max) | 7 mm |
| Head height k (max) | 4 mm |
| Hexagon socket width across flats s (key size) | 3 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) |
Row-level standard labelling: dimensions follow GB/T 70.1-2008 Socket Head Cap Screws (the current edition, 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 ISO series, cited for series reference only - no table values are reproduced from it. Two bases have to be kept apart: the GB/T 70.1-2008 / ISO 4762 size series runs from M1.6 to M64 and does not include M18 or M22, so 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 GB/T 70.1 / ISO 4762 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 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)
Matching Hex Key Dimensions (s=3 mm)
The socket width across flats s of the screw is the key size you need: M4 takes a 3 mm key. The table below gives the two arm lengths of the 3 mm key (long arm L1 / short arm L2) on the GB/T 5356-2021 standard length series (ISO 2936 standard series equivalent). 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) |
|---|---|---|
| 3 | 66 | 23 |
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 M4 screw has d=4 mm but a socket width across flats of s=3 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.
M4 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). The basis is identical 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, i.e. 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) |
|---|---|---|---|---|
| M4×8 | 8 | 0 | 2 | 2 |
| M4×10 | 10 | 0 | 2 | 2 |
| M4×12 | 12 | 0 | 2 | 2 |
| M4×16 | 14 | 2 | 2 | 2 |
| M4×20 | 14 | 6 | 3 | 3 |
| M4×25 | 14 | 11 | 3 | 3 |
| M4×30 | 14 | 16 | 4 | 4 |
Configuration basis (same source as the product catalog): shank = plain section pi/4 x d^2 x (L-b) plus threaded section pi/4 x d2^2 x b (d2 = d - 0.6495P, ISO basic profile pitch diameter); head = pi/4 x dk^2 x k x 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 (whole g below 100 g, 10 g steps below 1,000 g, 100 g steps 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
Dimensions come from fastenerCalc.SHCS_DIMS (GB/T 70.1 socket head cap screws, max nominal values) and fastenerCalc.HEX_KEY_DIMS (GB/T 5356-2021 hex key standard series arm lengths, ISO 2936 equivalent, 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 pi/4 x dk^2 x k x 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). No table values are reproduced from any copyrighted standard (for example ASME B18.3).
Boundary: (1) 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; (2) head dimensions are max nominal values and actual parts vary within tolerance - this chart is not an acceptance basis; (3) the lengths listed are the common range, and unlisted lengths are made to order; (4) 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.
Corresponding standard page
Open the GB/T 70.1 / DIN 912 standard product page: sizes, materials and finishes, and ordering questions.
Open the GB/T 70.1 / DIN 912 page
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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 M4 socket head cap screw (GB/T 70.1) has a socket width across flats of s=3 mm, so it takes a 3 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.
Which hex key fits a M4 socket head cap screw, and what are its dimensions?
A 3 mm hex key. On the GB/T 70.1 basis a M4 socket head cap screw has nominal diameter d=M4, coarse pitch 0.7 mm, head diameter dk=7 mm, head height k=4 mm and socket width across flats s=3 mm. The matching 3 mm hex key (GB/T 5356-2021 standard series) has a long arm of L1=66 mm and a short arm of L2=23 mm. Use the long arm to reach into a counterbore or to apply leverage, and the short arm for fast running in and for the final tightening turn.
What standard covers socket head cap screws, and how do GB/T 70.1, ISO 4762 and DIN 912 differ?
They are the Chinese, international and German series for the same form (socket head cap screw): GB/T 70.1 is the Chinese national standard, ISO 4762 the ISO standard and DIN 912 the German 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 GB/T 70.1 basis and cites 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.
How much does one M4 socket head cap screw weigh, and how is it calculated?
Computed from the socket head cap screw geometry (carbon steel at 7.85 g/cm3, coating excluded): M4x16 is about 2 g/pc (about 0.4409 lb per 100 pcs) and M4x100 about 0 g/pc. The calculation is full-diameter plain shank + pitch-diameter threaded section + head pi/4 x dk^2 x k x 0.85, the same parameters and formula as the spec_json weightCalc of the same size SKUs in our product catalog. 304 stainless runs about x1.010 and 316 about x1.017 (geometry unchanged, density ratio only). At the same diameter the weight grows with length; the length-series table on this page covers each size.
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.
Socket Head Cap Screw Category (GB/T 70.1)
M4-M24 product catalog with specification and material class options, including made-to-drawing lengths and finishes.
Disclaimer: This table is engineering estimate data for selection, quotation and freight estimation reference only, not a design basis or acceptance standard.