Socket head cap screw dimensions: ISO 4762 and inch chart
Published October 16, 2026 — ISO 4762 socket head cap screw dimensions from M1.6 to M36: head diameter and height, hex key, thread length, standard lengths. Plus ASME B18.3 inch screws, property classes 8.8, 10.9, 12.9 and a length and counterbore calculator.
In short: on an ISO 4762 socket head cap screw, the head height k equals the nominal diameter (8 mm for M8) and the head diameter dk is 1.5 times the diameter from M12 up (a little more below that: 13 mm for M8). Common hex keys: M4 → 3, M5 → 4, M6 → 5, M8 → 6, M10 → 8, M12 → 10. To sink the head, the counterbore is at least k deep and larger than dk in diameter.
1. ISO 4762 socket head cap screw dimensions, M1.6 to M36
Dimensions in millimetres, maximum values for the head (ISO 4762, 2004 edition). P: thread pitch (coarse). dk: head diameter. k: head height. s: hex key size, i.e. the socket width across flats. t: minimum socket depth. b: reference thread length of partially threaded screws.
Head diameters are for a plain head; a knurled head may be slightly larger (10.22 mm instead of 10 mm for M6). The standard goes up to M64. Short screws are threaded up to the head: the limit depends on the size, see the standard or the supplier's catalogue.
Printable version: the socket head cap screw chart as a PDF (A4, no sign-up), metric and inch, to pin up in the shop or the design office.
2. Length and counterbore calculator
Metric socket head cap screw driven into a tapped hole. Enter the clamped thickness under the head (the part, or what remains under the counterbore floor), the thickness of any washer and the thread engagement you want in the tapped hole. The minimum engagement depends on the tapped material and the screw's property class; VDI 2230 gives the calculation.
3. Inch socket head cap screws (ASME B18.3)
Inch-series socket head cap screws, UNC thread: 1/4-20 reads "1/4 inch diameter, 20 threads per inch". Head: maximum values in inches (mm equivalent in brackets, 1 inch = 25.4 mm). As in metric, the head height equals the diameter, and the head diameter is 1.5 times the diameter from 1/4 up.
- To convert any other inch dimension, see the inch to mm conversion.
4. Standard lengths and designation
The nominal length L of a socket head cap screw is measured from under the head to the end of the screw. ISO 4762 length series, in mm: 2.5 · 3 · 4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 25 · 30 · 35 · 40 · 45 · 50 · 55 · 60 · 65 · 70 · 80 · 90 · 100 · 110 · 120 · 130 · 140 · 150 · 160 · 180 · 200 · 220 · 240 · 260 · 280 · 300. Not every size exists in every length: the range depends on the diameter and the supplier.
- Designation: "Hexagon socket head cap screw ISO 4762 - M8 × 30 - 12.9", i.e. diameter, nominal length and property class. In the shop they are also called SHCS or Allen screws.
- Threaded portion: a partially threaded screw has a reference thread length b = 2·d + 12 mm (28 mm for M8); the plain shank must stay within the clamped part so that the thread works in the tapped hole.
- Fine thread: fine-pitch socket head cap screws (M8 × 1, M10 × 1.25…) are covered by ISO 12474.
5. Property classes 8.8, 10.9, 12.9 (ISO 898-1)
Steel socket head cap screws come in property classes 8.8, 10.9 and 12.9. The first number gives the nominal tensile strength (12 → 1,200 MPa), the second the ratio of yield to tensile strength (.9 → 90%). ISO 898-1 values, in MPa:
In stainless steel, socket head cap screws are most often A2-70 or A4-70 (ISO 3506-1): A2 or A4 is the austenitic grade, 70 a tensile strength of at least 700 MPa. The tightening torque depends on the class and on friction: see the bolt torque chart.
6. Counterbore, holes and other socket screws
- Counterbore depth: for a flush head, at least the head height k, i.e. the nominal diameter (8 mm for M8), plus any recess wanted below the surface.
- Counterbore diameter: larger than the head diameter dk, knurled head included (13.27 mm for M8), with the clearance needed for the key and position tolerances. Standard diameters are given in DIN 974-1; the tables published elsewhere do not all agree, hence no values here.
- Clearance and tapped holes: ISO 273 diameters (fine, medium, coarse series) and tap drills in the metric thread chart.
- In SOLIDWORKS, the Hole Wizard creates the counterbore and clearance hole of an ISO socket head cap screw from its database; check those dimensions against your company standard before releasing the drawing.
- Other socket screws: low head (DIN 7984, smaller key, when height is limited), button head (ISO 7380), countersunk head (ISO 10642): their dimensions differ from the chart above.
7. FAQ
What hex key size fits an M6, M8 or M10 socket head cap screw?
Per ISO 4762: a 5 mm key for M6, 6 mm for M8, 8 mm for M10, 10 mm for M12 and 14 mm for M16. The key is always smaller than the wrench for a hex head bolt of the same diameter.
What are the dimensions of an M8 socket head cap screw?
Pitch 1.25 mm, head 13 mm in diameter and 8 mm high, 6 mm hex key engaging at least 4 mm, reference thread length 28 mm for partially threaded screws.
How deep should the counterbore be for a socket head cap screw?
To sink the head, at least the head height k, which equals the nominal diameter: 6 mm for M6, 8 mm for M8, plus any recess you want below the surface. The counterbore diameter must exceed the head diameter dk (10 mm for M6); standard diameters are given in DIN 974-1.
What is the difference between DIN 912 and ISO 4762?
DIN 912 was withdrawn and replaced by ISO 4762 (EN ISO 4762) for coarse threads and by ISO 12474 for fine threads. Both names refer to the same screw family and still coexist in catalogues, but some dimensions and tolerances were revised: for a critical dimension, rely on ISO 4762.
What size Allen key for a 1/4-20 socket head cap screw?
A 3/16 inch key (4.76 mm) per ASME B18.3. The head is 0.375 inch in diameter (9.53 mm) and 0.250 inch high (6.35 mm).