Metric screw size chart M2–M12: pitch, tap drill, head dimensions and property classes 8.8 / 12.9

Reading a screw designation: what does M5 × 0.8 × 20 mean?

Metric screws are designated M plus the nominal (major) diameter — M5 is a 5 mm thread. The full form M5 × 0.8 × 20 adds the pitch P (distance between threads, mm) and the length L. Coarse pitch is the default, so a coarse screw is normally written M5 × 20; only fine threads must state the pitch, e.g. M10 × 1.25.

Length is measured from under the head for socket head, hex head, button head and pan head screws, but overall (including the head) for countersunk screws. Counting the head height is the most common reason for buying the wrong length.

Pitch diameter d₂ = d − 0.6495P　　Minor D₁ = d − 1.0825P　　Stress area As = π/4 · ((d₂ + d₃)/2)²

The ISO 68-1 basic profile is a 60° thread. The tensile stress area As is the reference area for strength: an M8 coarse thread has As = 36.6 mm², about 27% less than the 50.3 mm² of an 8 mm circle.

Pitch, tap drill and clearance hole chart (ISO 261 / ISO 273)

The tap drill rule of thumb is nominal diameter minus pitch, giving roughly 75–80% thread engagement — strong enough without breaking taps. It works for fine threads too: 8.75 mm for M10 × 1.25, 10.5 mm for M12 × 1.5. Clearance holes just let the screw pass — fine for located assemblies, medium for general machining.

Coarse or fine thread?

• Coarse: universal, quick to assemble, tolerant of dirt and dings — the first choice for aluminium, cast iron and plastics.
• Fine: larger stress area at the same diameter (M10 × 1.25: 61.2 mm² vs 58.0 mm² coarse), smaller lead angle for better vibration resistance, finer adjustment and thin-walled tubes.
• Fine threads strip more easily in soft materials and are shallower, so the tap drill size matters more.

TIPThe tap drill calculator gives the drill for any pitch and thread class. In aluminium use the table value; in hard steel or stainless go 0.1 mm larger to ease tapping.

Head dimensions: socket head, button head, hex head

Head diameter dk (or across-flats s) × head height k in mm, with the hex key size in brackets. ISO 4762 equals DIN 912 / JIS B 1176; ISO 4017 is the fully threaded hex screw (DIN 933); ISO 7380-1 button heads start at M3.

• Socket head cap screws have k = d and dk ≈ 1.5–1.7 d. They sink into counterbores and are the default for machines and fixtures.
• Button heads are low and rounded, but their socket is one size smaller, so don't over-torque them.
• Hex head screws take open-end spanners and sockets for high torque — structures and steelwork.

TIPCounterbore for socket head screws: diameter dk + 1 mm, depth k + 0.5 mm — e.g. ⌀9.5 × 5.5 mm deep for M5 with a ⌀5.5 mm through hole.

Nuts, washers and spanner sizes

The across-flats s of hex screws and nuts is the spanner size. ISO 4032 style-1 nuts are about 0.8–0.9 d tall so the bolt breaks before the thread strips. The nut property class (ISO 898-2) should match or exceed the bolt: class 8 nuts for 8.8 bolts.

TIPOld DIN 934 / JIS M10 and M12 nuts are 17 and 19 mm across flats; ISO uses 16 and 18 mm. On older equipment the same M10 may need a different spanner.

What do property classes 8.8, 10.9 and 12.9 mean?

ISO 898-1 marks carbon and alloy steel bolts with two numbers: the first × 100 is the nominal tensile strength Rm in MPa, the second × 10 is the yield-to-tensile ratio in percent. Class 8.8 therefore has Rm = 800 MPa and yield = 800 × 0.8 = 640 MPa. Stainless steel follows ISO 3506-1 (A2-70, A4-80), where the number × 10 is the tensile strength.

Minimum breaking load Fm = Rm,min × As　　e.g. M6 class 8.8 = 800 × 20.1 ≈ 16.1 kN

Higher is not always better: 12.9 is hard but less tough and prone to hydrogen embrittlement under shock or after plating. Stainless A2-70 yields at only 450 MPa — much lower than 8.8 — so check before substituting.

Estimating tightening torque — and fastening 3D-printed parts

T ≈ K · F · d　　(K ≈ 0.2 dry steel; oiled or anti-seize K ≈ 0.15)

F is the preload, typically 70% of yield × As. The table uses K = 0.2; real values depend on coating and lubrication, so use VDI 2230 or the supplier's chart for critical joints.

• Don't use these values on printed parts — the plastic crushes first. An M3 into PLA / PETG needs only about 0.3–0.5 N·m; add a washer to spread the load.
• For joints that are reassembled or loaded, heat-set inserts or a captive hex-nut pocket (across flats + 0.2–0.3 mm) outlast printed threads.
• Printed threads work from about M6 up; an M3's 0.5 mm pitch is beyond a 0.4 mm FDM nozzle.