ISO metric threads: pitches, tap drills and wrench sizes

November 3, 2026  ·  Indumor

A brand-new M10 bolt, threaded to class 6g, does not measure 10 mm: it measures somewhere between 9.732 and 9.968, and it never reaches ten. That is not a defect. ISO 965-1 takes 32 microns off the top as an allowance and gives the thread another 236 microns of tolerance below that, so it runs into any 6H nut even after plating or paint. The nominal diameter is a name; what defines the thread is the pitch and the profile.

This chapter is what you look up with the tap in your hand: the pitches from M1 to M64 and the fine pitches in common use, the tap drill, the pitch and minor diameters calculated from the ISO 724 formulas, the tensile stress area and the width across flats. Tightening torque and property classes are in chapter 18; inch threads in chapter 5; cutting threads on the lathe in chapter 8.

The ISO 68-1 profile

The flanks sit at 60° and every dimension comes from the pitch P. The height of the sharp V is H = 0.866·P; the bolt crest is truncated by H/8 and the nut root by H/4, which leaves 5/8 of H as working thread. Halfway down H sits the pitch diameter d₂, where thread and space are equally wide: that is what a thread micrometer measures and what governs the fit. The bolt's minor diameter d₃ sits below the nut's minor diameter because the bolt root is rounded, and the standard wants it that way: a sharp V root is a notch where a fatigue crack starts.

ISO 261 coarse series: M1 to M64

The coarse pitch is the one implied when the designation gives no pitch: M12 means M12×1.75. The table lists 33 diameters, with the pitch stepping up irregularly in a pattern that goes back to the 1898 Zurich congress, which took the 60° Sellers profile and recast it in millimeters.

The wrench column holds a well-known trap. ISO 4014 and ISO 4032 give 16 mm across flats for M10, 18 for M12, 21 for M14 and 34 for M22; the DIN 931/933/934 fasteners still common in Europe and Latin America use 17, 19, 22 and 32; and ASME B18.2.3.1M, the North American hex cap screw standard, still lists a non-standard 15 mm M10, made to order, with 16 mm as the default. Three wrench sizes for one bolt. A full metric set carries all of them, and an 18 mm nut turned with a 19 mm wrench ends up rounded.

Find a thread

d₂, d₃ and A_s calculated from ISO 724 and ISO 898-1. DIN wrench column: only where it differs from ISO (M10, M12, M14 and M22) and for M1 to M1.4 and M7, which ISO 4014/4032 do not cover; a dash means "same as ISO".

ISO 262 fine pitches

Fine pitch goes where coarse does not work: thin walls, parts that have to be adjusted (turnbuckles, lock nuts, fittings) and threads that live with vibration, since the smaller helix angle resists loosening. It has more stress area than the coarse thread of the same diameter (M24×2: 384 mm² against 353), which is why automotive work uses it almost everywhere. In exchange it cross-threads more easily and is less forgiving of the tap drill: half a millimeter oversize already leaves a thin thread. It is always designated with the pitch spelled out, M12×1.25.

The tap drill

The rule is tap drill = d − P, and the DIN 336 chart merely rounds it to a drill that exists: M8 gives 6.75 and the chart says 6.8; M12 gives 10.25 and the chart says 10.2.

That drill leaves what American charts call a 75 % thread. Those charts measure against 3/4 of H (0.6495·P per side): with d − P the thread is P/2 per side, 77 % of that reference. Against the basic ISO nut thread, 5/8 of H, the same drill leaves 92 %. That is why nobody drills smaller: from 75 % to 100 % the nut gains almost no strength, since the bolt breaks before the nut strips, and the tap works twice as hard. An M6 tap in a 5 mm hole runs by hand; in a 4.9 mm hole it snaps.

In hard steels, austenitic stainless and gray cast iron, go a tenth of a millimeter larger (8.6 for M10): the thread drops to about 72 % and the tap stops binding. In aluminum and soft bronze, which close in behind the drill, use the chart size. In thin sheet what matters is the length of engagement, not the percentage: with less than one diameter of thread the nut strips before the bolt breaks. The same arithmetic in inches, with the 0.0130 × % / tpi formula, is in chapter 5.

Designation and tolerance classes

A full designation gives diameter, pitch, tolerance class and hand. The class pairs a number (the grade: 4 fine, 6 medium, 8 coarse) with a letter (the position relative to the basic profile): lower case for the bolt, upper case for the nut. In h and H the tolerance zone starts at the basic profile, with no allowance; in g it starts 32 microns lower at 1.5 mm pitch, which is the figure behind the M10 at the top of the chapter; in e and f the allowance is larger, for thick coatings or threads that run hot. When a drawing says nothing, 6g and 6H are assumed.

For an M10×1.5 nut in class 6H, D₁ may run from 8.376 to 8.676: with those 300 microns, any 8.5 mm drill lands inside even if it ran a tenth oversize.

Stress area and calculator

The tensile stress area A_s is what you use to work out the load a bolt carries: force = steel strength × A_s. It is not the root area, which would be pessimistic, but the area of a circle whose diameter lies between pitch and minor diameters, because that is where the bolt breaks. ISO 898-1 defines it that way, and the values in the tables come from it: 58 mm² for M10, 84.3 for M12, 157 for M16, 245 for M20. An M12 in class 8.8, with 800 N/mm² tensile strength, breaks at 67 kN, nearly seven tonnes.

Frequently asked questions

What size drill for an M8 tap?

6.8 mm: 8 − 1.25 = 6.75, rounded to the stock 6.8 mm drill. The closest inch drill is 17/64 (6.75 mm). For M8×1 fine pitch, 7 mm.

What is the pitch of an M10 thread?

1.5 mm in the coarse series, which is the one implied. The fine pitches for M10 are 1.25 and 1 mm. The tap drill is 8.5 mm, and 1.5 mm pitch works out to 16.9 threads per inch.

What does 6g mean on a bolt?

Medium tolerance grade (6) in position g: the thread starts a few microns below the basic profile so it has clearance against a 6H nut. It is the normal class for commercial fasteners.

Sources: ISO 68-1:2023 (basic profile); ISO 261:1998 (general plan of diameters and pitches); ISO 262:1998 (selected sizes); ISO 724:1993 (basic dimensions); ISO 965-1:2013 (tolerances and classes); ISO 898-1 (tensile stress area A_s); ISO 4014, ISO 4017 and ISO 4032 (width across flats); DIN 13-1, DIN 931, DIN 933, DIN 934 and DIN 336 (tap drills); ASME B18.2.3.1M (metric hex cap screws).