114 metric threads — tap drills, minor diameters and stress areas

In M8×1.25 the 1.25 is not a thickness — it is the distance from one thread crest to the next. Given the outside diameter and that pitch, every other dimension follows.

The 35 coarse threads

Each diameter has one default pitch. When someone just says “M8”, they mean M8×1.25.

Choosing the tap drill

The shop rule is outside diameter minus pitch: M8×1.25 gives 6.75 mm, so you reach for a 6.8 mm drill. That is wider than the nut’s minor diameter, which is why the tap cuts without snapping.

The thread is a 60° triangle

ISO defines the thread profile as an equilateral 60° triangle. Its height H is 0.866 × pitch, and every other diameter is that triangle cut back by a fixed fraction of H.

H = 0.866 × Pitch · Pitch diameter = d − 0.75H · Minor diameter (nut) = d − 1.25H

The stress area sets the strength

A bolt’s load capacity is worked out from this area, not from the outside diameter. It is the area of a circle whose diameter is the average of the pitch and minor diameters.

M6×1 · 20.12 mm² M8×1.25 · 36.61 mm² M10×1.5 · 57.99 mm² M12×1.75 · 84.26 mm²

The 79 fine threads

A tighter pitch cuts shallower, leaving a thicker core. That is why fine threads suit thin walls and places that vibrate.

M2×0.25 ⌀1.75 M3×0.35 ⌀2.65 M4×0.5 ⌀3.5 M5×0.5 ⌀4.5 M6×0.75 ⌀5.25 M6×0.5 ⌀5.5 M7×0.75 ⌀6.25 M8×1 ⌀7 M8×0.75 ⌀7.25 M10×1.25 ⌀8.75 M10×1 ⌀9 M10×0.75 ⌀9.25 M12×1.5 ⌀10.5 M12×1.25 ⌀10.75 M12×1 ⌀11 M14×1.5 ⌀12.5 M14×1.25 ⌀12.75 M14×1 ⌀13 M16×1.5 ⌀14.5 M16×1 ⌀15 M18×2 ⌀16 M18×1.5 ⌀16.5 M18×1 ⌀17 M20×2 ⌀18 M20×1.5 ⌀18.5 M20×1 ⌀19 M22×2 ⌀20 M22×1.5 ⌀20.5 M22×1 ⌀21 M24×2 ⌀22 M24×1.5 ⌀22.5 M24×1 ⌀23 M27×2 ⌀25 M27×1.5 ⌀25.5 M27×1 ⌀26 M30×3 ⌀27 M30×2 ⌀28 M30×1.5 ⌀28.5 M30×1 ⌀29 M33×3 ⌀30 M33×2 ⌀31 M33×1.5 ⌀31.5 M36×3 ⌀33 M36×2 ⌀34 M36×1.5 ⌀34.5 M39×3 ⌀36 M39×2 ⌀37 M39×1.5 ⌀37.5 M42×4 ⌀38 M42×3 ⌀39 M42×2 ⌀40 M42×1.5 ⌀40.5 M45×4 ⌀41 M45×3 ⌀42 M45×2 ⌀43 M45×1.5 ⌀43.5 M48×4 ⌀44 M48×3 ⌀45 M48×2 ⌀46 M48×1.5 ⌀46.5 M52×4 ⌀48 M52×3 ⌀49 M52×2 ⌀50 M52×1.5 ⌀50.5 M56×4 ⌀52 M56×3 ⌀53 M56×2 ⌀54 M56×1.5 ⌀54.5 M60×4 ⌀56 M60×3 ⌀57 M60×2 ⌀58 M60×1.5 ⌀58.5 M64×4 ⌀60 M64×3 ⌀61 M64×2 ⌀62 M64×1.5 ⌀62.5 M1.6×0.2 ⌀1.4 M2.5×0.35 ⌀2.15 M3.5×0.35 ⌀3.15

How to read this

The number after M is the outside diameter in mm; the number after × is the pitch in mm.
H = 0.866 × pitch. The pitch diameter is d − 0.75H, the nut’s minor diameter d − 1.25H.
Take the tap drill as outside diameter minus pitch, then pick the nearest drill you own.
Load capacity comes from the stress area, not from the outside diameter.

Frequently asked questions

Q. If someone just says M8, which pitch is it?

The coarse thread, M8×1.25. Every diameter has one default pitch, and that is what an unqualified size means.

Q. Why use a fine thread?

It cuts shallower, so the core stays thicker and the thread fits into thin parts; it also backs off less per turn, which helps against vibration. The trade-off is a shallower thread that strips more easily.

Q. How do I pick the tap drill?

Subtract the pitch from the outside diameter. M6×1 gives 5 mm; M8×1.25 gives 6.75 mm, so you use a 6.8 mm drill.

Q. Why do bolt and nut have different minor diameters?

The bolt is cut deeper so the nut’s crests have somewhere to sit. That is why the bolt’s minor diameter is the smaller of the two.

Q. Can I size a bolt from its outside diameter?

No. It breaks at the root, so the strength calculation uses the stress area. For M8 that is 36.6 mm², not the 50.3 mm² of the full circle.