Metric threads·M18×1.5
M18×1.5
Tap drill
16.5 mm

M18×1.5 thread — 16.5 mm tap drill, 16.16 mm minor diameter

Fine

M18×1.5 is a fine thread: 18 mm across with a 1.5 mm pitch. The bolt’s minor diameter is 16.16 mm, the tap drill 16.5 mm and the stress area 216.24 mm².

Outside diameter
18 mm
Pitch
1.5 mm (Fine)
Pitch diameter
17.026 mm
Minor diameter (bolt)
16.16 mm
Minor diameter (nut)
16.376 mm
Thread height
0.92 mm
Stress area
216.24 mm²
Threads per inch
16.93

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 × 1.5 = 1.299 mm · 17.026 = 18 − 0.75 × 1.299

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.

Other pitches at this diameter

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

M18×2.5 ⌀15.5M18×2 ⌀16M18×1 ⌀17

One size up and down

M16×2 ⌀14M20×2.5 ⌀17.5

The 35 coarse threads

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

M1×0.25 ⌀0.75M2×0.4 ⌀1.6M3×0.5 ⌀2.5M4×0.7 ⌀3.3M5×0.8 ⌀4.2M6×1 ⌀5M7×1 ⌀6M8×1.25 ⌀6.75M10×1.5 ⌀8.5M12×1.75 ⌀10.25M14×2 ⌀12M16×2 ⌀14M18×2.5 ⌀15.5M20×2.5 ⌀17.5M22×2.5 ⌀19.5M24×3 ⌀21M27×3 ⌀24M30×3.5 ⌀26.5M33×3.5 ⌀29.5M36×4 ⌀32M39×4 ⌀35M42×4.5 ⌀37.5M45×4.5 ⌀40.5M48×5 ⌀43M52×5 ⌀47M56×5.5 ⌀50.5M60×5.5 ⌀54.5M64×6 ⌀58M1.2×0.25 ⌀0.95M1.4×0.3 ⌀1.1M1.6×0.35 ⌀1.25M1.8×0.35 ⌀1.45M2.2×0.45 ⌀1.75M2.5×0.45 ⌀2.05M3.5×0.6 ⌀2.9

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. What drill do I need to tap M18×1.5?

16.5 mm — the 18 mm outside diameter minus the 1.5 mm pitch. Use the nearest drill you have.

Q. What is the minor diameter of M18×1.5?

16.16 mm on the bolt and 16.376 mm in the nut, with the pitch diameter between them at 17.026 mm.

Q. Is it a coarse or a fine thread?

Fine. There is a coarse pitch for this diameter too, so the pitch must always be written out.

Q. How much load does it take?

Its stress area is 216.24 mm². Multiply that by the bolt’s strength grade to get the load it carries.