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ISO Metric Thread Pitch Chart

Guide · Updated October 28, 2026

ISO metric threads are called out by M, the nominal diameter in millimeters and the pitch in millimeters, such as M8×1.25. When the pitch is the standard coarse pitch it can be left off, so plain M8 means M8×1.25. Fine pitches must always be written out. The pitches below are the ISO 262 selected sizes for screws, bolts and nuts, and the profile formulas come from ISO 68-1.

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Step by step

  1. Measure the diameter: Mic the thread crests; the nominal diameter is the major diameter in mm.
  2. Measure the pitch: Use a metric pitch gage. Pitch is the distance from crest to crest in mm.
  3. Coarse or fine?: Compare with the chart. If the pitch is not the coarse value, it is a fine thread and must be called out.
  4. Find the tap drill: Tap drill ≈ D − P, for example M10×1.5 uses 8.5 mm.
  5. Work out basic diameters: Pitch diameter = D − 0.6495P; minor diameter = D − 1.0825P.
  6. Check the tolerance class: 6H for nuts and tapped holes and 6g for bolts is the medium general purpose fit.

Coarse and fine pitch chart (ISO 262)

First choice sizes are M1.6, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 24, 30, 36, 42, 48, 56 and 64. M14, M18, M22 and M27 are second choice sizes; use them only when the design needs them.

Common tap drills

Because the difference between the major and minor diameters is 1.0825P, subtracting one pitch from the major diameter gives a little under 100% of the theoretical depth in practice and lands near the usual 75 to 80% thread. M12×1.75 is often drilled 10.2 mm rather than the 10.25 mm the formula gives.

The profile formulas (ISO 68-1)

Worked example: M10×1.5

Pitch diameter = 10 − 0.6495 × 1.5 = 9.026 mm. Minor diameter = 10 − 1.0825 × 1.5 = 8.376 mm. Tap drill = 10 − 1.5 = 8.5 mm. Best wire = 0.57735 × 1.5 = 0.866 mm. M10×1.25 fine: pitch diameter 9.188 mm, minor 8.647 mm, tap drill 8.75 mm.

Worked example: M8×1.25 and M12×1.75

M8×1.25: pitch diameter = 8 − 0.812 = 7.188 mm, minor = 8 − 1.353 = 6.647 mm, tap drill 6.8 mm.

M12×1.75: pitch diameter = 12 − 1.137 = 10.863 mm, minor = 12 − 1.894 = 10.106 mm, tap drill 10.2 mm. On an inch lathe, 1.75 mm is 14.514 TPI, so you need metric transposing gears (127 tooth) or a lathe with metric threading; keep the half-nuts engaged for every pass.

Tolerance classes

ISO 965 classes combine a tolerance grade and a position letter. 6H/6g is the normal fit for general fasteners. 6H/6h has no allowance on the bolt. 4H5H/4h is a close fit; 7H/8g is coarse. Galvanized bolts need special oversize nuts or tapped holes.

Common problems

Assuming M8 bolts from an old Japanese machine are 1.25 pitch (pre-1967 JIS used 1.0 for M8 and 1.25 for M10), mixing M10×1.5 with M10×1.25 fine on automotive parts, and drilling an inch drill size close to a metric tap drill without checking. Check pitch with a gage before tapping a mating part.

FAQ

What is the pitch of an M8 bolt?

Coarse M8 is 1.25 mm. Fine pitches are 1.0 and 0.75 mm.

What size drill for M6?

5.0 mm for M6×1, about 75 to 80% thread.

What does M10×1.5-6g mean?

10 mm major diameter, 1.5 mm pitch, external thread tolerance class 6g.

How do I check a metric thread wire stack?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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