Half of all fastener mix-ups are not quality problems — they are thread problems. A screw that "looks like an M4" but is actually M3.5 will start fine by hand and wreck the joint under torque. This page is a working reference for the metric threads small-purchase buyers use most: what the numbers mean, which pitches are standard, and how to write the size into an RFQ so nobody guesses.

How to read an "M" callout

M4 × 0.70 × 12 breaks down as:

  • M4 — nominal thread outer diameter is 4 mm.
  • 0.70 — thread pitch: the distance from one thread crest to the next, in mm. If the pitch is omitted, the coarse pitch is assumed (for M4 that means 0.70).
  • 12 — length in mm (for countersunk heads, length is usually measured including the head; for most other heads, under the head).

Two screws can share the same diameter and still be incompatible: M4×0.70 and M4×0.50 will not smoothly share a nut or a pre-tapped hole. Always state the pitch when it is not the coarse default.

Coarse pitches (the normal series)

ISO 261 defines the coarse pitch for each diameter. These are the pitches you get on standard hardware-store bolts, machine screws and most nuts unless stated otherwise:

Size Coarse pitch (mm) Tapping drill ≈ (mm)
M2 0.40 1.6
M2.5 0.45 2.05
M3 0.50 2.5
M3.5 0.60 2.9
M4 0.70 3.3
M5 0.80 4.2
M6 1.00 5.0
M8 1.25 6.8
M10 1.50 8.5
M12 1.75 10.2

The tapping-drill column is the common approximation for steels and plastics (nominal diameter minus pitch). Soft materials like aluminium want a slightly larger hole; exact values depend on the material and the required thread engagement — check against your supplier's data before committing a production run.

Fine pitches (the ones you must write in full)

Fine series exist for thin walls, adjustment hardware, flanged joints and higher-stress thread roots. The callout must always include the pitch — an "M8" without a number is coarse M8×1.25 by definition:

Size Common fine pitches (mm)
M8 1.00 · 0.75
M10 1.25 · 1.00 · 0.75
M12 1.50 · 1.25 · 1.00

If you are replacing a part where fine thread was used as a feature (short engagement in thin plates, fine adjustment travel), match the pitch exactly — a coarse substitute will bind or strip.

Four traps that produce wrong orders

  1. M3 vs M3.5. Almost identical heads, 0.1 mm apart in diameter. In electronics housings they are interchangeable as screws but not as threads. Verify with a thread gauge or by driving one into known-good material, not by eye.
  2. Self-tapping "M-sizes" are not machine threads. A pan head self-tapper labelled M3×10 cuts its own thread in sheet; it is not a substitute for an M3×0.5 machine screw going into a tapped hole or a nut. See Self-Tapping vs Machine Screws for the full difference.
  3. Unit confusion. Some catalogs write small sizes in inches (4-40, 6-32). "Looks like M3" and is #3-40 are different threads. If the drawing has no unit, ask.
  4. Pitch vs threads-per-inch. Never convert between the two systems by approximation — buy the correct thread, or adapt the joint.

What to put in your RFQ

For each line, one string: type + M-size × pitch × length + head + drive + material + finish + quantity. Example: "pan head cross recess machine screw, M3 × 0.5 × 10, nylon insert not required, nickel plated carbon steel, 5,000 pcs". Where you have a drawing or a sample, attach it — a photo of a worn-out part beats a paragraph of assumptions. Our team quotes against exactly this structure; see How to Specify a Screw and How We Quote.

This page explains publicly defined ISO metric thread data for buyers doing their own spec work. It is not a statement about the stock or specifications of any particular item we quote — final dimensions for every order are confirmed in the quotation against your stated standard, drawing or sample.