How to Read a Metric Thread Callout (M8 x 1.25 and Friends)
A metric thread callout is a short code. The M means an ISO metric thread, the first number is the nominal diameter in millimetres, and the number after the x is the pitch in millimetres. So M8 x 1.25 is an 8 mm thread with 1.25 mm between crests. If the pitch is missing, as in plain M8, the thread has the standard coarse pitch for that diameter. If a third number follows, it is usually the length: M8 x 1.25 x 30 is 30 mm long. To check any size against the standard list, open the metric screw size chart.
Key takeaways
- M = metric, then diameter in mm, then pitch in mm. Pitch is the distance between neighbouring crests, not threads per inch.
- No pitch written means coarse pitch. Fine pitches must be written out.
- In M8 x 30, the 30 is the length, not the pitch.
- Letters such as 6g or 6H are the tolerance class. When omitted, the default applies.
The anatomy of a callout
Take M8 x 1.25 – 6g and read it left to right.
| Part | Meaning |
|---|---|
| M | ISO general-purpose metric thread |
| 8 | Nominal major diameter, 8 mm |
| x 1.25 | Pitch, 1.25 mm between crests |
| 6g | Tolerance class (lowercase g means an external thread, such as a bolt or screw) |
The separator varies. M8 x 1.25, M8×1.25 and M8-1.25 all mean the same thing, and catalogues use whichever their layout prefers. ISO 261 says a thread in conformity with it is designated according to ISO 965-1, which is where the class system lives.
Coarse pitch is the default; fine pitch is written out
Every standard diameter has one coarse pitch, listed in ISO 261 and its shorter selection ISO 262. It is the normal pitch for that diameter, so it can be left off: M8 means M8 x 1.25. A fine pitch is a deliberate choice and always appears in the callout.
| Callout | Diameter | Pitch | Series | Tap drill |
|---|---|---|---|---|
| M6 (or M6 x 1) | 6.0 mm | 1.0 mm | Coarse | 5.00 mm |
| M6 x 0.75 | 6.0 mm | 0.75 mm | Fine | 5.25 mm |
| M8 (or M8 x 1.25) | 8.0 mm | 1.25 mm | Coarse | 6.80 mm |
| M8 x 1 | 8.0 mm | 1.0 mm | Fine | 7.00 mm |
Look at the pitch of 1.0 mm. It is the coarse pitch on an M6 and a fine pitch on an M8. “x 1” has no fixed meaning on its own; it depends on the diameter in front of it. That is why a pitch gauge result has to be read against the diameter, never alone.
When the last number is the length
A screw order often adds the length, and that creates the most common misreading. M8 x 1.25 x 30 is diameter, pitch, length. But with the coarse pitch left out, the same screw is written M8 x 30, and now the number after the x is the length. Nobody makes an M8 with a 30 mm pitch. The quick test: pitches are small (about 0.2 mm to a few millimetres on common sizes), lengths are larger, and a pitch has to be one the standard lists for that diameter.
The tolerance class
The trailing code, such as 6g or 6H, describes how tightly the thread is made. The number is the tolerance grade, and the letter is the position of the tolerance band: lowercase for external threads (bolts and screws), uppercase for internal threads (nuts and tapped holes). When no class is given, the defaults are 6g for external and 6H for internal threads. A pair such as 5g6g gives the pitch-diameter class first, then the major-diameter class.
In practice the class explains why a caliper never reads exactly 8.00 mm on an M8. For class 6g the major diameter may fall between 7.760 mm and 7.972 mm. A left-hand thread adds LH at the end, as in M8 x 1.25 – LH; with no mark, the thread is right-hand.
Worked examples
| Callout | How to read it |
|---|---|
| M6 | 6 mm, coarse pitch 1.0 mm (not written) |
| M6 x 0.75 | 6 mm, fine pitch 0.75 mm |
| M8 x 30 | M8 coarse (1.25 mm), 30 mm long |
| M8 x 1 x 30 | M8 fine pitch 1.0 mm, 30 mm long |
| M8 x 1.25 – 6g | M8 coarse, external thread, class 6g |
| M8 x 1 – 6H | M8 fine, internal thread (nut or tapped hole), class 6H |
Metric or inch? Check the pitch
M6 is 6.0 mm across and 1/4 in is 6.35 mm, so a caliper can tell them apart. M8 and 5/16 are harder: 8.00 mm against 7.94 mm, a gap smaller than the thread’s own tolerance band. The pitch settles it. An M8 x 1.25 has about 20.3 threads per inch (25.4 ÷ 1.25), while a 5/16-18 has 18. To convert a metric pitch to threads per inch, divide 25.4 by the pitch. Measure pitch across ten crests and divide by ten, or fit a metric pitch gauge.
When you have the callout, the full metric table lists the coarse and fine pitches for each diameter with the tap drill and clearance hole. The screw size chart hub covers inch and wood families if the screw turns out not to be metric.
The thread system is defined in ISO 261 (general plan), ISO 262 (selected sizes), ISO 724 (basic dimensions) and ISO 965-1 (tolerances and designation). Sources and method explains how this site checks its figures.
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