Hong Life CalculatorsUnderstanding the screw thread chart — concept and principle
This chart gives the ISO metric coarse pitch for common bolt sizes from M3 to M24 and the nearest Unified inch thread (UNC or UNF), with its threads per inch (TPI) and that pitch in millimetres. Use it to identify a bolt, choose a tap or die, or check whether a part from an inch-based machine can be replaced with metric hardware.
How it is calculated
The metric size and coarse pitch are looked up in the table. For the inch thread, pitch in mm = 25.4 ÷ TPI, because one inch is exactly 25.4 mm. A metric designation such as M8 × 1.25 means an 8 mm nominal major diameter and 1.25 mm from one thread crest to the next; when the pitch is left out (just "M8"), the coarse pitch is meant. An inch designation such as 5/16-18 UNC means a 5/16 in nominal diameter and 18 threads per inch.
The principle
Both systems use the same 60° thread profile, but they are built on different grids: metric threads step in round millimetres of diameter and pitch, while Unified threads step in fractions of an inch and whole numbers of threads per inch. A size pair can therefore look almost identical yet never match exactly. The pitch error adds up with every turn, so a nut that starts by hand will bind after a few threads.
Worked example
Default input: M8.
- Look up the coarse pitch for M8: 1.25 mm.
- Closest inch thread: 5/16-18 UNC (5/16 in = 7.94 mm, very close to 8 mm).
- Convert its pitch: 25.4 ÷ 18 = 1.411 mm.
- Result: M8 has a 1.25 mm coarse pitch; the nearest inch thread is 5/16-18 UNC with 18 TPI, or 1.411 mm.
The difference is 0.161 mm per thread, so after about eight turns the threads are already more than 1 mm out of step. They are not interchangeable.
How to read the result
Use the metric pitch to choose the correct tap and nut. A common rule of thumb for the tap drill is major diameter minus pitch, which for M8 × 1.25 gives about 6.8 mm. Use the inch column only to recognise an imperial fastener, for example when a thread gauge reads 18 TPI on a bolt that measures about 7.9 mm.
Common mistakes
- Forcing an M6 bolt into a 1/4-20 hole or the reverse; the diameters are close, the pitches are not.
- Assuming every M8 bolt is 1.25 mm; fine-pitch M8 × 1.0 exists, especially in automotive parts.
- Measuring pitch over one thread instead of over ten threads and dividing by ten.
FAQ
Frequently asked questions
Is M6 the same as 1/4-20?
No. 1/4 in is 6.35 mm and 20 TPI is a 1.27 mm pitch, while M6 is 6 mm with a 1.0 mm pitch.
What is the pitch of an M8 bolt?
The coarse pitch is 1.25 mm; the common fine pitch is 1.0 mm.
How do I measure thread pitch without a gauge?
Measure the length of ten threads with a caliper and divide by ten, or count threads in one inch for TPI.
Sources: ISO 261, ISO general purpose metric screw threads — General plan · ISO 262, ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts · ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form)