Thread Pitch Converter

Convert between inch threads (threads per inch) and metric thread pitch (mm between threads).

How to use the Thread Pitch Converter

Choose TPI → mm or mm → TPI.
Enter the value.
Read the equivalent and closest metric coarse thread.

How it works

Pitch (mm) = 25.4 ÷ TPI. A 1/4-20 bolt has 20 threads per inch, a pitch of 1.27 mm; an M6 bolt has a coarse pitch of 1.0 mm. Inch and metric threads also differ in thread angle conventions and diameters, so similar numbers do not mean the parts fit together — use a thread gauge to identify unknown fasteners.

Common threads

Thread	TPI	Pitch (mm)
1/4-20 UNC	20	1.270
5/16-18 UNC	18	1.411
3/8-16 UNC	16	1.588
1/2-13 UNC	13	1.954
M6 × 1.0	25.4	1.000
M8 × 1.25	20.3	1.250
M10 × 1.5	16.9	1.500
M12 × 1.75	14.5	1.750

Frequently asked questions

What is 20 TPI in mm?

1.27 mm pitch.

What TPI is a 1.5 mm pitch?

About 16.9 TPI.

Is M6 the same as 1/4-20?

No — they are close in size but the threads do not match.

How do I measure thread pitch?

Use a thread pitch gauge or count threads over one inch with a ruler.