Thread Pitch Calculator

The Thread Pitch Calculator converts between threads per inch (TPI) and thread pitch in millimeters, and estimates thread depth. It also provides a reference table of standard ISO metric thread sizes with their nominal pitches, major diameters, and minor diameters for quick lookup.

TPI to Pitch Conversion

Thread pitch and TPI are inverse measurements. Pitch is the distance between adjacent thread crests in millimeters. TPI is the number of threads in one inch. The conversion formula is:

$$\text{Pitch (mm)} = \frac{25.4}{\text{TPI}} \qquad \text{TPI} = \frac{25.4}{\text{Pitch (mm)}}$$

For example, 20 TPI equals a pitch of 25.4 / 20 = 1.27 mm. Conversely, an M10 × 1.5 thread has a pitch of 1.5 mm, which equals 25.4 / 1.5 ≈ 16.9 TPI.

Thread Depth Estimation

The approximate thread depth for ISO metric threads is calculated as 0.614 times the pitch:

$$\text{Depth} = 0.614 \times P$$

This is the working depth of the thread engagement (the distance the external and internal threads overlap). For an M12 × 1.75 thread, the depth is approximately 0.614 × 1.75 = 1.07 mm.

For full thread dimension calculations including tolerance grades and positions, use the Thread Calculator. For pitch diameter limits, see the Pitch Diameter Calculator.

Frequently Asked Questions

How do I convert TPI to metric pitch?

Divide 25.4 by the TPI value. For example, 24 TPI = 25.4 / 24 = 1.058 mm pitch. 13 TPI = 25.4 / 13 = 1.954 mm pitch. Use the calculator for instant conversion in either direction.

What is the standard pitch for an M8 bolt?

The standard coarse pitch for an M8 bolt is 1.25 mm. Fine pitch variants include M8 × 1.0 and M8 × 0.75. The reference table in the calculator shows standard pitches for common metric thread sizes from M3 to M24.

What is the difference between coarse and fine thread pitch?

Coarse threads have a larger pitch (fewer TPI) and are more resistant to stripping, faster to assemble, and less likely to cross-thread. Fine threads have a smaller pitch (more TPI), provide better adjustment resolution, and have higher tensile strength for the same major diameter.

How deep should I cut threads in a lathe?

For ISO metric threads, the compound rest infeed depth is approximately 0.614 × pitch for the thread depth (single-point threading with 60° tool). For an M12 × 1.75 thread, cut to a depth of about 1.07 mm measured perpendicular to the workpiece axis.

What does thread depth mean for thread engagement?

Thread depth (or working depth) is the distance external and internal threads overlap radially. A minimum engagement of 1.0 × the bolt diameter is recommended for full strength. For example, an M10 bolt should engage at least 10 mm of thread length.

Can I use this calculator for UNC threads?

The TPI-to-pitch conversion works for any thread system including UNC, UNF, BSW, and metric. The depth formula (0.614 × P) is specific to ISO metric 60° threads. UNC/UNF threads use 60° as well but have different root truncation, so the depth may vary slightly.

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Thread Pitch Calculator