Thread Pitch Calculator

A thread pitch calculator converts between thread pitch and threads per inch (TPI) for a screw thread. It works with imperial threads, measured in TPI, and metric threads, measured in millimeters of pitch.

What is thread pitch?

Thread pitch is the distance from one thread crest to the next, measured along the axis of a screw or bolt. A crest is the highest point of a thread ridge. A smaller pitch means the threads sit closer together; a larger pitch means they sit farther apart.

Imperial threads (used mainly in the United States) are usually described by threads per inch, or TPI, rather than pitch directly. Metric threads are described by pitch in millimeters. The two values are reciprocals of each other.

How to calculate thread pitch

Thread pitch formula

Pitch equals 1 divided by the number of threads per unit length:

Pitch = 1 ÷ Threads Per Unit

For imperial threads, this is Pitch (inches) = 1 ÷ TPI. For metric threads measured in threads per millimeter, it is Pitch (mm) = 1 ÷ Threads Per mm.

Threads-per-unit formula

The reverse calculation just flips the fraction:

Threads Per Unit = 1 ÷ Pitch

Imperial vs. metric thread systems

Imperial screws are named by diameter and TPI. A 1/4"-20 screw is 1/4 inch across with 20 threads per inch. Metric screws are named by diameter and pitch. An M6 × 1.0 screw is 6 mm across with a pitch of 1.0 mm.

The two systems can be converted using the exact inch-to-millimeter factor, 1 inch = 25.4 mm:

• Metric to imperial: TPI = 25.4 ÷ pitch (mm)
• Imperial to metric: pitch (mm) = 25.4 ÷ TPI

Thread pitch vs. thread lead

Thread pitch and thread lead are related but not always the same thing. Pitch is the distance between adjacent thread crests. Lead is the distance a screw travels along its axis in one full turn.

On a single-start thread, the ordinary kind found on most bolts, pitch and lead are equal. On a multi-start thread, which has two or more parallel thread grooves, lead equals pitch multiplied by the number of starts. A two-start thread with a 1 mm pitch has a 2 mm lead.

Example: TPI to pitch

A standard 1/4"-20 UNC (Unified National Coarse) bolt has 20 threads per inch.

Pitch = 1 ÷ 20 = 0.0500 inches

A finer 3/8"-24 UNF (Unified National Fine) bolt has 24 threads per inch.

Pitch = 1 ÷ 24 = 0.0417 inches

Example: pitch to threads per unit

A standard M10 bolt has a coarse pitch of 1.5 mm.

Threads per mm = 1 ÷ 1.5 = 0.6667

A fine M8 thread has a pitch of 1.0 mm, which gives exactly 1 thread per mm.

Example: converting between systems

A 1.5 mm metric pitch converts to an imperial TPI value as follows:

TPI = 25.4 ÷ 1.5 = 16.9

This is close to, but not the same as, a standard imperial thread count, which is one reason metric and imperial fasteners are not interchangeable even at similar diameters.

Standard thread pitches for common bolt sizes

Imperial (UNC coarse series):

Metric (ISO coarse series):

How to measure thread pitch

A thread pitch gauge is the most reliable tool. It is a set of thin metal blades, each cut with a different pitch, mounted on a folding handle like a pocketknife. The user tries blades against the thread until one meshes cleanly with no gaps of light, then reads the pitch stamped on that blade.

Without a gauge, pitch can be measured with a ruler or caliper. Measure the distance across ten complete threads, then divide by ten. Measuring across more threads reduces the effect of small reading errors.

History of thread standards

Screw threads were cut by hand for centuries before any standard existed, so a bolt from one workshop rarely fit a nut from another. British engineer Joseph Whitworth proposed the first national thread standard in 1841, using a 55-degree thread angle. In the United States, William Sellers proposed a rival 60-degree standard in 1864, which became the basis of the American National thread form.

During the Second World War, American and British engineers reconciled their separate standards into the Unified Thread Standard, still used for inch-based fasteners today. Metric threads are now defined by the International Organization for Standardization (ISO), using a 60-degree thread angle similar to the American standard.

Frequently asked questions

How do you calculate thread pitch from TPI?

Divide 1 by the TPI value. A thread with 20 TPI has a pitch of 1 ÷ 20 = 0.050 inches.

What is the difference between thread pitch and TPI?

TPI counts how many threads fit in one inch. Pitch measures the distance between two adjacent threads. They are reciprocals: pitch = 1 ÷ TPI, and TPI = 1 ÷ pitch.

Can metric pitch be converted to imperial TPI?

Yes. Divide 25.4 by the pitch in millimeters. A 1.5 mm pitch equals 25.4 ÷ 1.5, or about 16.9 TPI.

How can thread pitch be measured without a gauge?

Measure the distance across ten complete threads with a ruler or caliper, then divide that distance by ten to get the pitch.

Are pitch and lead the same measurement?

Only on single-start threads, which are the most common type. On multi-start threads, lead equals pitch times the number of starts.

Why do coarse and fine threads both exist?

Coarse threads (larger pitch) go in and out faster and resist stripping in soft materials. Fine threads (smaller pitch) hold more tightly under vibration and allow finer adjustment, but they are more easily cross-threaded.

References

1. American Society of Mechanical Engineers. (2003). ASME B1.1: Unified Inch Screw Threads (UN and UNR Thread Form).
2. International Organization for Standardization. (1998). ISO 68-1: ISO general purpose screw threads — Basic profile — Metric screw threads.
3. Oberg, E., Jones, F. D., Horton, H. L., & Ryffel, H. H. (2016). Machinery's Handbook (30th ed.). Industrial Press.
4. National Institute of Standards and Technology. SP 811: Guide for the Use of the International System of Units.