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Thread Pitch Calculator

Construction

Convert threads per inch (TPI) to thread pitch in inches and millimeters instantly. Free tool for machinists, engineers, and DIY builders working with screws.

Reviewed by the thecalcu.com team · Last updated July 8, 2026

480

Thread Pitch (mm)

1.27
Thread Pitch (in)
0.05

This calculator computes your Thread Pitch (mm), Thread Pitch (in) from the values you enter.

Inputs
Threads Per Inch (TPI)
Outputs
Thread Pitch (mm)Thread Pitch (in)

What is a Thread Pitch?

A Thread Pitch Calculator converts threads per inch (TPI), the standard way imperial fasteners specify thread spacing, into thread pitch expressed in both inches and millimeters. This lets you quickly cross-reference an imperial fastener against metric thread charts, taps, dies, or nuts without doing the conversion math by hand.

Pair this calculator with the Pitch Diameter Calculator when you need the full thread geometry, or the Bolt Torque Calculator once you've confirmed pitch and are ready to tighten the fastener to spec.

Why Use a Thread Pitch Calculator?

Fastener catalogs, taps, dies, and thread gauges from different countries mix TPI and millimeter pitch conventions, and mixing them up leads to cross-threading, stripped threads, or ordering the wrong replacement part. This calculator removes the guesswork by giving you both values side by side from a single TPI input.

It's also a fast way to double-check a thread pitch gauge reading or verify a fastener spec sheet before placing an order.

Who Should Use This Calculator?

Machinists use this calculator to cross-reference TPI against metric tap and die charts when working across imperial and metric shop equipment. Mechanical engineers use it while specifying fasteners on drawings that mix unit systems. Maintenance technicians use it in the field to identify a replacement bolt or nut when only a TPI count is available. DIY builders and hobbyists use it to match hardware store fasteners to metric parts on imported equipment. Purchasing and procurement staff use it to verify fastener specifications before ordering stock.

What Insights Does the Thread Pitch Calculator Give You?

The Thread Pitch (mm) is the primary result, the distance between adjacent thread crests in millimeters, useful for matching against metric thread charts, calipers, or CAD drawings that specify pitch in millimeters. The Thread Pitch (in) output gives the same spacing in inches, which is the native unit most imperial thread standards (like UNC and UNF) are built around and is useful when working directly with inch-based thread pitch gauges.

How to use this Thread Pitch calculator

  1. Enter or slide to your fastener's Threads Per Inch (TPI) value, check the fastener packaging, a thread pitch gauge, or a thread chart if unknown.
  2. Review the Thread Pitch (mm) result to cross-reference against metric thread charts, taps, or dies.
  3. Review the Thread Pitch (in) result if you're working with imperial thread pitch gauges or specifications.
  4. Use the result to confirm you have the correct matching nut, tap, die, or replacement fastener before purchasing or machining.
Show formula & methodology ↓Show less ↑

Formula & Methodology

The calculator converts threads per inch directly to pitch using the reciprocal relationship between TPI and pitch:

Pitch (in) = 1 ÷ Threads Per Inch

Pitch (mm) = Pitch (in) × 25.4

Worked example: For a fastener with 20 TPI:

Pitch (in) = 1 ÷ 20 = 0.05 in

Pitch (mm) = 0.05 × 25.4 = 1.27 mm

Frequently Asked Questions

What is thread pitch?
Thread pitch is the distance between two adjacent thread crests, measured along the axis of a screw or bolt, typically expressed in millimeters for metric fasteners or as threads per inch (TPI) for imperial fasteners. It determines how far a fastener advances into a mating thread with each full turn.
How do I convert TPI to pitch in millimeters?
Divide 1 by the threads-per-inch value to get pitch in inches, then multiply that result by 25.4 to convert to millimeters. For example, 20 TPI gives a pitch of 0.05 inches, which equals 1.27 mm.
Why do imperial and metric fasteners use different pitch conventions?
Imperial fasteners (like UNC and UNF) specify threads per inch because that convention grew out of inch-based machining standards in the US and UK, while metric fasteners (like the M series) specify pitch directly in millimeters since the entire system is based on the millimeter. Converting between the two is necessary whenever you're matching a fastener to a nut, tap, or die from the other system.
What's the difference between coarse and fine threads?
Coarse threads have a larger pitch (fewer threads per inch) and are generally stronger, faster to assemble, and more forgiving of minor damage or debris, while fine threads have a smaller pitch (more threads per inch) and offer finer adjustment, higher clamping force for a given torque, and better resistance to loosening under vibration.
How accurate does thread pitch need to be for a matching nut or tap?
Thread pitch must match essentially exactly, since even a small pitch mismatch will cause a fastener to bind, cross-thread, or fail to engage at all. Always verify pitch with a thread pitch gauge or caliper measurement rather than relying on a visual estimate.
Can I measure thread pitch with a caliper instead of a gauge?
Yes, measure the distance across several threads (for example, 10 threads) with a caliper, then divide that distance by the number of threads to get pitch, which is often more accurate than a single-thread measurement due to caliper reading error. A dedicated thread pitch gauge is faster for shop use, but a caliper measurement is a reliable cross-check.
What TPI values are common for standard bolts?
Common UNC (coarse) TPI values include 20 TPI for 1/4-20 bolts, 13 TPI for 1/2-13 bolts, and 8 TPI for 1-8 bolts, while UNF (fine) threads run higher, such as 28 TPI for 1/4-28 bolts. Always check the markings on the fastener head or reference a thread chart to confirm before ordering a matching nut or tap.
Does thread pitch affect fastener strength?
Thread pitch affects the tensile stress area and thread engagement, which in turn influence a fastener's strength, finer threads generally have a slightly larger stress area for a given nominal diameter, giving a small strength advantage, though the fastener's grade and material typically matter more than pitch alone.
How is thread pitch related to lead and starts on multi-start screws?
On a single-start thread, pitch and lead are the same value, the distance the fastener advances in one full turn. On multi-start threads, lead equals pitch multiplied by the number of starts, since each turn advances the fastener by the combined distance of all the parallel thread starts.
Why does my TPI value need to be within a specific range for calculation?
This calculator accepts TPI values from 4 to 80 to cover the practical range used in standard fasteners, from very coarse threads on large lag bolts down to very fine threads on precision instrument screws. Values outside this range are uncommon in general mechanical and construction work.
How does this calculator relate to other fastener and GD&T tools?
Use this Thread Pitch Calculator to confirm the pitch of a fastener before ordering a matching nut, tap, or die, then check hole clearance with the [Clearance Hole Calculator](/clearance-hole-calculator/) or verify tightening torque with the [Bolt Torque Calculator](/bolt-torque-calculator/). If you're inspecting hole position on a drawing, the [True Position Calculator](/true-position-calculator/) covers the related GD&T position tolerance calculation.
Also known as
TPI to pitch calculatorthreads per inch converterthread pitch in mm calculatorscrew thread pitch calculatorbolt thread pitch converter