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Pitch Diameter Calculator

Construction

Calculate the pitch diameter of a 60° screw thread from major diameter and threads per inch. Free tool for machinists, toolmakers, and thread gauging.

0.054
480

Pitch Diameter

0.218
Thread Pitch
0.05

This calculator computes your Pitch Diameter, Thread Pitch from the values you enter.

Inputs
Major DiameterThreads Per Inch (TPI)
Outputs
Pitch DiameterThread Pitch

What is a Pitch Diameter?

A Pitch Diameter Calculator computes the pitch diameter of a screw thread — the theoretical diameter, measured partway down the thread flank, where the thread width equals the groove width. Given a thread's major diameter and threads per inch (TPI), the calculator applies the standard 60-degree thread form relationship to return both the pitch diameter and the thread pitch itself.

Pitch diameter is the dimension that governs how tightly or loosely an external thread mates with an internal thread. It sits between the major diameter (the crest-to-crest measurement) and the minor diameter (the root-to-root measurement), and it is the reference dimension used in nearly every thread tolerance class, from commercial-grade fasteners to precision aerospace threads.

This tool is scoped narrowly to pitch diameter. If you also need the minor diameter of the same thread, use the broader Thread Calculator, which reports both pitch diameter and minor diameter side by side.

How to use this Pitch Diameter calculator

  1. Enter the Major Diameter in inches, using the slider or number field. This is the nominal outside diameter of the thread, such as 0.25 in for a 1/4-inch fastener.

  2. Enter the Threads Per Inch (TPI) for the thread you're checking. Common values include 20 for 1/4-20 UNC or 28 for 1/4-28 UNF.

  3. Read the Pitch Diameter in the highlighted result card. This is the theoretical pitch diameter for a standard 60-degree thread form at the sizes you entered.

  4. Check the Thread Pitch shown beneath the headline result if you need the raw pitch value for gear-train setup or comparison against a metric pitch specification.

  5. Cross-reference against a thread standard table (such as ASME B1.1) if you need a manufacturing tolerance range rather than the theoretical nominal value calculated here.

Formula & Methodology

The calculator uses the standard theoretical relationship for a 60-degree (Unified/ISO-style) thread form:

Step 1 — Thread Pitch:

> p = 1 ÷ TPI

Where:
- p = thread pitch in inches
- TPI = threads per inch

Step 2 — Pitch Diameter:

> PD = D − 0.64952 × p

Where:
- PD = pitch diameter in inches
- D = major diameter in inches
- 0.64952 = the standard 60-degree thread form constant (the height, in units of pitch, subtracted twice to move from the crest to the pitch line on each thread flank)

Worked example — 1/4-20 thread:

- p = 1 ÷ 20 = 0.05 in
- PD = 0.25 − (0.64952 × 0.05) = 0.25 − 0.03248 = 0.2175 in

This theoretical value matches published pitch diameter references for a 1/4-20 UNC thread. For the same thread's minor diameter as well, use the Thread Calculator, which applies the related constant 1.29904 to compute the root diameter alongside pitch diameter.

Frequently Asked Questions

Pitch diameter is the diameter of an imaginary cylinder that passes through the threads at the point where the width of the thread and the width of the groove are equal. It sits between the major diameter (the outside of the thread) and the minor diameter (the root of the thread). Pitch diameter is the single most important dimension for determining whether an external and internal thread will fit together correctly.
The constant 0.64952 comes from the geometry of a standard 60-degree thread form, which is used by both Unified (UN/UNC/UNF) and ISO metric threads. It represents the vertical distance, in units of pitch, from the crest of the thread down to the pitch line on each side. Multiplying this constant by the thread pitch and subtracting it twice from the major diameter yields the theoretical pitch diameter for a sharp-vee 60-degree thread.
Pitch is the reciprocal of threads per inch (TPI). For example, a 1/4-20 thread has 20 threads per inch, so its pitch is 1 divided by 20, which equals 0.05 inches. This calculator performs that division automatically whenever you change the TPI value.
This calculator is set up for inch-based threads defined by threads per inch, which is standard for Unified (UN) thread callouts used across the US and much of the English-speaking world. Metric threads are defined directly by pitch in millimeters rather than TPI, so a metric thread would need its pitch converted to an equivalent inch value before being entered here.
Major diameter is the largest diameter of a screw thread, measured across the crests of an external thread or the roots of an internal thread. Pitch diameter is always smaller than major diameter for an external thread, because it is measured partway down the thread flank rather than at the outer crest. The difference between the two is directly proportional to the thread pitch.
Thread fit classes, such as 2A/2B or 3A/3B in the Unified system, are defined by tolerance ranges on pitch diameter rather than on major or minor diameter. Two fasteners can have identical major diameters yet fit very differently if their pitch diameters differ, because pitch diameter controls how much thread flank contact and clearance exists between mating parts.
Yes. If you know the nominal major diameter and TPI of a thread specification, this calculator gives you the theoretical pitch diameter to compare against a three-wire measurement or thread micrometer reading. Keep in mind that real-world thread gauges report a measured pitch diameter that can vary slightly within its tolerance class, so treat the calculator's output as the nominal target rather than an exact go/no-go value.
Common Unified coarse (UNC) threads range from about 4 TPI on very large bolts down to 20 TPI on a quarter-inch bolt, while Unified fine (UNF) threads run finer for the same diameter, such as 28 TPI on a quarter-inch bolt. Small machine screws can run as fine as 80 TPI. The calculator's slider covers this full practical range from 4 to 80 TPI.
The formula used is the standard theoretical relationship for a sharp-vee 60-degree thread form defined in ASME B1.1 and ISO 68-1. It is accurate for design and estimation purposes, but actual manufactured threads have a flat or rounded root and crest rather than a perfectly sharp vee, which shifts the practical pitch diameter by a small, standardized allowance. For precision manufacturing tolerances, consult the relevant thread standard tables.
Yes, pitch diameter and effective diameter are commonly used as synonyms in thread terminology, particularly in older or British engineering references. Both terms describe the same imaginary cylinder used to define thread fit and tolerance.
Increasing TPI decreases the pitch, since pitch equals 1 divided by TPI. A smaller pitch means a smaller reduction is subtracted from the major diameter, so pitch diameter moves closer to major diameter as TPI increases. This is why fine-thread (UNF) fasteners have a pitch diameter closer to their major diameter than coarse-thread (UNC) fasteners of the same nominal size.
The formula applies to the nominal (basic) pitch diameter shared by both external and internal threads of the same size, since a bolt and its mating nut are designed around the same nominal pitch diameter before tolerances are applied. Actual external and internal pitch diameters diverge slightly once class-of-fit tolerances are added, with external threads sized slightly under nominal and internal threads sized slightly over nominal.
Also known as
screw pitch diameter calculatorthread pitch diameter calculator60 degree thread pitch diameterunified thread pitch diameterthread gauge pitch diameter