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# How do I calculate pipe thread size?

## How do I calculate pipe thread size?

To determine the size of a pipe, the outside diameter of each pipe or fitting must be measured and compared to the table for size identification. For example, a 3/4″ NPT pipe thread has an outside diameter of 1.050 inches. Each thread size has a defined number of threads per inch (TPI).

Table of Contents

## What size is 3/8 pipe thread?

Basic Dimensions, American National Standard Taper Pipe Threads, NPT, (ANSI B2. 1-1968)

Nominal Pipe Size Outside Diameter of Pipe Threads per Inch
1/16 0.313 27
1/8 0.405 27
1/4 0.540 18
3/8 0.675 18

How do you measure NPT thread size?

Despite the variation, you can still measure an NPT fitting. For the male end, measure across the bottom O.D. of the threads. For the female end, measure across the top I.D. of the threads. Your measurement may be different than what is listed in the table.

### How are threads measured?

General Instructions

1. Measure screw diameter, or screw size, by measuring the width of the threaded part of the screw. This corresponds to the first number of the thread size.
2. Measure thread spacing. This corresponds to the second number of the thread size.

### What is the thread dimension?

Threads are designated or named by the external thread major diameter and a pitch measurement. The major diameter is the outer diameter at the top of the thread crests. Thread sizes are given in nominal sizes, not in the actual measurement. The exact measurement is slightly below the named or nominal size.

How do you specify pipe threads?

Pipe thread sizes are based on an inside diameter (ID) or flow size. For example, “1/2–14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14 threads to the inch, made according to the NPT standard.

## How is 1/4 NPT measured?

As a general “rule of thumb” an NPT thread is approximately 1/4″ (0.25”) larger than its “name.” For a 1/4″ NPT fitting the “nominal” OD is 0.533”. NPT fittings are slightly tapered so the “nominal” diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads.