How To Measure Screw Thread Size - McMaster-Carr

In this guide, we demonstrate how to measure screw thread size for inch and metric systems of measurement.

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. For inches, any diameter smaller than 1/4" is indicated by a number, referenced in the table below (e.g. 8 corresponds to 0.164"). For metric screws, this is the number of millimeters preceded by the letter M (e.g. M6 corresponds to 6 mm).

2. Measure thread spacing. This corresponds to the second number of the thread size. For inches, this is done by counting the number of threads in an inch. For metric screws, this is the length in millimeters between threads.

Inch

Inch thread sizes are specified by diameter and threads per inch. Diameter is also known as screw size. For those with a diameter smaller than 1/4", screw size is indicated by a number (e.g., #10 or No.10). For example, a 10-24 screw has 0.190" thread diameter and has 24 threads per inch.

Inch Equivalents for Numbered Screw Sizes

No.0	No.1	No.2	No.3	No.4	No.5	No.6	No.8	No.10	No.12
.060"	.073"	.086"	.099"	.112"	.125"	.138"	.164"	.190"	.216"

Metric

Metric thread sizes are specified by diameter and pitch, which is the space between threads. For the size M6 × 1 mm, the pitch is coarse; while the size M6 × 0.75 mm has a fine pitch. If you don't know the pitch, choose coarse threads; they're the industry standard. For example, a M6 x 1 mm has a 6 mm thread diameter and has 1 mm between threads.

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