ANSI Thread Sizes

Screw Diameter Specifications

• The ANSI standard describes screw thread dimensions in terms of three diameters. The "major" diameter is the diameter of the screw from the outside of a thread on one side to the outside of the thread on the other side. The "minor" diameter is the diameter of the screw at its thinnest point, the bottom of the ridges between the threads. The "pitch" diameter is the diameter at which the width of the thread is equal to the space between the threads. The thicker the thread, the greater the pitch diameter.

Thread Classes

• The dimensional tolerance for threads are designated by their "thread class." The two most common thread classes are 2A and 2B. The actual dimensions of class 2B threads are equal to their stated (or "nominal") dimensions, while the actual dimensions of a class 2A thread are slightly smaller to allow for the width of an additional coating which may be applied to the screw.

Single-Digit Screw Specifications

• Below 1/4 of an inch in major diameter, screws in the ANSI standard are named by single digits, from 0 to 10. Size 0 screws have a major diameter of 0.0600 inch, a minor diameter of 0.0447 inch and a pitch diameter of 0.0519 inch. On the other end of the spectrum, Size 10 screws have a major diameter of 0.1900 inch, a minor diameter of 0.1517 inch, and a pitch diameter of 0.1697 inch.

Fractional Screw Specifications

• Above 1/4 in major diameter, the ANSI standard designates screws at their nominal major dimension. For example, a 1/4-20 screw will have a major diameter of 0.25 inch, a 1 1/2 -12 screw will have a major diameter of 1.5 inches, and so on. Other diameters will be proportionately smaller. A 1-12 screw, for instance, will have a major diameter of 1 inch, a minor diameter of 0.8978 inch, and a pitch diameter of 0.9459 inch.