Glossary & Reference
Addendum is just the top half of thread height — the part of the crest that sits above the pitch line, and the first thing a worn cutting edge gives away.
Thread addendum is the radial distance from the pitch line out to the crest of an external screw thread — the portion of the thread profile that sits outside the theoretical pitch diameter established on our Pitch Diameter page. It's designated has. Because the pitch-line concept only applies meaningfully to the projecting thread ridge, internal threads don't have an addendum of their own.
The pitch line splits a thread profile into two regions. Everything above it, out to the crest, is the addendum. Everything below it, down to the root, is the dedendum. Thread height (see our Thread Height / Depth page) is simply addendum plus dedendum added together — addendum is the upper half of that same total depth, not a separate measurement.
For the same 60° symmetric thread form used by ISO metric and Unified inch threads, addendum is measured from the major diameter down to the pitch diameter: has = (d − d2) / 2, where d is major diameter and d2 is pitch diameter. Substituting the standard relationship between pitch diameter and pitch reduces this to:
has = 0.5 × (d − d2) ≈ 0.32476 × P
Like thread height, addendum depends solely on pitch — and it's noticeably smaller than full thread height (0.54127 × P), since it only covers the crest-side portion of the total depth.
The same term shows up in gear design, where addendum means the radial distance from a gear's pitch circle out to the tip (outside) circle of the tooth — conceptually the same idea as in threads: the projecting portion of the profile measured outward from the reference pitch line or circle.
Addendum matters most during inspection. A dedicated addendum gauge — a flat reference resting on the crest plus a ball-tipped indicator — measures it directly, and it's often the fastest way to catch a thread cut with a worn or chipped tool: the crest is usually the first part of a threading insert to wear, and that shows up as an addendum error before it shows up anywhere else on the part.