Glossary & Reference
Thread height is how deep the thread groove actually cuts into the part — a different question entirely from whether two threaded parts fit together.
Thread height (also called thread depth) is the radial distance between the crest and the root of a screw thread, measured perpendicular to the thread axis. It's designated hs for external threads and hn for internal threads. It's a different measurement than pitch diameter: pitch diameter (see our Pitch Diameter page) tells you where the thread crosses its own half-depth point, which controls how two threaded parts fit and engage. Thread height tells you how much material was actually removed to cut the profile in the first place.
Every straight thread has a major diameter (crest to crest, the largest) and a minor diameter (root to root, the smallest). Thread height is simply half the difference between the two: h = (d − d1) / 2 for an external thread. Because either the crest or the root is always deliberately relieved rather than cut to a sharp point, thread height is treated as a reference dimension — it's usually not directly toleranced or inspected on its own.
Both ISO metric and Unified (UN/UNC/UNF) threads use a symmetric 60° thread form. The theoretical "sharp-V" height of that 60° triangle works out to 0.866 × pitch, but a sharp V is never actually produced — the crest and root are flattened to avoid a fragile knife-edge and to leave working clearance. Once the top (roughly 1/8) and bottom (roughly 1/4) of that triangle are truncated, the real, standard thread height that remains is:
hn = hs = 0.5 × (d − d1) ≈ 0.54127 × P
This applies to both external and internal 60° threads and holds regardless of diameter — a fine pitch cuts a shallow thread, a coarse pitch cuts a deep one, independent of how large the fastener is.
Thread height is effectively how deep a threading tool has to plunge to form a complete, in-spec thread — set by the tap drill and tap geometry on an internal thread, or the total infeed of a single-point tool or die on an external one. Because it depends only on pitch, a coarser-pitch thread demands a deeper total cut than a fine-pitch thread of the same diameter, which usually means more threading passes and higher cutting force per pass on the lathe.