What is pitch diameter?
The fetched Omni pitch diameter page explains that pitch diameter is the thread diameter where the thread thickness and thread gap are equal.
Different pitch diameter formulas
d2 = d - 0.6495 x P
External: d2max = d2 + es ; d2min = d2 + es - Td2
Internal: D2max = D2 + EI + TD2 ; D2min = D2 + EI
ISO metric thread scope
Like Omni, this tool is limited to 60-degree ISO metric threads. It uses the tolerance-grade and tolerance-position equations shown on the fetched pitch diameter and thread pages.