Diameter alone does not distinguish M10 × 1.5 from M10 × 1.25, and pitch alone does not tell you the size. A thread pitch gauge — a fan of toothed leaves — settles the pitch in seconds and costs very little.
Then the standard: metric coarse and fine, UNC and UNF, BSP for pipe. Pipe threads are the common trap because their nominal size bears no relation to their measured diameter.
The questions welders ask
Why does a BSP thread measure wrong?
Because pipe sizes are named for a nominal bore, not the thread diameter. A "half-inch BSP" fitting measures about 21 mm (0.83 in) across the thread. Trying to match it by measurement alone leads people to the wrong size every time, which is why the standard has to be identified first.
Last reviewed 2026-08-14