How to measure thread pitch without an off-by-one error

Metric: measure the spaces

Place one reference line on a thread crest, then find a crest several threads away. Measure the distance between those two crests and divide by the number of spaces between them.

Five visible crests contain four spaces. If the first-to-fifth crest distance is 4.0 mm, the pitch is 4.0 ÷ 4 = 1.0 mm. Dividing by five would create the classic off-by-one error.

Inch: count over a known distance

TPI means threads per inch. If a full inch of clean thread is available, count the repeating spaces over that inch. If only half an inch is available, count the spaces and multiply by two.

A pitch gauge is faster and usually safer: hold the leaf or on-screen lines against several crests. The correct pitch stays aligned across the run; a near miss gradually drifts.

Using an on-screen thread comparator

1. Lay the bolt so the thread crests touch the screen without rolling.
2. Look straight down over the compared section.
3. Adjust until the blue reference lines sit in the same repeating position on several threads.
4. Ignore a single damaged crest; judge the pattern across the full visible run.
5. If two candidates look close, keep both until diameter and a known nut or hole settle it.