Give us a diameter, a thread pitch, and a length and we can almost always find your part. Diameter and thread are measured the same way no matter the fastener — so we’ll cover those here. Length is the one that trips people up, because it’s measured differently depending on the fastener and its head. Once you’ve got the first two, pick your type below for the right way to measure length.
Measure straight across the shank (the round body), not across the threads’ outer points if you can help it — though for a quick read, across the thread crests is close enough. This is the first number in a size: the ¼ in “¼–20.”
Smaller fasteners use gauge numbers instead of fractions — #6, #8, #10 — where a bigger number means a bigger screw. Above about ¼″, sizes switch to fractions of an inch.
No caliper? Lay the fastener on a ruler and read the width, or stand it next to one you already know. Even “a hair over ¼″” helps us narrow it down.
Thread pitch is the second number in a size — the 20 in “¼–20” — and it means threads per inch. To read it, line the threads up against a ruler and count how many crests fall in one inch. More threads packed in = a fine thread; fewer = a coarse thread.
You don’t have to nail this exactly. Coarse and fine come in a fixed set of values for each diameter, so once you have the size, our thread pitch & tap-drill chart tells you which pitches are even possible.
Easiest of all: bring the fastener to either counter and we’ll drop it on a thread gauge — an instant, exact read.
Here’s the part that causes wrong orders. A bolt’s length is measured from under the head, but a flat-head screw is measured overall, and a stud is measured a different way again. The rule follows how the fastener sits when it’s installed — so pick yours for the right method and a diagram.
Hex, round, pan, button — any head that sits on top.
Length: under head to tip →Any head that sinks flush into the surface.
Length: overall, head included →No head — threaded rod, double-end, or tap-end.
Length: end to end (with a catch) →The U-shaped one — with the pipe-size trap.
Width, leg length & thread →No ruler handy? Print this page at 100% scale (not “fit to page”) and the strip below comes out true to size — a quick six inches for measuring at the bench. Check it against a known ruler before you trust a tight measurement.
Printer scaling varies — always sanity-check the printed inch against a real ruler before measuring anything critical.
A sample beats any measurement. Bring the fastener (or what it came out of) to either counter and we’ll size the diameter, thread, and length on the spot — the right way for that fastener.
