Three measurements identify almost any fastener: how wide, how the threads are spaced, and how long. Two of those work the same for everything — the third depends on what you’re holding.
Three numbers find almost any part. They read left to right in every size:
Diameter and thread are measured the same way on everything. Length is the one that trips people up — it changes with the head. Start with the first two:
Diameter — how wide
Measure straight across the shank (the round body). Across the thread tips is close enough for a quick read. It’s the first number in a size: the ¼ in “¼–20.”
Small screws use gauge numbers — #6, #8, #10 — and a bigger number means a bigger screw. Above about ¼″, sizes switch to fractions of an inch.
No caliper? Lay it on a ruler, or stand it next to one you already know. Even “a hair over ¼″” helps us narrow it down.
Thread pitch — how the threads are spaced
The second number — the 20 in “¼–20” — is threads per inch. Hold it to a ruler and count the crests in one inch. More = fine; fewer = coarse.
You don’t have to nail it. Each diameter only comes in a few pitches, so once you have the size, the thread pitch & tap-drill chart shows which are possible.
Easiest of all: bring it to either counter and we’ll drop it on a thread gauge — an instant, exact read.
3. Length — it depends on your fastener
This is what causes wrong orders. The rule: measure the part that ends up buried in the work.
Pick yours for the full method and a bigger diagram:
Bolt / hex cap screw
Hex, round, pan, button — any head that sits on top.
Length: under head to tip →Flat-head (countersunk) screw
Any head that sinks flush into the surface.
Length: overall, head included →Stud / all-thread
No head — threaded rod, double-end, or tap-end.
Length: end to end (with a catch) →U-bolt
The U-shaped one — with the pipe-size trap.
Width, leg length & thread →Printable inch ruler
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.
Still not sure? Bring it in.
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.