Screw and Bolt Size Gauge with Thread Pitch Chart

A free tool for reading the size of an unknown screw or bolt. A metric fastener is named by its major diameter and thread pitch — M4 × 0.7, for instance — while imperial machine screws carry a gauge number and a threads-per-inch count such as #8-32. Lay your fastener on a calibrated ruler or a printed 1:1 strip, match the diameter and the pitch, and the tool names the size in whichever standard fits.

Reading a screw size step by step

1
Measure across the thread crests

Lay the threaded shank on the diameter scale and read the widest point — the major diameter the M-number is built on.

2
Count the pitch

Hold the threads against the 60-degree combs (or count threads over 10 mm) until one profile sits crest-for-crest with yours.

3
Take the length

Run the fastener along the length scale — from under the head for a countersunk screw, from the very top for a pan or cheese head.

4
Read both standards

The tool combines diameter and pitch into one designation and offers the nearest gauge-number equivalent for an imperial screw.

Work out the M-designation of any fastener from its threads

What this gauge does

Names the full M-spec
Returns a complete designation such as M5 × 0.8, not just a diameter, because the pitch is what decides whether a nut threads on.
Coarse and fine pitch
Distinguishes the coarse and fine variants that share a diameter — M8 × 1.25 coarse versus M8 × 1.0 fine — which standard charts often blur together.
Metric ↔ gauge-number cross-reference
Bridges metric M1.6–M12 and imperial #2 to 1/4" UNC/UNF machine screws so a #8-32 reads alongside its nearest M-size.
60-degree pitch strips
The printed sheet carries life-size ISO 60-degree thread profiles from 0.35 to 1.75 mm — lay the threads against each comb until the crests line up.
Major versus nominal diameter
Measures the crest-to-crest major diameter (the figure the M-number refers to), not the smaller core, so the reading maps to the standard.

Why this beats guessing from a chart

A printed size chart only helps once you already know the diameter and pitch; the hard part is measuring them off a loose screw with no caliper. This tool turns a calibrated screen or a 1:1 printout into both a diameter rule and a pitch comb, then does the metric-to-imperial matching for you — so you walk into the hardware aisle knowing it is an M5 × 0.8, not "about 5 millimetres".

Screw sizing questions

What does the "M" in a screw size actually mean?

The M is the ISO metric thread prefix, and the number after it is the major diameter in millimetres: an M5 screw measures 5 mm across the thread crests. A second number gives the pitch, so M5 × 0.8 is a 5 mm screw with 0.8 mm between threads.

Two screws look the same diameter but one will not thread on — why?

They almost certainly have different pitches. M8 is sold in a 1.25 mm coarse pitch and a 1.0 mm fine pitch; the diameters are identical but the threads are not, so a coarse nut binds on a fine bolt. Matching the pitch comb is what prevents this.

How do I count threads per inch on an imperial screw?

Line the threads up with a ruler and count how many crests fall within one inch — a #8-32 screw has 32. Pair that with the gauge number and you have the full call-out; the tool also shows the closest metric size.

Should I measure the major or the minor diameter?

Use the major diameter — the outer crest-to-crest width — because that is the figure the M-number and the gauge number both refer to. The minor (root) diameter is always smaller and would read as the wrong size.

What is the difference between a machine screw and a wood screw here?

This gauge is built around constant-diameter machine-screw and bolt threads that mate with a nut or tapped hole. Tapered wood and self-tapping screws can be measured for diameter and length, but their threads do not follow a fixed metric pitch.

My screw falls between two sizes — which do I pick?

Threads wear and plating adds a little width, so round to the nearest standard pitch and diameter rather than an exact reading. If the pitch comb matches one size cleanly, trust the pitch over a borderline diameter.

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