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A loose bolt with no markings is a guessing game until you measure it. A thread is defined by two numbers, its diameter and its pitch, and together they name it. This takes your measured major diameter and the pitch or threads per inch and tells you the closest standard thread, metric or inch, so you can buy the nut, chase the thread or order more.
Two numbers name a thread
Every standard thread is a diameter and a pitch. The diameter is across the outside of the threads, the pitch is how far apart they are, and the name ties them together: an M6 by 1.0 is six millimeters across with a one millimeter pitch. Measure both and you can look up the rest. One number alone is not enough, because the same diameter comes in coarse and fine pitches, and a 6 mm metric looks a lot like a quarter inch until you check the thread spacing.
Metric or inch, and how they are called
The world runs two thread systems, and they are named differently. Metric threads give the diameter and pitch in millimeters, like M8 by 1.25. Inch threads, UNC coarse and UNF fine, give a size and threads per inch, like quarter twenty. This tool reads either, since you might measure pitch directly in millimeters or count threads per inch with a gauge, and it converts between them so you can compare a mystery thread against both systems at once.
How to measure it
Use calipers across the outside of the threads for the major diameter, and a thread pitch gauge for the spacing: press the leaves against the thread until one matches cleanly, and read it off. No gauge, then count the threads across a measured length and work out the pitch or the threads per inch. Clean the thread first, because grit and burrs throw the reading, and measure an unworn section, since a chewed up end reads small.
When it is between sizes
If the measurement lands between two standard threads, the usual culprit is wear, dirt or a slightly off caliper reading, so measure again on a clean, good section. Metric and inch threads can sit close in both diameter and pitch, a 6 mm against a quarter inch for instance, so when two candidates are near, the surest test is to thread a known nut on by hand. It should spin on smoothly with no force; if it binds or wobbles, it is the other one.
More MakerKiln tools
- tap drillTap Drill Size CalculatorMetric tap drill at the percent of thread you want, plus the clearance hole.→
- bolt torqueBolt Torque CalculatorTightening torque for a metric bolt from its size, grade and lubrication.→
- countersinkCountersink Depth CalculatorPlunge depth so a flat-head screw sits flush, by head, hole and angle.→
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