Hand Tools / SELECTION GUIDE
How to Identify Nut and Bolt Threads: Metric, Inch and Pitch
Read diameter and pitch together, check gently, and keep size identification separate from choosing a replacement fastener.
To identify a loose nut or bolt, find its thread system, diameter and pitch. A labeled nut-and-bolt thread checker can help you find a matching size. For a bolt, use the checker's threaded hole; for a nut, use its threaded stud. Read the complete marking rather than judging the fastener by its head or by one turn of engagement.
Read the size as a pair of numbers
The diameter alone is not enough. Metric fasteners normally pair a nominal diameter with a pitch in millimeters. Common inch fasteners pair a nominal diameter with a thread count in threads per inch, or TPI. Bolt Depot's fastener notation guide explains these separate fields.
| Marking | Diameter field | Thread spacing field | What is still missing? |
|---|---|---|---|
| M6 × 1.0 | 6 mm nominal diameter | 1.0 mm pitch | Bolt length, head, material and other application requirements |
| 1/4-20 | 1/4 in nominal diameter | 20 threads per inch | Bolt length, head, material and other application requirements |
The number after “M6” is a distance, while the number after “1/4” is a count per inch. Bolt Depot's pitch and TPI explanation sets out that distinction.
A thread checker, pitch gauge and caliper answer different questions
A screw-on checker provides labeled mating threads. A leaf-style pitch gauge compares its teeth with the spacing of an exposed thread. A caliper measures a dimension such as a bolt's outside thread diameter. Do not treat a pitch reading alone as a complete fastener identification.
One manufacturer example is S&W Manufacturing's Original Thread Checker, which puts a nut checker and a bolt checker on each labeled gauge. Read the labels and size chart for your own checker; manufacturers use different layouts and color coding.
Before starting, read the instructions supplied with your own checker and confirm which thread systems and sizes it contains. A kit's piece count is not a substitute for its size chart.
Work through a loose fastener without forcing a match
- Look at the part first. Check whether it is a machine-threaded nut or bolt and whether the threads are visibly dirty, deformed or damaged. Do not interpret binding on a damaged thread as proof of a different size. If you cannot expose a sound thread without altering the part, set it aside for inspection.
- Choose a plausible marked size. Use the checker chart and, where useful, an approximate outside-diameter measurement to narrow the candidates. Keep metric and inch candidates separate in your notes.
- Present the correct side squarely. Offer a bolt to the female threaded opening, or a nut to the male threaded stud. Start gently by hand. Do not use a wrench, pliers or a powered driver to make a candidate engage.
- Stop if it binds or feels wrong. An initial catch is not a confirmed match. Withdraw the part, recheck alignment, condition and the complete size marking. Do not force the part farther to “prove” the size.
- Record the full marking and any uncertainty. Write the diameter and pitch or TPI, then record length and the remaining replacement requirements separately. If the fit is ambiguous, leave the identification unconfirmed.
Thread Check's gage-care guidance advises gentle engagement and warns that forcing a gauge can damage the part and gauge.
What to confirm before buying a replacement
A plausible size identification is useful when sorting a box of loose hardware or asking a supplier for a specific thread. It does not establish the fastener's strength grade, material, coating, head dimensions, suitability for reuse or required tightening torque.
For example, finding a thread marked M6 × 1.0 gives you a diameter-and-pitch description to investigate. It does not tell you which length will avoid bottoming in a blind hole or what material the equipment requires. Those answers come from the equipment documentation or its responsible supplier.
If a drawing specifies thread tolerances or a formal GO/NO-GO inspection, use the specified inspection equipment and procedure. A general-purpose checker is not evidence that a part meets that drawing. Keep thread identification and acceptance inspection as separate tasks.
If none of the sizes fits
| Observation | Possible explanation to investigate | Next step |
|---|---|---|
| The approximate diameter looks familiar, but engagement binds | A different pitch, poor alignment or damaged threads | Recheck the full marking and condition; do not force it |
| No candidate is close | The required size or thread family may be outside the kit's range | Compare the actual kit chart with the part documentation |
| The part is a pipe fitting or a special connection | This loose machine-fastener workflow may not apply | Identify the connection system from its documentation before choosing a gauge |
If the size is still uncertain, take the fastener and your measurements to a supplier. Keep it separate from hardware you have already identified.
Choose a checker by its thread list, not its piece count
Our nut and bolt thread checker listing offers 26-, 44-, 46-, 48-, 55-, 66- and 70-piece configurations. Compare the included thread sizes for the exact configuration you select; the gauge selection and layout vary between sets.
Before selecting one, write down the thread systems and diameter-and-pitch pairs you need. Ask for the exact size chart for that option and compare it with your list. A larger set is useful only if its extra gauges cover the threads you need.
Once you have identified the thread and know whether your next task is cutting, cleaning or repairing it, our thread repair kit vs tap and die guide explains the different tool purposes. Choose the repair tool after you have confirmed the thread and the work required.