Thread Size Charts & Gauge Selection Tables

How to Use a Thread Chart to Pick the Right Gauge

A thread chart is most useful when you read it in a fixed order. Before ordering a gauge, work through these steps with the drawing and the chart side by side.

1. Identify the thread system. Metric (M), Unified (UNC, UNF, UNEF), pipe (NPT, BSP G or R) or trapezoidal/Acme. The prefix or suffix on the drawing usually tells you.
2. Read the nominal size and pitch. For metric threads the pitch is in mm (M10×1.25); for inch threads it is threads per inch (3/8-24). If a metric size has no pitch, it is coarse pitch.
3. Find the tolerance class. 6H or 6g for metric, 2B or 2A for Unified. The class decides which GO/NO-GO gauge you need.
4. Decide internal or external. Internal threads are checked with plug gauges; external threads with ring gauges.
5. Check special requirements. Left-hand thread, blind-hole depth, coating, or a customer-specified gauging standard.

Reading across the wrong row is a common slip on large charts, especially between coarse and fine pitch rows of the same diameter. Confirm the pitch twice before ordering.

With these five items, the gauge specification is complete. Missing any of them is the main reason for wrong gauges and repeat orders.

Tap Drill Size vs Gauge Size

Thread charts often list tap drill sizes next to thread sizes, and the two are sometimes confused. The tap drill sets the hole’s minor diameter before tapping; it is not a gauge dimension. The thread plug gauge checks the finished pitch diameter and form after tapping.

A correct drill size does not guarantee a good thread. Tap wear, runout and chip packing can still put the pitch diameter out of tolerance, which only a gauge will detect. A plain plug gauge is sometimes used to check the minor diameter separately.

Common Look-Alike Threads

Some thread sizes from different systems are close enough to start by hand, then bind or leak. These pairs cause the most mix-ups in incoming inspection.

Hand tools separate these pairs, but the final check should always be the correct GO/NO-GO gauge. For pipe threads, see our NPT vs BSP gauge comparison; for Unified series, see UNC vs UNF vs UNEF explained.

Where to Find Tolerance Limits

A thread chart gives nominal sizes and pitches, but a gauge is made to tolerance limits: minimum and maximum pitch diameter, major and minor diameter for the specified class. These come from ISO 965 for metric threads, ASME B1.1 for Unified threads, and the pipe standards for NPT and BSP. Gauge dimensions themselves are defined in ISO 1502 (metric) and ASME B1.2 (Unified).

For metric sizes, our metric thread plug gauge size chart with tolerance limits lists pitch diameter limits by size. If your thread is non-standard, send the drawing and we will quote a custom gauge.

Thread Chart Questions

Is the tap drill size the same as the gauge size?

No. The tap drill size sets the hole diameter before tapping and roughly controls the thread’s minor diameter. A thread plug gauge checks the finished thread’s pitch diameter and form. You need the thread size, pitch and tolerance class to order a gauge; the tap drill size is not used in the gauge specification.

What does it mean if a metric thread size has no pitch?

A metric size written without a pitch, such as M10, means the ISO coarse pitch, which is 1.5 mm for M10. Fine pitches are always written in full, for example M10×1.25. When ordering gauges, write the pitch explicitly to avoid mistakes, even for coarse threads.

Can I use a thread chart to check if a part is in tolerance?

Not directly. A chart gives nominal dimensions and pitches, which help identify the thread. Tolerance acceptance needs pitch diameter limits for the class and a gauge or measurement system. For production inspection, use GO/NO-GO thread gauges made to ISO 1502 or ASME B1.2.

Why does a nut start on a bolt but then bind?

The most common cause is a look-alike thread from a different system, such as an M10×1.5 nut on a 3/8-16 UNC bolt. The diameters are close and the pitch difference is small, so the first turns engage before the threads drift out of phase. Other causes are damaged threads, burrs or oversize coatings.