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Reference Chart

Thread Chart

Look up major, minor, and pitch diameters for common UNC threads. This chart covers Class 2A (external) and Class 2B (internal) threads from #2 through 3/4 inch.

Showing 24 of 24 threads
Thread Size ▲ Class ▲ Major Dia Max (in) ▲ Major Dia Min (in) ▲ Pitch Dia Max (in) ▲ Pitch Dia Min (in) ▲ Minor Dia (in) ▲
#2-56 UNC 2A 0.0860 0.0744 0.0744 0.0692 0.0628
#2-56 UNC 2B 0.0860 — 0.0759 0.0700 0.0667
#4-40 UNC 2A 0.1120 0.0985 0.0958 0.0894 0.0795
#4-40 UNC 2B 0.1120 — 0.0982 0.0911 0.0849
#6-32 UNC 2A 0.1380 0.1218 0.1177 0.1101 0.0974
#6-32 UNC 2B 0.1380 — 0.1202 0.1120 0.1042
#8-32 UNC 2A 0.1640 0.1478 0.1437 0.1353 0.1234
#8-32 UNC 2B 0.1640 — 0.1460 0.1371 0.1302
#10-24 UNC 2A 0.1900 0.1719 0.1629 0.1534 0.1359
#10-24 UNC 2B 0.1900 — 0.1654 0.1555 0.1449
1/4"-20 UNC 2A 0.2500 0.2300 0.2175 0.2067 0.1850
1/4"-20 UNC 2B 0.2500 — 0.2211 0.2094 0.1959
5/16"-18 UNC 2A 0.3125 0.2912 0.2764 0.2643 0.2403
5/16"-18 UNC 2B 0.3125 — 0.2800 0.2674 0.2524
3/8"-16 UNC 2A 0.3750 0.3525 0.3344 0.3209 0.2938
3/8"-16 UNC 2B 0.3750 — 0.3382 0.3242 0.3073
7/16"-14 UNC 2A 0.4375 0.4131 0.3911 0.3762 0.3447
7/16"-14 UNC 2B 0.4375 — 0.3955 0.3800 0.3602
1/2"-13 UNC 2A 0.5000 0.4745 0.4500 0.4337 0.4001
1/2"-13 UNC 2B 0.5000 — 0.4548 0.4378 0.4167
9/16"-12 UNC 2A 0.5625 0.5354 0.5084 0.4907 0.4542
9/16"-12 UNC 2B 0.5625 — 0.5135 0.4951 0.4723
5/8"-11 UNC 2A 0.6250 0.5963 0.5660 0.5468 0.5069
5/8"-11 UNC 2B 0.6250 — 0.5715 0.5516 0.5266
3/4"-10 UNC 2A 0.7500 0.7195 0.6850 0.6640 0.6201
3/4"-10 UNC 2B 0.7500 — 0.6912 0.6693 0.6417
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Quick Answer: What Is a Thread Size Chart?

A thread size chart lists the major, minor and pitch diameters for each standard screw thread. It tells you the exact dimensions a thread must hold to fit its mating part. Machinists, designers and inspectors use it to pick a thread, set a tap drill and check parts against a gage.

This chart uses ASME B1.1 Unified Inch Screw Threads to give: major diameter (max and min), pitch diameter (max and min), and minor diameter. It covers UNC sizes from #2 through 3/4″, in Class 2A (external) and Class 2B (internal).

Common sizes: 1/4″–20 UNC major 0.2500″ | 3/8″–16 UNC major 0.3750″ | 1/2″–13 UNC major 0.5000″ | Class 2A is the bolt, Class 2B is the nut.

Notes on This Chart

  • All values are per ASME B1.1 threading standard for Unified Inch Screw Threads.
  • Class 2A/2B is the most common fit for general-purpose fasteners.
  • A dash (—) means that value does not apply for the given class.
  • Internal thread major diameter minimums are not controlled by the nut.
  • Always check your thread gage before production to confirm fit.

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Frequently Asked Questions

What is the difference between Class 2A and Class 2B threads?

Class 2A is for external threads. These go on bolts and screws. Class 2B is for internal threads. These go inside nuts and tapped holes. Class 2A has a small gap built in so the bolt can slide into the nut. These two classes are the most common in general machining.

What are major, minor, and pitch diameters?

The major diameter is the biggest diameter of the thread. The minor diameter is the smallest, at the root. The pitch diameter sits between the two. At the pitch diameter, the thread width and the groove width are the same. The pitch diameter is what controls how tight or loose a thread fits.

What is UNC vs UNF?

UNC stands for Unified National Coarse. It has fewer threads per inch and is the most common type. UNF stands for Unified National Fine. It has more threads per inch. UNF gives a tighter fit and more holding power. Use UNC for general work. Use UNF when you need finer control.

How do I pick the right tap drill size?

The tap drill size is based on the minor diameter of the internal thread. A good rule is to aim for about 75% thread depth. This gives a strong thread without putting too much load on the tap. Check a tap drill chart for the exact drill size for each thread.

Thread & Fastener Reference Series

Six connected thread references — jump to the one you need:

Thread Size ChartThread Pitch ChartMetric ↔ Imperial ConversionThreading Standards GuideNPT Pipe Thread ChartThread Engagement Calculator

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