UNC Thread Chart

Basic dimensions for Unified National Coarse threads (ASME B1.1), from #2-56 to 4"-4 — threads per inch, major, pitch and minor diameter, and recommended tap drill size.

UNC (Unified National Coarse) is the coarse-pitch thread of the Unified Thread Standard — the inch-based counterpart to ISO metric coarse threads, standardized in ASME/ANSI B1.1. It shares the same 60° symmetric V profile as metric threads, but uses inch major diameters and threads per inch (TPI) instead of millimetres and pitch.

UNC is the default general-purpose fastener thread across North American industry: automotive, machinery, structural work and castings. Its coarser pitch starts more easily, resists cross-threading, and tolerates minor damage or corrosion better than a fine thread — which is why it's preferred for tapped holes in cast iron, aluminium, plastic and other soft or brittle materials.

Sizes below 1/4" use numbered screw gauges (#2 to #12) instead of a fraction — designation stays "size-TPI", e.g. 1/4-20 UNC means a 1/4" major diameter at 20 threads per inch. UNC has no exact metric equivalent: even close-looking sizes like 3/8-16 UNC and M10 are not interchangeable.

Size TPI Major Ø (in) Major Ø (mm) Pitch Ø (in) Minor Ø — bolt (in) Minor Ø — nut (in) Tap drill Tap drill (mm)
#2-56 56 0.0860 2.184 0.0744 0.0641 0.0667 #50 1.78
#4-40 40 0.1120 2.845 0.0958 0.0813 0.0849 #43 2.26
#6-32 32 0.1380 3.505 0.1177 0.0997 0.1042 #36 2.71
#8-32 32 0.1640 4.166 0.1437 0.1257 0.1302 #29 3.45
#10-24 24 0.1900 4.826 0.1629 0.1389 0.1449 #25 3.80
#12-24 24 0.2160 5.486 0.1889 0.1649 0.1709 #16 4.50
1/4-20 20 0.2500 6.350 0.2175 0.1887 0.1959 #7 5.11
5/16-18 18 0.3125 7.938 0.2764 0.2443 0.2524 F 6.53
3/8-16 16 0.3750 9.525 0.3344 0.2983 0.3073 5/16 7.94
7/16-14 14 0.4375 11.113 0.3911 0.3499 0.3602 U 9.35
1/2-13 13 0.5000 12.700 0.4500 0.4056 0.4167 27/64 10.72
9/16-12 12 0.5625 14.288 0.5084 0.4603 0.4723 31/64 12.30
5/8-11 11 0.6250 15.875 0.5660 0.5135 0.5266 17/32 13.49
3/4-10 10 0.7500 19.050 0.6850 0.6273 0.6418 21/32 16.67
7/8-9 9 0.8750 22.225 0.8028 0.7387 0.7547 49/64 19.45
1-8 8 1.0000 25.400 0.9188 0.8466 0.8647 7/8 22.23
1-1/8-7 7 1.1250 28.575 1.0322 0.9497 0.9704 63/64 25.00
1-1/4-7 7 1.2500 31.750 1.1572 1.0747 1.0954 1-7/64 28.18
1-3/8-6 6 1.3750 34.925 1.2667 1.1705 1.1946 1-7/32 30.96
1-1/2-6 6 1.5000 38.100 1.3917 1.2955 1.3196 1-11/32 34.13
1-3/4-5 5 1.7500 44.450 1.6201 1.5046 1.5335 1-9/16 39.69
2-4.5 4.5 2.0000 50.800 1.8557 1.7274 1.7594 1-25/32 45.24
2-1/4-4.5 4.5 2.2500 57.150 2.1057 1.9774 2.0094 2-1/32 51.59
2-1/2-4 4 2.5000 63.500 2.3376 2.1933 2.2294 2-1/4 57.15
2-3/4-4 4 2.7500 69.850 2.5876 2.4433 2.4794 2-1/2 63.50
3-4 4 3.0000 76.200 2.8376 2.6933 2.7294 2-3/4 69.85
3-1/4-4 4 3.2500 82.550 3.0876 2.9433 2.9794 3 76.20
3-1/2-4 4 3.5000 88.900 3.3376 3.1933 3.2294 3-1/4 82.55
3-3/4-4 4 3.7500 95.250 3.5876 3.4433 3.4794 3-1/2 88.90
4-4 4 4.0000 101.600 3.8376 3.6933 3.7294 3-3/4 95.25

Values are ASME B1.1 basic (theoretical) dimensions. Tap drill sizes target roughly 75% thread engagement — the standard used in general mechanical design. This table lists the most common UNC sizes; the full ASME B1.1 series also includes #1-64, #3-48 and #5-40. Data provided for reference — always verify against the current standard for critical or safety-related applications.

Understanding the columns

TPI (Threads Per Inch)
The number of thread crests per axial inch of the fastener. Pitch, in the metric sense, equals 1/TPI.
Numbered screw sizes
Gauge numbers (#2–#12) used for fasteners below 1/4" major diameter; roughly, major diameter (in) = 0.060 + 0.013 × N (gauge #11 is skipped — it was never a standard size).
Minor Ø — bolt vs. nut
The bolt (external) minor diameter is the root diameter of the male thread — the value to use when checking a screw's own strength. The nut (internal) minor diameter is the corresponding root diameter of the mating female thread, which is always larger to provide clearance.
UNC vs UNF vs UNEF
Coarse, Fine and Extra-Fine series of the same 60° thread form, differing only in TPI for a given diameter. UNC is the general-purpose default; UNF and UNEF trade easier assembly for higher stress area and finer adjustment.
Class of fit
Tolerance grade for mating tightness: 1A/2A/3A for external (bolt) threads, 1B/2B/3B for internal (nut) threads, loosest to tightest. 2A/2B is the standard commercial fit.

Frequently asked questions

What does "1/2-13 UNC" mean?

A 1/2" nominal major (bolt) diameter with 13 threads per inch, in the Unified National Coarse series.

Why choose UNC over UNF?

UNC assembles faster, resists stripping and cross-threading better, and suits tapped holes in cast iron, aluminum, plastic or other soft/brittle materials; it's also more tolerant of nicks and corrosion. UNF gives greater tensile strength for a given diameter and finer positioning, so it's preferred in thin-walled or hardened parts.

How does UNC compare to ISO metric coarse threads?

Both use an identical 60° V profile; UNC is inch/TPI-based while ISO metric is mm/pitch-based. There is no exact metric equivalent for most UNC sizes (e.g. no metric size matches 3/8-16 UNC), even where diameters are close.

Why isn't the tap drill simply the thread's minor diameter?

Standard tap drills target ~75% thread engagement, not 100%, because 75% gives roughly 93–95% of full thread strength while being much easier to tap and far less likely to break the tap. That's why 1/4-20 uses a #7 drill (0.201 in) rather than the smaller 100%-engagement minor diameter.

References & further reading

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More thread charts

Looking for the fine-pitch series instead? See the UNF Thread Chart. Metric, BSP and NPT are on the thread charts overview page.