Unified Thread Dimensions Chart
Basic major, pitch and minor diameters for Unified inch screw threads from #0-80 to 1-1/2, computed from the ASME B1.1 basic profile, with the usual tap drill for each size. Inches on top, millimeters underneath. Jump to UNC coarse, UNF fine, UNEF extra fine.
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UNC coarse (UNC)
| Thread | TPI | Major dia | Pitch dia | Minor dia, external (UNR) | Minor dia, internal | Tap drill |
|---|---|---|---|---|---|---|
| #1-64 | 64 | 0.07301.854 mm | 0.06291.596 mm | 0.05381.367 mm | 0.05611.425 mm | #53 (0.0595)1.511 mm |
| #2-56 | 56 | 0.08602.184 mm | 0.07441.890 mm | 0.06411.628 mm | 0.06671.693 mm | #50 (0.0700)1.778 mm |
| #3-48 | 48 | 0.09902.515 mm | 0.08552.171 mm | 0.07341.865 mm | 0.07641.942 mm | #47 (0.0785)1.994 mm |
| #4-40 | 40 | 0.11202.845 mm | 0.09582.432 mm | 0.08132.066 mm | 0.08492.157 mm | #43 (0.0890)2.261 mm |
| #5-40 | 40 | 0.12503.175 mm | 0.10882.763 mm | 0.09432.396 mm | 0.09792.488 mm | #38 (0.1015)2.578 mm |
| #6-32 | 32 | 0.13803.505 mm | 0.11772.990 mm | 0.09972.531 mm | 0.10422.646 mm | #36 (0.1065)2.705 mm |
| #8-32 | 32 | 0.16404.166 mm | 0.14373.650 mm | 0.12573.192 mm | 0.13023.306 mm | #29 (0.1360)3.454 mm |
| #10-24 | 24 | 0.19004.826 mm | 0.16294.139 mm | 0.13893.528 mm | 0.14493.680 mm | #25 (0.1495)3.797 mm |
| #12-24 | 24 | 0.21605.486 mm | 0.18894.799 mm | 0.16494.188 mm | 0.17094.341 mm | #16 (0.1770)4.496 mm |
| 1/4-20 | 20 | 0.25006.350 mm | 0.21755.525 mm | 0.18874.792 mm | 0.19594.975 mm | #7 (0.2010)5.105 mm |
| 5/16-18 | 18 | 0.31257.938 mm | 0.27647.021 mm | 0.24436.206 mm | 0.25246.410 mm | F (0.2570)6.528 mm |
| 3/8-16 | 16 | 0.37509.525 mm | 0.33448.494 mm | 0.29837.577 mm | 0.30737.806 mm | 5/16" (0.3125)7.938 mm |
| 7/16-14 | 14 | 0.437511.113 mm | 0.39119.934 mm | 0.34998.887 mm | 0.36029.148 mm | U (0.3680)9.347 mm |
| 1/2-13 | 13 | 0.500012.700 mm | 0.450011.431 mm | 0.405610.303 mm | 0.416710.585 mm | 27/64" (0.4219)10.716 mm |
| 9/16-12 | 12 | 0.562514.288 mm | 0.508412.913 mm | 0.460311.691 mm | 0.472311.996 mm | 31/64" (0.4844)12.303 mm |
| 5/8-11 | 11 | 0.625015.875 mm | 0.566014.375 mm | 0.513513.042 mm | 0.526613.375 mm | 17/32" (0.5313)13.494 mm |
| 3/4-10 | 10 | 0.750019.050 mm | 0.685017.400 mm | 0.627315.934 mm | 0.641716.300 mm | 21/32" (0.6563)16.669 mm |
| 7/8-9 | 9 | 0.875022.225 mm | 0.802820.392 mm | 0.738718.763 mm | 0.754719.170 mm | 49/64" (0.7656)19.447 mm |
| 1-8 | 8 | 1.000025.400 mm | 0.918823.338 mm | 0.846621.505 mm | 0.864721.963 mm | 7/8" (0.8750)22.225 mm |
| 1-1/8-7 | 7 | 1.125028.575 mm | 1.032226.218 mm | 0.949724.123 mm | 0.970424.647 mm | 63/64" (0.9844)25.003 mm |
| 1-1/4-7 | 7 | 1.250031.750 mm | 1.157229.393 mm | 1.074727.298 mm | 1.095427.822 mm | 1-7/64" (1.1094)28.178 mm |
| 1-3/8-6 | 6 | 1.375034.925 mm | 1.266732.175 mm | 1.170529.731 mm | 1.194630.342 mm | 1-7/32" (1.2188)30.956 mm |
| 1-1/2-6 | 6 | 1.500038.100 mm | 1.391735.350 mm | 1.295532.906 mm | 1.319633.517 mm | 1-11/32" (1.3438)34.131 mm |
UNF fine (UNF)
| Thread | TPI | Major dia | Pitch dia | Minor dia, external (UNR) | Minor dia, internal | Tap drill |
|---|---|---|---|---|---|---|
| #0-80 | 80 | 0.06001.524 mm | 0.05191.318 mm | 0.04471.134 mm | 0.04651.180 mm | 3/64" (0.0469)1.191 mm |
| #1-72 | 72 | 0.07301.854 mm | 0.06401.625 mm | 0.05601.421 mm | 0.05801.472 mm | #53 (0.0595)1.511 mm |
| #2-64 | 64 | 0.08602.184 mm | 0.07591.927 mm | 0.06681.697 mm | 0.06911.755 mm | #50 (0.0700)1.778 mm |
| #3-56 | 56 | 0.09902.515 mm | 0.08742.220 mm | 0.07711.958 mm | 0.07972.024 mm | #45 (0.0820)2.083 mm |
| #4-48 | 48 | 0.11202.845 mm | 0.09852.501 mm | 0.08642.196 mm | 0.08942.272 mm | #42 (0.0935)2.375 mm |
| #5-44 | 44 | 0.12503.175 mm | 0.11022.800 mm | 0.09712.467 mm | 0.10042.550 mm | #37 (0.1040)2.642 mm |
| #6-40 | 40 | 0.13803.505 mm | 0.12183.093 mm | 0.10732.726 mm | 0.11092.818 mm | #33 (0.1130)2.870 mm |
| #8-36 | 36 | 0.16404.166 mm | 0.14603.707 mm | 0.12993.300 mm | 0.13393.402 mm | #29 (0.1360)3.454 mm |
| #10-32 | 32 | 0.19004.826 mm | 0.16974.310 mm | 0.15173.852 mm | 0.15623.967 mm | #21 (0.1590)4.039 mm |
| #12-28 | 28 | 0.21605.486 mm | 0.19284.897 mm | 0.17224.373 mm | 0.17734.504 mm | #14 (0.1820)4.623 mm |
| 1/4-28 | 28 | 0.25006.350 mm | 0.22685.761 mm | 0.20625.237 mm | 0.21135.368 mm | #3 (0.2130)5.410 mm |
| 5/16-24 | 24 | 0.31257.938 mm | 0.28547.250 mm | 0.26146.639 mm | 0.26746.792 mm | I (0.2720)6.909 mm |
| 3/8-24 | 24 | 0.37509.525 mm | 0.34798.838 mm | 0.32398.227 mm | 0.32998.379 mm | Q (0.3320)8.433 mm |
| 7/16-20 | 20 | 0.437511.113 mm | 0.405010.288 mm | 0.37629.554 mm | 0.38349.738 mm | 25/64" (0.3906)9.922 mm |
| 1/2-20 | 20 | 0.500012.700 mm | 0.467511.875 mm | 0.438711.142 mm | 0.445911.325 mm | 29/64" (0.4531)11.509 mm |
| 9/16-18 | 18 | 0.562514.288 mm | 0.526413.371 mm | 0.494312.556 mm | 0.502412.760 mm | 33/64" (0.5156)13.097 mm |
| 5/8-18 | 18 | 0.625015.875 mm | 0.588914.958 mm | 0.556814.144 mm | 0.564914.347 mm | 37/64" (0.5781)14.684 mm |
| 3/4-16 | 16 | 0.750019.050 mm | 0.709418.019 mm | 0.673317.102 mm | 0.682317.331 mm | 11/16" (0.6875)17.463 mm |
| 7/8-14 | 14 | 0.875022.225 mm | 0.828621.047 mm | 0.787419.999 mm | 0.797720.261 mm | 13/16" (0.8125)20.638 mm |
| 1-12 | 12 | 1.000025.400 mm | 0.945924.025 mm | 0.897822.803 mm | 0.909823.109 mm | 59/64" (0.9219)23.416 mm |
| 1-1/8-12 | 12 | 1.125028.575 mm | 1.070927.200 mm | 1.022825.978 mm | 1.034826.284 mm | 1-3/64" (1.0469)26.591 mm |
| 1-1/4-12 | 12 | 1.250031.750 mm | 1.195930.375 mm | 1.147829.153 mm | 1.159829.459 mm | 1-11/64" (1.1719)29.766 mm |
| 1-3/8-12 | 12 | 1.375034.925 mm | 1.320933.550 mm | 1.272832.328 mm | 1.284832.634 mm | 1-19/64" (1.2969)32.941 mm |
| 1-1/2-12 | 12 | 1.500038.100 mm | 1.445936.725 mm | 1.397835.503 mm | 1.409835.809 mm | 1-27/64" (1.4219)36.116 mm |
UNEF extra fine (UNEF)
| Thread | TPI | Major dia | Pitch dia | Minor dia, external (UNR) | Minor dia, internal | Tap drill |
|---|---|---|---|---|---|---|
| #12-32 | 32 | 0.21605.486 mm | 0.19574.971 mm | 0.17774.513 mm | 0.18224.627 mm | #13 (0.1850)4.699 mm |
| 1/4-32 | 32 | 0.25006.350 mm | 0.22975.834 mm | 0.21175.376 mm | 0.21625.491 mm | 7/32" (0.2188)5.556 mm |
| 5/16-32 | 32 | 0.31257.938 mm | 0.29227.422 mm | 0.27426.964 mm | 0.27877.078 mm | 9/32" (0.2813)7.144 mm |
| 3/8-32 | 32 | 0.37509.525 mm | 0.35479.009 mm | 0.33678.551 mm | 0.34128.666 mm | 11/32" (0.3438)8.731 mm |
| 7/16-28 | 28 | 0.437511.113 mm | 0.414310.523 mm | 0.393710.000 mm | 0.398810.130 mm | 13/32" (0.4063)10.319 mm |
| 1/2-28 | 28 | 0.500012.700 mm | 0.476812.111 mm | 0.456211.587 mm | 0.461311.718 mm | 15/32" (0.4688)11.906 mm |
| 3/4-20 | 20 | 0.750019.050 mm | 0.717518.225 mm | 0.688717.492 mm | 0.695917.675 mm | 45/64" (0.7031)17.859 mm |
| 7/8-20 | 20 | 0.875022.225 mm | 0.842521.400 mm | 0.813720.667 mm | 0.820920.850 mm | 53/64" (0.8281)21.034 mm |
| 1-20 | 20 | 1.000025.400 mm | 0.967524.575 mm | 0.938723.842 mm | 0.945924.025 mm | 61/64" (0.9531)24.209 mm |
Basic sizes from the ASME B1.1 profile, before allowance and tolerance. External minor is the UNR rounded-root design minor (d - 1.226869P); a flat-root UN thread with the narrowest permitted root flat bottoms out lower, at d - 1.299038P. Tap drills leave roughly 65 to 80 percent thread; see the tap and drill size chart for engagement guidance. 9/16-24 and 5/8-24 UNEF and UNEF sizes over 1 in are not listed.
How the diameters are calculated
Unified threads share one 60 degree basic profile. The pitch is P = 1 / TPI and the sharp-V height is H = 0.866025P. The profile is cut back H/8 at the major diameter and H/4 at the minor diameter, so every basic diameter follows from the major diameter d and the pitch:
- Pitch diameter:
d2 = d - 0.649519P - Internal (basic) minor diameter:
D1 = d - 1.082532P - External minor diameter, UNR root:
d3 = d - 1.226869P - Numbered sizes:
#n major = 0.060 + 0.013n in(#10 = 0.190 in)
Thread classes: 2A/2B vs 3A/3B
Every Unified callout ends in a class. A is an external thread (bolt, screw, stud) and B is an internal thread (tapped hole, nut). The number sets how much pitch diameter tolerance the thread gets and whether there is an allowance.
Class 2A/2B is the default for commercial fasteners and most machined parts. A 2A thread is made under basic by a small allowance (0.0011 in on a 1/4-20), so parts assemble freely and there is room for a thin plating. A 2B thread has no allowance: its smallest pitch diameter is the basic size in the tables above, and its tolerance is 30 percent wider than the matching 2A tolerance.
Class 3A/3B tightens the pitch diameter tolerance (3A gets 75 percent of the 2A tolerance) and 3A has no allowance, so a 3A screw can sit right at basic size. Use it where the thread has to locate, carries high or fatigue loads, or where an aerospace or precision spec requires it. Classes can be mixed, such as a 2A screw in a 3B hole. Class 1A/1B, the loosest, is rare outside quick-assembly hardware.
Plating eats into the fit. A 2A allowance covers a thin zinc or black oxide finish; heavier coatings, and any coating on a 3A thread, need the thread cut undersize before plating so the finished part still gauges. UNR external threads have a mandatory rounded root and gauge the same as UN. UNJ (ASME B1.15) uses a larger root radius and a raised internal minor diameter for fatigue-critical aerospace parts.
Frequently asked questions
- What is the difference between UNC, UNF and UNEF?
- They are pitch series for the same diameters. UNC (coarse) has the fewest threads per inch, assembles fastest and tolerates dings and soft materials best. UNF (fine) has a larger minor diameter and a larger stress area, so it is stronger in tension and easier to adjust finely. UNEF (extra fine) is for thin walls, short engagement and precision adjusters.
- What does 1/4-20 UNC-2A mean?
- A 1/4 inch nominal (basic major) diameter, 20 threads per inch, the coarse series, class 2A, which is an external thread with the standard allowance and tolerance. Threads are right-hand unless the callout ends in LH.
- How do you calculate the pitch diameter of a Unified thread?
- Basic pitch diameter = major diameter - 0.649519 / TPI. For 1/4-20 that is 0.2500 - 0.649519 / 20 = 0.2175 in. The internal minor diameter is major - 1.082532 / TPI (0.1959 in for 1/4-20).
- Why does my bolt measure under the major diameter in the chart?
- The chart shows basic sizes. A class 2A thread is made under basic by an allowance and then toleranced below that: a 1/4-20 UNC-2A bolt has a major diameter of about 0.2408 to 0.2489 in. Only class 3A external threads can reach the basic size.
- How do I check the pitch diameter of an external thread?
- Use a thread micrometer or three wires. With the best-size wire, W = 0.57735 x P, the measurement over the wires is M = E - 0.866025 x P + 3W, where E is the pitch diameter. Go and no-go ring gauges remain the acceptance test for most prints.
