Constant-pitch UN threads (8UN, 12UN, 16UN): tap drill chart
Full chart with the tap drill for every size. Press “Calculate” on any row to open the calculator with that thread.
The constant-pitch UN series keeps the same number of threads per inch on every diameter: 8UN, 12UN and 16UN. It is used on large diameters, bearing nuts, shafts and thin-walled parts, where the UNC pitch would be too coarse. When a size matches UNC, UNF or UNEF (1"-8 is UNC), it is designated with that series and does not appear here.
| Thread | Threads per inch | Ø major | Inch drill | Metric drill | Drill for forming tap | Calculate |
|---|---|---|---|---|---|---|
| 1-1/8"-8 UN | 8 | 1.125" · 28.575 mm | 1"1" · 25.4 mm | ⌀ 25.5 | 1-1/16"1.0625" · 26.99 mm | |
| 1-1/4"-8 UN | 8 | 1.25" · 31.75 mm | 1-1/8"1.125" · 28.58 mm | ⌀ 28.5 | 1-3/16"1.1875" · 30.16 mm | |
| 1-3/8"-8 UN | 8 | 1.375" · 34.925 mm | 1-1/4"1.25" · 31.75 mm | ⌀ 32 | 1-5/16"1.3125" · 33.34 mm | |
| 1-1/2"-8 UN | 8 | 1.5" · 38.1 mm | 1-3/8"1.375" · 34.92 mm | ⌀ 35 | 1-7/16"1.4375" · 36.51 mm | |
| 1-5/8"-8 UN | 8 | 1.625" · 41.275 mm | 1-1/2"1.5" · 38.1 mm | ⌀ 38 | 1-9/16"1.5625" · 39.69 mm | |
| 1-3/4"-8 UN | 8 | 1.75" · 44.45 mm | 1-5/8"1.625" · 41.28 mm | ⌀ 41.5 | 1-11/16"1.6875" · 42.86 mm | |
| 1-7/8"-8 UN | 8 | 1.875" · 47.625 mm | 1-3/4"1.75" · 44.45 mm | ⌀ 44.5 | 1-13/16"1.8125" · 46.04 mm | |
| 2"-8 UN | 8 | 2" · 50.8 mm | 1-7/8"1.875" · 47.63 mm | ⌀ 47.5 | 1-15/16"1.9375" · 49.21 mm |
| Thread | Threads per inch | Ø major | Inch drill | Metric drill | Drill for forming tap | Calculate |
|---|---|---|---|---|---|---|
| 5/8"-12 UN | 12 | 0.625" · 15.875 mm | 35/64"0.5469" · 13.89 mm | ⌀ 14 | 37/64"0.5781" · 14.68 mm | |
| 3/4"-12 UN | 12 | 0.75" · 19.05 mm | 43/64"0.6719" · 17.07 mm | ⌀ 17 | 45/64"0.7031" · 17.86 mm | |
| 7/8"-12 UN | 12 | 0.875" · 22.225 mm | 51/64"0.7969" · 20.24 mm | ⌀ 20 | 53/64"0.8281" · 21.03 mm | |
| 1-1/16"-12 UN | 12 | 1.0625" · 26.988 mm | 63/64"0.9844" · 25 mm | ⌀ 25 | 1-1/64"1.0156" · 25.8 mm | |
| 1-5/8"-12 UN | 12 | 1.625" · 41.275 mm | 1-35/64"1.5469" · 39.29 mm | ⌀ 39 | 1-37/64"1.5781" · 40.08 mm | |
| 1-3/4"-12 UN | 12 | 1.75" · 44.45 mm | 1-43/64"1.6719" · 42.47 mm | ⌀ 42.5 | 1-45/64"1.7031" · 43.26 mm | |
| 1-7/8"-12 UN | 12 | 1.875" · 47.625 mm | 1-51/64"1.7969" · 45.64 mm | ⌀ 45.5 | 1-53/64"1.8281" · 46.43 mm | |
| 2"-12 UN | 12 | 2" · 50.8 mm | 1-59/64"1.9219" · 48.82 mm | ⌀ 48.5 | 1-61/64"1.9531" · 49.61 mm |
| Thread | Threads per inch | Ø major | Inch drill | Metric drill | Drill for forming tap | Calculate |
|---|---|---|---|---|---|---|
| 7/16"-16 UN | 16 | 0.4375" · 11.113 mm | V0.377" · 9.58 mm | ⌀ 9.6 | 13/32"0.4063" · 10.32 mm | |
| 1/2"-16 UN | 16 | 0.5" · 12.7 mm | 7/16"0.4375" · 11.11 mm | ⌀ 11.2 | 15/32"0.4688" · 11.91 mm | |
| 9/16"-16 UN | 16 | 0.5625" · 14.288 mm | 1/2"0.5" · 12.7 mm | ⌀ 12.7 | 17/32"0.5313" · 13.49 mm | |
| 5/8"-16 UN | 16 | 0.625" · 15.875 mm | 9/16"0.5625" · 14.29 mm | ⌀ 14.5 | 19/32"0.5938" · 15.08 mm | |
| 7/8"-16 UN | 16 | 0.875" · 22.225 mm | 13/16"0.8125" · 20.64 mm | ⌀ 20.5 | 27/32"0.8438" · 21.43 mm | |
| 1"-16 UN | 16 | 1" · 25.4 mm | 15/16"0.9375" · 23.81 mm | ⌀ 24 | 31/32"0.9688" · 24.61 mm | |
| 1-1/8"-16 UN | 16 | 1.125" · 28.575 mm | 1-1/16"1.0625" · 26.99 mm | ⌀ 27 | 1-3/32"1.0938" · 27.78 mm | |
| 1-1/4"-16 UN | 16 | 1.25" · 31.75 mm | 1-3/16"1.1875" · 30.16 mm | ⌀ 30 | 1-7/32"1.2188" · 30.96 mm | |
| 1-3/8"-16 UN | 16 | 1.375" · 34.925 mm | 1-5/16"1.3125" · 33.34 mm | ⌀ 33.5 | 1-11/32"1.3438" · 34.13 mm | |
| 1-1/2"-16 UN | 16 | 1.5" · 38.1 mm | 1-7/16"1.4375" · 36.51 mm | ⌀ 36.5 | 1-15/32"1.4688" · 37.31 mm | |
| 1-3/4"-16 UN | 16 | 1.75" · 44.45 mm | 1-11/16"1.6875" · 42.86 mm | ⌀ 43 | 1-23/32"1.7188" · 43.66 mm | |
| 2"-16 UN | 16 | 2" · 50.8 mm | 1-15/16"1.9375" · 49.21 mm | ⌀ 49.5 | 1-31/32"1.9688" · 50.01 mm |
What about UNS threads?
UNS (Unified Special) is not a series with fixed sizes: it is any diameter and pitch combination that is not in the standard series, for example 1/4"-24 UNS. That is why it has no table. To calculate one, open the tap drill calculator and use "Custom thread" with the diameter and threads per inch from the drawing: the profile and formula are the same as for UNC and UNF.
How to read the chart
Inch drill: the drill from an inch drill set closest to 75 % thread, the same one the ASME tables give. It can be a number drill (#7), a letter drill (F) or a fraction drill (5/16").
Metric drill: the millimetre drill closest to the same 75 %. It is the one to use when the shop only has metric drills; the difference from the inch drill is a few hundredths.
Forming tap drill: the tap does not cut, it displaces the material, so the hole is larger. Drilling with the cutting-tap drill and tapping with a forming tap breaks it.
"Calculate" opens the calculator with that size: G84 cycle, feed, depth in a blind hole and the recommended method for the material. Guide values for a 2B thread; on critical parts, check against the tap maker's table.
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