Unified National Coarse is the default thread series for general machined parts. Tap drill values are for cutting taps at approximately 75% thread.[1]
| Thread Size | Tap Drill | Tap Dec. (in) | Close Fit | Close Dec. (in) | Free Fit | Free Dec. (in) |
|---|---|---|---|---|---|---|
| #1-64 | #53 | 0.0595 | #48 | 0.0760 | #46 | 0.0810 |
| #2-56 | #50 | 0.0700 | #43 | 0.0890 | #41 | 0.0960 |
| #3-48 | #47 | 0.0785 | #37 | 0.1040 | #35 | 0.1100 |
| #4-40 | #43 | 0.0890 | #32 | 0.1160 | #30 | 0.1285 |
| #5-40 | #38 | 0.1015 | #30 | 0.1285 | #29 | 0.1360 |
| #6-32 | #36 | 0.1065 | #27 | 0.1440 | #25 | 0.1495 |
| #8-32 | #29 | 0.1360 | #18 | 0.1695 | #16 | 0.1770 |
| #10-24 | #25 | 0.1495 | #9 | 0.1960 | #7 | 0.2010 |
| #12-24 | #16 | 0.1770 | #2 | 0.2210 | #1 | 0.2280 |
| 1/4-20 | #7 | 0.2010 | F | 0.2570 | H | 0.2660 |
| 5/16-18 | F | 0.2570 | P | 0.3230 | Q | 0.3320 |
| 3/8-16 | 5/16 | 0.3125 | W | 0.3860 | X | 0.3970 |
| 7/16-14 | U | 0.3680 | 29/64 | 0.4531 | 15/32 | 0.4688 |
| 1/2-13 | 27/64 | 0.4219 | 33/64 | 0.5156 | 17/32 | 0.5312 |
| 9/16-12 | 31/64 | 0.4844 | 37/64 | 0.5781 | 19/32 | 0.5938 |
| 5/8-11 | 17/32 | 0.5312 | 41/64 | 0.6406 | 21/32 | 0.6562 |
| 3/4-10 | 21/32 | 0.6562 | 49/64 | 0.7656 | 25/32 | 0.7812 |
| 7/8-9 | 49/64 | 0.7656 | 57/64 | 0.8906 | 29/32 | 0.9062 |
| 1"-8 | 7/8 | 0.8750 | 1-1/64 | 1.0156 | 1-1/32 | 1.0312 |
Unified National Fine threads offer more engagement per length and finer adjustment. Common in aerospace hardware and thin sections. Clearance drills are the same as the equivalent UNC size.[1]
| Thread Size | Tap Drill | Tap Dec. (in) | Close Fit | Close Dec. (in) | Free Fit | Free Dec. (in) |
|---|---|---|---|---|---|---|
| #0-80 | 3/64 | 0.0469 | #52 | 0.0635 | #50 | 0.0700 |
| #1-72 | #53 | 0.0595 | #48 | 0.0760 | #46 | 0.0810 |
| #2-64 | #50 | 0.0700 | #43 | 0.0890 | #41 | 0.0960 |
| #3-56 | #45 | 0.0820 | #37 | 0.1040 | #35 | 0.1100 |
| #4-48 | #42 | 0.0935 | #32 | 0.1160 | #30 | 0.1285 |
| #5-44 | #37 | 0.1040 | #30 | 0.1285 | #29 | 0.1360 |
| #6-40 | #33 | 0.1130 | #27 | 0.1440 | #25 | 0.1495 |
| #8-36 | #29 | 0.1360 | #18 | 0.1695 | #16 | 0.1770 |
| #10-32 | #21 | 0.1590 | #9 | 0.1960 | #7 | 0.2010 |
| #12-28 | #14 | 0.1820 | #2 | 0.2210 | #1 | 0.2280 |
| 1/4-28 | #3 | 0.2130 | F | 0.2570 | H | 0.2660 |
| 5/16-24 | I | 0.2720 | P | 0.3230 | Q | 0.3320 |
| 3/8-24 | Q | 0.3320 | W | 0.3860 | X | 0.3970 |
| 7/16-20 | 25/64 | 0.3906 | 29/64 | 0.4531 | 15/32 | 0.4688 |
| 1/2-20 | 29/64 | 0.4531 | 33/64 | 0.5156 | 17/32 | 0.5312 |
| 9/16-18 | 33/64 | 0.5156 | 37/64 | 0.5781 | 19/32 | 0.5938 |
| 5/8-18 | 37/64 | 0.5781 | 41/64 | 0.6406 | 21/32 | 0.6562 |
| 3/4-16 | 11/16 | 0.6875 | 49/64 | 0.7656 | 25/32 | 0.7812 |
| 7/8-14 | 13/16 | 0.8125 | 57/64 | 0.8906 | 29/32 | 0.9062 |
| 1"-12 | 59/64 | 0.9219 | 1-1/64 | 1.0156 | 1-1/32 | 1.0312 |
Metric coarse tap drills follow the simple rule of nominal diameter minus pitch. Clearance values are the "normal" (medium) series per ISO 273.[2]
| Thread Size | Tap Drill (mm) | Tap Dec. (in) | Clear. (mm) | Clear. (in) | Nearest Inch Drill | Inch Dec. (in) | Diff. (in) | % Thread (inch drill) |
|---|---|---|---|---|---|---|---|---|
| M2 × 0.4 | 1.60 | 0.0630 | 2.4 | 0.0945 | 1/16" | 0.0625 | −0.0005 | 79% |
| M2.5 × 0.45 | 2.05 | 0.0807 | 2.9 | 0.1142 | #46 | 0.0810 | +0.0003 | 76% |
| M3 × 0.5 | 2.50 | 0.0984 | 3.4 | 0.1339 | #40 | 0.0980 | −0.0004 | 79% |
| M4 × 0.7 | 3.30 | 0.1299 | 4.5 | 0.1772 | #30 | 0.1285 | −0.0014 | 81% |
| M5 × 0.8 | 4.20 | 0.1654 | 5.5 | 0.2165 | #19 | 0.1660 | +0.0006 | 75% |
| M6 × 1.0 | 5.00 | 0.1969 | 6.6 | 0.2598 | #9 | 0.1960 | −0.0009 | 79% |
| M8 × 1.25 | 6.80 | 0.2677 | 9.0 | 0.3543 | H | 0.2660 | −0.0017 | 77% |
| M10 × 1.5 | 8.50 | 0.3346 | 11.0 | 0.4331 | Q | 0.3320 | −0.0026 | 80% |
| M12 × 1.75 | 10.20 | 0.4016 | 13.5 | 0.5315 | Y | 0.4040 | +0.0024 | 76% |
| M16 × 2.0 | 14.00 | 0.5512 | 17.5 | 0.6890 | 35/64" | 0.5469 | −0.0043 | 81% |
| M20 × 2.5 | 17.50 | 0.6890 | 22.0 | 0.8661 | 11/16" | 0.6875 | −0.0015 | 78% |
Thread percentage. The tap drills above target roughly 75% of full thread height, the standard for cutting taps in common materials. Full 100% thread adds very little strength but multiplies tapping torque and tap breakage risk. In tough materials such as stainless steels, titanium, and nickel superalloys, shops often drop to 50 to 60% thread by drilling one size larger; the joint loses almost no strength and taps survive.[1]
The formulas behind the chart. For a 75% thread with a cutting tap, the inch tap drill is approximately major diameter − 0.974 / TPI, and the metric tap drill is approximately D − P, rounded to the nearest stock drill (M8 × 1.25 works out to 6.75, so the chart shows 6.8 mm).[1][2]
Roll form taps. Form (roll) taps displace material instead of cutting it and need a larger hole than the chart values. A common approximation for a form tap is nominal diameter minus half the pitch, which gives roughly 70 to 75% thread; for 65% use nominal minus 0.44 × pitch, and always check the tap maker's chart before drilling.
Close fit vs free fit. Close fit suits dowel-adjacent hardware and parts that locate on their fasteners. Free fit is the default for general bolted assemblies and allows for position tolerance between mating parts. If holes on both parts are drilled free fit, standard fasteners assemble without fuss.
Thread depth. Threads deeper than about 1.5 to 2 diameters (steel) or 2 to 3 diameters (aluminum and plastics) add cost without adding strength. See the thread guidance in our DFM Guide for CNC Milling for how depth callouts drive cycle time.
Send us your drawing. We machine and tap everything on this chart daily, from #2-56 in aluminum to 3/4-10 in Inconel, with AS9100D traceability.