A tap drill makes the hole that a tap cuts its thread into. A 1/4-20 thread takes a #7 drill (0.201 in), 10-24 a #25, 3/8-16 a 5/16 in drill and M6 × 1 a 5 mm drill. The rule of thumb is thread size minus pitch, which leaves about 75% of a full thread.
How to use the chart
- Find your thread. For an inch screw, that is its size and threads per inch (a 1/4-20 bolt is 1/4 in across with 20 threads per inch). For metric, it is the size and pitch (M6 × 1 is 6 mm across, 1 mm from thread to thread).
- Drill the tap drill where the thread will be cut. Number drills (#1 to #80) and letter drills (A to Z) fill the gaps between fraction sizes, which is why so many tap drills are numbers or letters.
- Drill the clearance hole in the part the screw passes through, so it slides in and pulls the parts together. Close fit locates parts accurately; free fit allows for holes that don’t line up perfectly.
The rule: thread size minus pitch
A tap drill is close to the screw’s size minus the distance between threads. For 1/4-20, that is 0.250 − 1/20 = 0.200 in, and the nearest standard drill, a #7, is 0.201 in. For M6 × 1, it is 6 − 1 = 5 mm.
The exact share of a full thread a drill leaves follows the formula OSG, a tap maker, publishes:
For metric threads the same formula reads 76.98 × (size − drill) ÷ pitch. These lines follow the thread picked in the tool above.
How much thread you need
A full 100% thread is hard to tap and rarely needed. OSG advises using the largest drill that still gives adequate strength, and says a thread of 55 to 65% is enough for most jobs. Its suggested ranges:
| Material | Average work | Deep holes | Thin sheet |
|---|---|---|---|
| Free-cutting: aluminum, brass, bronze, cast iron, copper, mild steel, tool steel | 65–70% | 60–70% | 75–85% |
| Hard or tough: stainless steel, cast steel, forgings, nickel steel | 60–70% | 55–65% | – |
Source: OSG, Tap Drill Size and Pitch Diameter Limit chart (2024). Deep holes are deeper than about 1½ times the hole diameter.
The standard chart drills mostly give about 65 to 80%. In stainless steel or a deep hole, a slightly larger drill is easier on the tap: OSG’s advice is the largest drill that still leaves enough thread, for fewer tapping problems and longer tap life. The tool’s “easier tapping” line gives the stocked drill nearest 65%.
Inch threads: UNC and UNF
| Thread | Series | Tap drill | Inches | Thread | Close fit | Free fit |
|---|---|---|---|---|---|---|
| #0-80 | UNF | 3/64 in | .0469 | 81% | #52 · .0635 | #50 · .0700 |
| #1-64 | UNC | #53 | .0595 | 67% | #48 · .0760 | #46 · .0810 |
| #1-72 | UNF | #53 | .0595 | 75% | #48 · .0760 | #46 · .0810 |
| #2-56 | UNC | #50 | .0700 | 69% | #43 · .0890 | #41 · .0960 |
| #2-64 | UNF | #50 | .0700 | 79% | #43 · .0890 | #41 · .0960 |
| #3-48 | UNC | #47 | .0785 | 76% | #37 · .1040 | #35 · .1100 |
| #3-56 | UNF | #45 | .0820 | 73% | #37 · .1040 | #35 · .1100 |
| #4-40 | UNC | #43 | .0890 | 71% | #32 · .1160 | #30 · .1285 |
| #4-48 | UNF | #42 | .0935 | 68% | #32 · .1160 | #30 · .1285 |
| #5-40 | UNC | #38 | .1015 | 72% | #30 · .1285 | #29 · .1360 |
| #5-44 | UNF | #37 | .1040 | 71% | #30 · .1285 | #29 · .1360 |
| #6-32 | UNC | #36 | .1065 | 78% | #27 · .1440 | #25 · .1495 |
| #6-40 | UNF | #33 | .1130 | 77% | #27 · .1440 | #25 · .1495 |
| #8-32 | UNC | #29 | .1360 | 69% | #18 · .1695 | #16 · .1770 |
| #8-36 | UNF | #29 | .1360 | 78% | #18 · .1695 | #16 · .1770 |
| #10-24 | UNC | #25 | .1495 | 75% | #9 · .1960 | #7 · .2010 |
| #10-32 | UNF | #21 | .1590 | 76% | #9 · .1960 | #7 · .2010 |
| #12-24 | UNC | #16 | .1770 | 72% | #2 · .2210 | #1 · .2280 |
| #12-28 | UNF | #14 | .1820 | 73% | #2 · .2210 | #1 · .2280 |
| 1/4-20 | UNC | #7 | .2010 | 75% | F · .2570 | H · .2660 |
| 1/4-28 | UNF | #3 | .2130 | 80% | F · .2570 | H · .2660 |
| 5/16-18 | UNC | F | .2570 | 77% | P · .3230 | Q · .3320 |
| 5/16-24 | UNF | I | .2720 | 75% | P · .3230 | Q · .3320 |
| 3/8-16 | UNC | 5/16 in | .3125 | 77% | W · .3860 | X · .3970 |
| 3/8-24 | UNF | Q | .3320 | 79% | W · .3860 | X · .3970 |
| 7/16-14 | UNC | U | .3680 | 75% | 29/64 · .4531 | 15/32 · .4688 |
| 7/16-20 | UNF | 25/64 in | .3906 | 72% | 29/64 · .4531 | 15/32 · .4688 |
| 1/2-13 | UNC | 27/64 in | .4219 | 78% | 33/64 · .5156 | 17/32 · .5312 |
| 1/2-20 | UNF | 29/64 in | .4531 | 72% | 33/64 · .5156 | 17/32 · .5312 |
| 9/16-12 | UNC | 31/64 in | .4844 | 72% | 37/64 · .5781 | 19/32 · .5938 |
| 9/16-18 | UNF | 33/64 in | .5156 | 65% | 37/64 · .5781 | 19/32 · .5938 |
| 5/8-11 | UNC | 17/32 in | .5312 | 79% | 41/64 · .6406 | 21/32 · .6562 |
| 5/8-18 | UNF | 37/64 in | .5781 | 65% | 41/64 · .6406 | 21/32 · .6562 |
| 3/4-10 | UNC | 21/32 in | .6562 | 72% | 49/64 · .7656 | 25/32 · .7812 |
| 3/4-16 | UNF | 11/16 in | .6875 | 77% | 49/64 · .7656 | 25/32 · .7812 |
| 7/8-9 | UNC | 49/64 in | .7656 | 76% | 57/64 · .8906 | 29/32 · .9062 |
| 7/8-14 | UNF | 13/16 in | .8125 | 67% | 57/64 · .8906 | 29/32 · .9062 |
| 1-8 | UNC | 7/8 in | .8750 | 77% | 1-1/64 · 1.0156 | 1-1/32 · 1.0312 |
| 1-12 | UNF | 59/64 in | .9219 | 72% | 1-1/64 · 1.0156 | 1-1/32 · 1.0312 |
| 1-14 | UNS | 15/16 in | .9375 | 67% | 1-1/64 · 1.0156 | 1-1/32 · 1.0312 |
Tap drills: Starrett’s tap drill chart. Clearance drills: University of Florida tap and clearance chart. Thread percentage: our calculation with OSG’s formula. Shaded rows are the most common sizes.
Metric threads
| Thread | Tap drill | Inches | Thread | Close fit | Free fit |
|---|---|---|---|---|---|
| M1.6 x 0.35 | 1.25 mm | .0492 | 77% | 1.70 mm | 1.75 mm |
| M2 x 0.4 | 1.6 mm | .0630 | 77% | 2.10 mm | 2.20 mm |
| M2.5 x 0.45 | 2.05 mm | .0807 | 77% | 2.65 mm | 2.75 mm |
| M3 x 0.5 | 2.5 mm | .0984 | 77% | 3.15 mm | 3.30 mm |
| M3.5 x 0.6 | 2.9 mm | .1142 | 77% | 3.70 mm | 3.85 mm |
| M4 x 0.7 | 3.3 mm | .1299 | 77% | 4.20 mm | 4.40 mm |
| M5 x 0.8 | 4.2 mm | .1654 | 77% | 5.25 mm | 5.50 mm |
| M6 x 1 | 5 mm | .1969 | 77% | 6.30 mm | 6.60 mm |
| M8 x 1.25 | 6.8 mm | .2677 | 74% | 8.40 mm | 8.80 mm |
| M8 x 1 fine | 7 mm | .2756 | 77% | 8.40 mm | 8.80 mm |
| M10 x 1.5 | 8.5 mm | .3346 | 77% | 10.50 mm | 11.00 mm |
| M10 x 1.25 fine | 8.8 mm | .3465 | 74% | 10.50 mm | 11.00 mm |
| M12 x 1.75 | 10.3 mm | .4055 | 75% | 12.60 mm | 13.20 mm |
| M12 x 1.25 fine | 10.8 mm | .4252 | 74% | 12.60 mm | 13.20 mm |
Tap drills are the stocked size nearest 75% thread (0.05 mm steps under 3 mm, 0.1 mm above), and match the University of Florida chart. Starrett lists drills 0.1 mm smaller for M8 × 1.25 (6.7 mm), M10 × 1.25 (8.7 mm) and M12 × 1.75 (10.2 mm), which give about 80% thread. Clearance drills: University of Florida.
Pipe threads: NPT
| Pipe thread | Tap drill | Inches | Millimeters |
|---|---|---|---|
| 1/8-27 NPT | 11/32 in | .3438 | 8.73 mm |
| 1/4-18 NPT | 7/16 in | .4375 | 11.11 mm |
| 3/8-18 NPT | 37/64 in | .5781 | 14.68 mm |
| 1/2-14 NPT | 23/32 in | .7188 | 18.26 mm |
| 3/4-14 NPT | 59/64 in | .9219 | 23.42 mm |
| 1-11.5 NPT | 1-5/32 in | 1.1562 | 29.37 mm |
| 1-1/4-11.5 NPT | 1-1/2 in | 1.5000 | 38.10 mm |
| 1-1/2-11.5 NPT | 1-3/4 in | 1.7500 | 44.45 mm |
| 2-11.5 NPT | 2-7/32 in | 2.2188 | 56.36 mm |
Source: Starrett. Pipe threads are tapered, so these drills are for tapping straight after drilling, without reaming the hole to the taper first.
Clearance holes: close or free fit
A clearance hole lets the screw pass through one part and thread into the next. A close-fit hole is just larger than the screw and holds the parts in place accurately. A free-fit hole is a little larger again, so screws still go in when the holes in two parts are slightly out of line. For a 1/4 in bolt, the close fit is an F drill (0.257 in) and the free fit an H (0.266 in).
Check the drill against a drill gauge or with calipers, and tap a test hole in scrap of the same material. Number and letter drills look alike, and a hole one size too large can leave a thread that strips.
Questions people ask
What size drill bit for a 1/4-20 tap?
A #7 drill, 0.201 in, which leaves about 75% thread. A 13/64 in drill (0.2031 in) is the nearest fraction and leaves about 72%.
What size drill for a 10-24 tap?
A #25 drill, 0.1495 in. The nearest fraction, 5/32 in, leaves only about 62% thread.
What size drill for a 3/8-16 tap?
A 5/16 in drill, 0.3125 in, about 77% thread.
What size drill for a 5/16-18 tap?
An F drill, 0.257 in, about 77% thread. A 17/64 in drill leaves about 65%.
What size drill for an 8-32 tap?
A #29 drill, 0.136 in, about 69% thread.
What size drill for an M6 tap?
A 5 mm drill for the standard M6 × 1 thread, about 77% thread.
What size drill for an M8 tap?
A 6.8 mm drill for M8 × 1.25, about 74% thread, and a 7 mm drill for the fine M8 × 1.
What is the difference between UNC and UNF?
UNC (coarse) has fewer threads per inch, UNF (fine) more. A 1/4 in bolt is 20 threads per inch in UNC and 28 in UNF, so it takes a different tap drill: #7 or #3.
Sources
- Starrett, Inch/Metric Tap Drill Sizes & Decimal Equivalents, Bulletin 1214 (copy hosted by Montana State University)
- University of Florida, Department of Mechanical and Aerospace Engineering, Tap & Drill Chart (EML2322L)
- OSG, Tap Drill Size and Pitch Diameter Limit chart, Vol. 3 (2024): percentage of full thread formula and suggested percentages
Checked October 12, 2026. Where the sources differ, the chart notes say which value we used. Millimeter and inch values are converted at 25.4 mm to the inch.