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Tap Drill Size Chart: UNC, UNF, Metric & Clearance Holes
Quick answer: For imperial threads, look the size up below — 1/4-20 takes a #7 (0.201"), 10-24 takes a #25 (0.1495"). For metric there is no table needed: tap drill = major diameter − pitch, so M6 × 1.0 takes a 5.0 mm bit.
Every tap drill below is sized for roughly 75% thread engagement — the standard target for steel, aluminium and brass. The last column shows the actual percentage each drill gives, so you can see exactly how much margin you have before switching to a substitute bit.
Unified coarse thread (UNC) tap drill chart
The default series for general fastening: hardware-store bolts, machine screws and anything going into softer material where a coarse thread grips better.
| Thread | Major dia (in) | Tap drill | Decimal (in) | mm | % thread |
|---|---|---|---|---|---|
| #0-80 | 0.0600 | 3/64 | 0.0469 | 1.19 | 81% |
| #1-64 | 0.0730 | #53 | 0.0595 | 1.51 | 67% |
| #2-56 | 0.0860 | #50 | 0.0700 | 1.78 | 69% |
| #3-48 | 0.0990 | #47 | 0.0785 | 1.99 | 76% |
| #4-40 | 0.1120 | #43 | 0.0890 | 2.26 | 71% |
| #5-40 | 0.1250 | #38 | 0.1015 | 2.58 | 72% |
| #6-32 | 0.1380 | #36 | 0.1065 | 2.71 | 78% |
| #8-32 | 0.1640 | #29 | 0.1360 | 3.45 | 69% |
| #10-24 | 0.1900 | #25 | 0.1495 | 3.80 | 75% |
| #12-24 | 0.2160 | #16 | 0.1770 | 4.50 | 72% |
| 1/4-20 | 0.2500 | #7 | 0.2010 | 5.11 | 75% |
| 5/16-18 | 0.3125 | F | 0.2570 | 6.53 | 77% |
| 3/8-16 | 0.3750 | 5/16 | 0.3125 | 7.94 | 77% |
| 7/16-14 | 0.4375 | U | 0.3680 | 9.35 | 75% |
| 1/2-13 | 0.5000 | 27/64 | 0.4219 | 10.72 | 78% |
| 9/16-12 | 0.5625 | 31/64 | 0.4844 | 12.30 | 72% |
| 5/8-11 | 0.6250 | 17/32 | 0.5312 | 13.49 | 79% |
| 3/4-10 | 0.7500 | 21/32 | 0.6562 | 16.67 | 72% |
| 7/8-9 | 0.8750 | 49/64 | 0.7656 | 19.45 | 76% |
| 1-8 | 1.0000 | 7/8 | 0.8750 | 22.22 | 77% |
Unified fine thread (UNF) tap drill chart
More threads per inch means a shallower thread, a smaller cutting allowance and a larger tap drill for the same nominal size. Fine threads hold better under vibration and strip more easily if you overtighten them.
| Thread | Major dia (in) | Tap drill | Decimal (in) | mm | % thread |
|---|---|---|---|---|---|
| #0-80 | 0.0600 | 3/64 | 0.0469 | 1.19 | 81% |
| #1-72 | 0.0730 | #53 | 0.0595 | 1.51 | 75% |
| #2-64 | 0.0860 | #50 | 0.0700 | 1.78 | 79% |
| #3-56 | 0.0990 | #45 | 0.0820 | 2.08 | 73% |
| #4-48 | 0.1120 | #42 | 0.0935 | 2.37 | 68% |
| #5-44 | 0.1250 | #37 | 0.1040 | 2.64 | 71% |
| #6-40 | 0.1380 | #33 | 0.1130 | 2.87 | 77% |
| #8-36 | 0.1640 | #29 | 0.1360 | 3.45 | 78% |
| #10-32 | 0.1900 | #21 | 0.1590 | 4.04 | 76% |
| #12-28 | 0.2160 | #14 | 0.1820 | 4.62 | 73% |
| 1/4-28 | 0.2500 | #3 | 0.2130 | 5.41 | 80% |
| 5/16-24 | 0.3125 | I | 0.2720 | 6.91 | 75% |
| 3/8-24 | 0.3750 | Q | 0.3320 | 8.43 | 79% |
| 7/16-20 | 0.4375 | 25/64 | 0.3906 | 9.92 | 72% |
| 1/2-20 | 0.5000 | 29/64 | 0.4531 | 11.51 | 72% |
| 9/16-18 | 0.5625 | 33/64 | 0.5156 | 13.10 | 65% |
| 5/8-18 | 0.6250 | 37/64 | 0.5781 | 14.68 | 65% |
| 3/4-16 | 0.7500 | 11/16 | 0.6875 | 17.46 | 77% |
| 7/8-14 | 0.8750 | 13/16 | 0.8125 | 20.64 | 67% |
| 1-12 | 1.0000 | 59/64 | 0.9219 | 23.42 | 72% |
Metric coarse thread tap drill chart
Metric is the easy one. Tap drill = major diameter − pitch. Every row below follows that rule, so you can work out any size you don't see listed.
| Thread | Major dia (mm) | Pitch (mm) | Tap drill | Decimal (in) |
|---|---|---|---|---|
| M1.6 × 0.35 | 1.6 | 0.35 | 1.25 mm | 0.0492 |
| M2 × 0.4 | 2 | 0.4 | 1.6 mm | 0.0630 |
| M2.5 × 0.45 | 2.5 | 0.45 | 2.05 mm | 0.0807 |
| M3 × 0.5 | 3 | 0.5 | 2.5 mm | 0.0984 |
| M3.5 × 0.6 | 3.5 | 0.6 | 2.9 mm | 0.1142 |
| M4 × 0.7 | 4 | 0.7 | 3.3 mm | 0.1299 |
| M5 × 0.8 | 5 | 0.8 | 4.2 mm | 0.1654 |
| M6 × 1 | 6 | 1 | 5 mm | 0.1969 |
| M7 × 1 | 7 | 1 | 6 mm | 0.2362 |
| M8 × 1.25 | 8 | 1.25 | 6.8 mm | 0.2677 |
| M10 × 1.5 | 10 | 1.5 | 8.5 mm | 0.3346 |
| M12 × 1.75 | 12 | 1.75 | 10.2 mm | 0.4016 |
| M14 × 2 | 14 | 2 | 12 mm | 0.4724 |
| M16 × 2 | 16 | 2 | 14 mm | 0.5512 |
| M18 × 2.5 | 18 | 2.5 | 15.5 mm | 0.6102 |
| M20 × 2.5 | 20 | 2.5 | 17.5 mm | 0.6890 |
| M22 × 2.5 | 22 | 2.5 | 19.5 mm | 0.7677 |
| M24 × 3 | 24 | 3 | 21 mm | 0.8268 |
Metric fine thread tap drill chart
Same rule, finer pitch, larger drill. Fine metric threads turn up in automotive and machine work where the coarse equivalent would loosen under vibration.
| Thread | Major dia (mm) | Pitch (mm) | Tap drill | Decimal (in) |
|---|---|---|---|---|
| M8 x 1.0 | 8 | 1 | 7 mm | 0.2756 |
| M10 x 1.25 | 10 | 1.25 | 8.8 mm | 0.3465 |
| M10 x 1.0 | 10 | 1 | 9 mm | 0.3543 |
| M12 x 1.5 | 12 | 1.5 | 10.5 mm | 0.4134 |
| M12 x 1.25 | 12 | 1.25 | 10.8 mm | 0.4252 |
| M14 x 1.5 | 14 | 1.5 | 12.5 mm | 0.4921 |
| M16 x 1.5 | 16 | 1.5 | 14.5 mm | 0.5709 |
| M18 x 1.5 | 18 | 1.5 | 16.5 mm | 0.6496 |
| M20 x 1.5 | 20 | 1.5 | 18.5 mm | 0.7283 |
| M24 x 2.0 | 24 | 2 | 22 mm | 0.8661 |
Clearance hole chart
The other half of the job, and the one people skip. A clearance hole is drilled larger than the fastener so the bolt drops through the top piece and clamps it down. Drill it the same size as the bolt and the threads bite the top piece, which stops it pulling tight and is the usual reason a joint stays loose no matter how hard you torque it.
| Screw size | Close fit | Decimal (in) | Free fit | Decimal (in) |
|---|---|---|---|---|
| #4 | #30 | 0.1285 | #27 | 0.1440 |
| #6 | #27 | 0.1440 | #18 | 0.1695 |
| #8 | #18 | 0.1695 | #9 | 0.1960 |
| #10 | #9 | 0.1960 | #2 | 0.2210 |
| 1/4 | 17/64 | 0.2656 | 9/32 | 0.2812 |
| 5/16 | 21/64 | 0.3281 | 11/32 | 0.3438 |
| 3/8 | 25/64 | 0.3906 | 13/32 | 0.4062 |
| 7/16 | 29/64 | 0.4531 | 15/32 | 0.4688 |
| 1/2 | 33/64 | 0.5156 | 17/32 | 0.5312 |
| 5/8 | 41/64 | 0.6406 | 21/32 | 0.6562 |
| 3/4 | 49/64 | 0.7656 | 25/32 | 0.7812 |
Close fit for locating parts precisely; free fit where you need room to align holes across two pieces.
How much thread engagement do you actually need?
The percentage column is not decoration — it is the number that decides whether your tap survives.
| % thread | Use it when | Trade-off |
|---|---|---|
| 50–60% | Hard or tough material, deep blind holes, tapping by hand in stainless | Roughly 90% of full thread strength, far less tapping torque |
| 70–75% | Standard for steel, aluminium, brass — every value in the tables above | The accepted balance of strength and tap survival |
| 85–100% | Thin sheet where you only get one or two threads of depth | Adds ~5% strength for ~3× the tapping torque; breaks taps |
The formula behind the column, if you want to check a size that isn't listed:
For 1/4-20 with a #7 drill: (0.2500 − 0.2010) ÷ (1.299038 ÷ 20) × 100 = 75.4%
If you don't have the exact bit
Go up to the next size, never down. A slightly oversize hole gives a slightly weaker thread; an undersize hole gives a tap that binds and snaps off inside the work, which is a far worse afternoon. The nearest fractional substitutes for the most common tap drills:
| Thread | Correct bit | Fractional substitute | Resulting % thread |
|---|---|---|---|
| 1/4-20 | #7 (0.2010") | 13/64" (0.2031") | 72% |
| 10-24 | #25 (0.1495") | 5/32" (0.1562") | 62% |
| 8-32 | #29 (0.1360") | 9/64" (0.1406") | 58% |
| 6-32 | #36 (0.1065") | 7/64" (0.1094") | 71% |
| 5/16-18 | F (0.2570") | 1/4" (0.2500") | 87% |
Note the last row: substituting 1/4" for an F drill pushes you to 87% thread. That is a lot of tapping torque for no useful strength — get the F bit.
Tapping order that works
- Centre punch — a tap drill that walks puts the thread off axis and there is no recovering it.
- Drill the tap drill size, straight, ideally in a press or with a drill guide.
- Chamfer the mouth lightly with a larger bit or countersink. This lets the tap start square and stops the first thread lifting a burr.
- Start with a taper tap, square to the hole. Check with a small square from two directions on the first two turns.
- Quarter turn forward, half turn back, with cutting fluid, to break the chip. In a blind hole, back the tap out and clear it every few turns.
- Finish with a bottoming tap if you need thread all the way down a blind hole.
Frequently asked questions
What size drill do I use for a 1/4-20 tap?
A #7 drill, 0.201 in (5.11 mm). That gives about 75% thread engagement, which is the standard target for steel and aluminium. A 13/64 in bit (0.2031 in) is the usual substitute if you have no number drills.
What is the tap drill for a 10-32 thread?
A #21 drill, 0.159 in (4.04 mm). For the coarse 10-24 version it is a #25, 0.1495 in — the two are easy to confuse and a 10-24 tap will not start cleanly in a #21 hole.
What drill size for an M6 tap?
5.0 mm. The metric rule is simple: tap drill = major diameter minus pitch. M6 has a 1.0 mm coarse pitch, so 6 − 1 = 5 mm. The same rule gives 8.5 mm for M10 × 1.5 and 10.2 mm for M12 × 1.75.
Why is 75% thread the standard, not 100%?
Going from 75% to 100% thread engagement adds only about 5% to the strength of the joint but roughly triples the torque needed to turn the tap — which is how taps break. In practice a 75% thread fails by stripping the bolt, not the hole, so the extra material buys nothing.
Can I use a clearance drill and a tap drill interchangeably?
No — they do opposite jobs. The tap drill is smaller than the fastener and leaves material for threads to be cut into. The clearance drill is larger than the fastener so the bolt passes through freely. A 1/4-20 bolt needs a #7 (0.201 in) hole to be tapped, and a 17/64 in (0.2656 in) hole to pass through.