Size charts
Tap Drill Size Chart (UNC, UNF, Metric)
In short
This tap drill size chart gives the drill bit for every common tap. For inch threads, use the 75% thread column. Common sizes: #8-32 → #29, #10-24 → #25, #10-32 → #21, 1/4-20 → #7, 5/16-18 → F, 3/8-16 → 5/16 in, 1/2-13 → 27/64 in. For metric, drill the diameter minus the pitch: M6 x 1 → 5.0 mm, M8 x 1.25 → 6.8 mm, M10 x 1.5 → 8.5 mm.
- Tap drill: makes the hole you thread.
- Clearance drill: makes a hole the screw passes through. Close fit = tight alignment. Free fit = some slop.
- 50% thread drill: a larger drill for tough or stainless steel, where tap breakage is the bigger risk.
Inch tap drill chart: #0 to 1/2 in (UNC and UNF)
| Thread | 75% tap drill | Decimal (in) | 50% tap drill | Close-fit clearance (in) | Free-fit clearance (in) |
|---|---|---|---|---|---|
| #0-80 | 3/64 | 0.0469 | #55 | #52 (0.0635) | #50 (0.0700) |
| #1-64 | #53 | 0.0595 | 1/16 | — ² | — ² |
| #1-72 | #53 | 0.0595 | #52 | — ² | — ² |
| #2-56 | #50 | 0.0700 | #49 | #43 (0.0890) | #41 (0.0960) |
| #2-64 | #50 | 0.0700 | #48 | #43 (0.0890) | #41 (0.0960) |
| #3-48 | #47 | 0.0785 | #44 | — ² | — ² |
| #3-56 | #45 ¹ | 0.0820 | #43 | — ² | — ² |
| #4-40 | #43 | 0.0890 | #41 | #32 (0.1160) | #30 (0.1285) |
| #4-48 | #42 | 0.0935 | #40 | #32 (0.1160) | #30 (0.1285) |
| #5-40 | #38 | 0.1015 | 7/64 | — ² | — ² |
| #5-44 | #37 | 0.1040 | #35 | — ² | — ² |
| #6-32 | #36 | 0.1065 | #32 | #27 (0.1440) | #25 (0.1495) |
| #6-40 | #33 | 0.1130 | #31 | #27 (0.1440) | #25 (0.1495) |
| #8-32 | #29 | 0.1360 | #27 | #18 (0.1695) | #16 (0.1770) |
| #8-36 | #29 | 0.1360 | #26 | #18 (0.1695) | #16 (0.1770) |
| #10-24 | #25 ¹ | 0.1495 | #20 | #9 (0.1960) | #7 (0.2010) |
| #10-32 | #21 | 0.1590 | #18 | #9 (0.1960) | #7 (0.2010) |
| #12-24 | #16 | 0.1770 | #12 | #2 (0.2210) | #1 (0.2280) |
| #12-28 | #14 ¹ | 0.1820 | #10 | #2 (0.2210) | #1 (0.2280) |
| 1/4-20 | #7 | 0.2010 | 7/32 | F (0.2570) | H (0.2660) |
| 1/4-28 | #3 | 0.2130 | #1 | F (0.2570) | H (0.2660) |
| 5/16-18 | F | 0.2570 | J | P (0.3230) | Q (0.3320) |
| 5/16-24 | I | 0.2720 | 9/32 | P (0.3230) | Q (0.3320) |
| 3/8-16 | 5/16 | 0.3125 | Q | W (0.3860) | X (0.3970) |
| 3/8-24 | Q | 0.3320 | S | W (0.3860) | X (0.3970) |
| 7/16-14 | U | 0.3680 | 25/64 | — ² | — ² |
| 7/16-20 | 25/64 | 0.3906 | 13/32 | — ² | — ² |
| 1/2-13 | 27/64 | 0.4219 | 29/64 | 33/64 (0.5156) | 17/32 (0.5312) |
| 1/2-20 | 29/64 | 0.4531 | 15/32 | 33/64 (0.5156) | 17/32 (0.5312) |
¹ Tameson's chart lists the next smaller number drill for these: #46 for 3-56, #26 for 10-24, #15 for 12-28. Little Machine Shop (and, for 10-24 and 12-28, the University of Pennsylvania) list the values shown, which are also the drills closest to 75% thread.
² Clearance sizes are shown only where two charts agree. For these sizes only one of our sources lists them; ask your fastener supplier or use a free fit slightly over the screw's diameter.
Inch tap drill chart: 9/16 to 1 in
| Thread | 75% tap drill | Decimal (in) |
|---|---|---|
| 9/16-12 | 31/64 | 0.4844 |
| 9/16-18 | 33/64 | 0.5156 |
| 5/8-11 | 17/32 | 0.5312 |
| 5/8-18 | 37/64 | 0.5781 |
| 3/4-10 | 21/32 | 0.6562 |
| 3/4-16 | 11/16 | 0.6875 |
| 7/8-9 | 49/64 | 0.7656 |
| 7/8-14 | 13/16 | 0.8125 |
| 1-8 | 7/8 | 0.8750 |
| 1-12 | 59/64 ³ | 0.9219 |
³ Tameson lists 59/64 in for 1-12; Little Machine Shop lists 15/16 in (0.9375 in), which works out to about 58% thread. 59/64 in gives about 72% and matches the 75%-thread target more closely.
Metric tap drill chart (ISO coarse and fine)
| Thread | Tap drill (mm) | Tap drill (in) |
|---|---|---|
| M1.6 x 0.35 | 1.25 | 0.0492 |
| M2 x 0.4 | 1.6 | 0.0630 |
| M2.5 x 0.45 | 2.05 | 0.0807 |
| M3 x 0.5 | 2.5 | 0.0984 |
| M3.5 x 0.6 | 2.9 | 0.1142 |
| M4 x 0.7 | 3.3 | 0.1299 |
| M5 x 0.8 | 4.2 | 0.1654 |
| M6 x 1.0 | 5.0 | 0.1969 |
| M8 x 1.25 | 6.8 ⁴ | 0.2677 |
| M8 x 1.0 (fine) | 7.0 | 0.2756 |
| M10 x 1.5 | 8.5 | 0.3346 |
| M10 x 1.25 (fine) | 8.8 ⁴ | 0.3465 |
| M12 x 1.75 | 10.2 ⁴ | 0.4016 |
| M12 x 1.5 (fine) | 10.5 | 0.4134 |
| M12 x 1.25 (fine) | 10.8 | 0.4252 |
| M14 x 2.0 | 12.0 ⁴ | 0.4724 |
| M14 x 1.5 (fine) | 12.5 | 0.4921 |
| M16 x 2.0 | 14.0 | 0.5512 |
| M16 x 1.5 (fine) | 14.5 | 0.5709 |
| M18 x 2.5 | 15.5 | 0.6102 |
| M20 x 2.5 | 17.5 | 0.6890 |
| M20 x 1.5 (fine) | 18.5 | 0.7283 |
| M22 x 2.5 | 19.5 | 0.7677 |
| M24 x 3.0 | 21.0 | 0.8268 |
| M24 x 2.0 (fine) | 22.0 | 0.8661 |
⁴ Charts differ by 0.1 mm on a few sizes. Tameson lists 6.7 mm for M8 x 1.25 and 8.7 mm for M10 x 1.25. The University of Pennsylvania chart lists 10.3 mm for M12 x 1.75 and 12.1 mm for M14 x 2.0. The values shown follow the diameter-minus-pitch rule and Fractory's chart.
Pipe tap drills (NPT)
Pipe threads are tapered, and published tap drill sizes for them differ. Allied Machine (a drill maker) and Tameson list the smaller drill; Wikipedia's NPT table lists the larger one.
| NPT size | Threads per inch | Tap drill: Allied Machine | Tap drill: Wikipedia table |
|---|---|---|---|
| 1/8 | 27 | Q (0.332 in) | R (0.339 in) |
| 1/4 | 18 | 7/16 in | 7/16 in |
| 3/8 | 18 | 9/16 in | 37/64 in |
| 1/2 | 14 | 45/64 in | 23/32 in |
| 3/4 | 14 | 29/32 in | 59/64 in |
| 1 | 11.5 | 1-9/64 in | 1-5/32 in |
Allied's values are for drilling without a taper reamer. Every NPT size, with the pipe ODs, is on the NPT thread size chart.
How to use this chart
- Identify the thread. Measure the diameter with calipers and the pitch with a thread gauge, or check the bolt's packaging. Not sure? Snap it.
- Pick the column. 75% for aluminum, brass, plastic and most mild steel. 50% for tough or stainless steel if the tap keeps binding.
- Drill straight, chamfer, tap with cutting fluid, and back the tap off every half to full turn to break the chip.
- Thread-forming (roll) taps need bigger holes. Use the size printed on the tap or the maker's chart, not this one.
Common mistakes
- Using the coarse-thread drill for a fine thread. 1/4-20 takes a #7, but 1/4-28 takes a #3. Check the TPI.
- Using the clearance size as a tap drill. F (0.257 in) is a 5/16-18 tap drill and a 1/4 in clearance hole. Read the right column.
- Mixing inch and metric look-alikes. 1/4-20 vs M6 x 1.0, 10-32 vs M5 x 0.8, 3/8-16 vs M10 x 1.5. They are close in size but don't interchange.
- Treating NPT like a straight thread. Pipe taps cut a taper, so how deep you tap matters as much as the drill. Test-fit the actual fitting as you go.
Not sure what thread you have? Snap it
Snap the old screw or bolt next to a ruler and Mechy tells you the size, thread and length. Then come back here for the drill. If the thread has to be exact, confirm it with a thread gauge before you drill. Identify a fastener from a photo.
Questions
How do I figure out the tap drill size?
Metric: subtract the pitch from the diameter (M8 x 1.25 → 6.8 mm). Inch: subtract 1 ÷ TPI from the diameter (1/4-20 → 0.250 − 0.050 = 0.200 in, so a #7 at 0.201 in). Both give roughly 75% thread.
What size drill bit for a 3/8-16 tap?
A 5/16 in drill bit (0.3125 in). For the fine 3/8-24 thread, use a Q (0.332 in).
What size drill bit for a 5/16-18 tap?
An F drill (0.257 in). For 5/16-24, use an I (0.272 in).
What size drill bit for a 1/2-13 tap?
A 27/64 in drill bit (0.4219 in). For 1/2-20, use 29/64 in (0.4531 in).
What does 75% thread mean?
The thread is cut to 75% of its full theoretical depth. It is the usual target because deeper threads make the tap much harder to turn and more likely to break, for little gain.
Sources
- Drill and Tap Chart (Inch and Metric) – MEAM Labs, University of Pennsylvania
- Tap Drill Sizes – Little Machine Shop
- Tap Drill Size Charts – Tameson
- Metric Tap Drill Chart – Fractory
- Tap Drill Charts: National Pipe Threads (PDF) – Allied Machine & Engineering
- National pipe thread – Wikipedia
- Unified Thread Standard – Wikipedia
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