Tap Drill Sizes Explained: How to Choose the Right Drill
A tap drill is the drill that makes the hole before a tap cuts the thread. It is smaller than the screw, because the tap needs wall material to cut a thread into. The common charts pick the drill that leaves about 75% of full thread depth: a #8-32 screw uses a #29 drill (0.136 in), a 1/4-20 uses a #7, and an M8 x 1.25 uses 6.80 mm. Do not confuse the tap drill with the clearance hole, which is the larger hole the screw passes through. To get the right drill for any size, use the drill-bit finder, or read the chart for inch machine screws or metric screws.
Key takeaways
- The tap drill makes the hole that will be threaded. The clearance hole is for the part the screw passes through. They are different holes with different sizes.
- Inch shortcut: tap drill ≈ major diameter − 0.975 ÷ TPI. Metric shortcut: tap drill ≈ major diameter − pitch.
- A finer thread is shallower, so it needs a larger tap drill.
- If the tap’s maker prints a recommended drill, use it; charts can differ by a drill size.
What a tap drill does
A tap does not make a hole; it cuts a thread into the wall of one that already exists. That hole must be smaller than the screw’s major diameter. If it were as wide as the screw, the tap would find nothing to cut. If it were much narrower, the tap would have to remove too much material, and tapping becomes hard work. The usual compromise is a hole that leaves roughly 75% of the full thread depth, which is what standard tap drill charts are built on.
Common sizes at a glance
| Screw | Series | Tap drill |
|---|---|---|
| #8-32 | UNC | #29 (0.136 in / 3.454 mm) |
| #10-24 | UNC | #25 (0.1495 in) |
| #10-32 | UNF | #21 (0.159 in) |
| 1/4-20 | UNC | #7 (0.201 in) |
| 1/4-28 | UNF | #3 (0.213 in) |
| M6 x 1 | Coarse | 5.00 mm |
| M6 x 0.75 | Fine | 5.25 mm |
| M8 x 1.25 | Coarse | 6.80 mm |
| M8 x 1 | Fine | 7.00 mm |
The finer series always gets the larger drill at the same diameter: #21 against #25 on a #10, #3 against #7 on a 1/4 in, 7.00 mm against 6.80 mm on an M8. A finer thread is shallower, so the hole can sit closer to the full diameter.
A shortcut you can check in your head
You can estimate the drill without a chart. In inch sizes, take the major diameter and subtract 0.975 divided by the threads per inch. In metric sizes, subtract the pitch from the major diameter. The 0.975 comes from taking 75% of the thread height on each side of the hole, the convention behind the common charts.
- #8-32: 0.1640 − 0.975 ÷ 32 = 0.1335 in. The chart picks the #29 (0.136 in), a nearby standard drill.
- #10-24: 0.1900 − 0.975 ÷ 24 = 0.1494 in, which is the #25 (0.1495 in).
- 1/4-20: 0.2500 − 0.975 ÷ 20 = 0.2013 in, which is the #7 (0.201 in).
- M8 x 1.25: 8 − 1.25 = 6.75 mm, so the standard 6.8 mm drill.
- M6 x 0.75: 6 − 0.75 = 5.25 mm, a drill size in its own right.
Round to the nearest drill the chart offers. The shortcut shows you whether a chart value is sensible, but for a real job, read the chart.
Tap drill vs clearance hole
One screw needs two different holes when it joins two parts. Take a #8-32 machine screw: the part that holds the thread gets the #29 tap drill (0.136 in) and is then tapped. The part the screw passes through gets a clearance hole, which for a close fit is a #18. The tap drill is deliberately smaller than the screw; the clearance hole is deliberately larger. Using one for the other either leaves no thread or leaves the screw rattling.
Practical notes
- Substitutes: a 3.5 mm drill (0.1378 in) is close to a #29 (0.136 in). It is a little larger, so it leaves slightly less thread than the chart intends.
- Charts can differ: two published charts sometimes list drills one size apart for the same thread, because they choose slightly different engagement. If the tap’s maker prints a recommended drill, use it.
- Blind holes: drill deeper than the thread length you need so the tap and its chips have room.
- Cutting oil: lubricate when tapping metal, and back the tap out regularly to break the chip.
- Wood is different: a pilot hole for a wood screw follows a separate table. Never use a tap drill as a wood pilot.
The thread series behind these numbers are defined in ASME B1.1 for inch threads and ISO 261 for metric. The drill sizes follow the standard 75% thread tap drill chart. Sources and method explains how this site checks its figures.
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