Tap drill size calculator and chart
Type a thread to get the drill that cuts it, and how much thread each nearby drill leaves. 45 common threads are built in, and any other size works too.
Find your thread
Every thread we cover, with its standard drill. Click a row for that thread's full page, or see the full chart, the metric, inch and screw size charts.
Screw sizes10 threads
Inch, fractional18 threads
Common sizes at a glance
| Thread | Standard drill | Drill size | Thread left |
|---|---|---|---|
| 1/4-20 | #7 | 0.2010 in (5.11 mm) | 75% |
| #6-32 | #36 | 0.1065 in (2.71 mm) | 78% |
| #8-32 | #29 | 0.1360 in (3.45 mm) | 69% |
| #10-24 | #25 | 0.1495 in (3.80 mm) | 75% |
| #10-32 | #21 | 0.1590 in (4.04 mm) | 76% |
| M6 | 5.0 mm | 0.1969 in | 77% |
| M8 | 6.8 mm | 0.2677 in | 74% |
| M10 | 8.5 mm | 0.3346 in | 77% |
Tap drill basics
Why the hole is smaller than the screw
A tap cuts a thread into the wall of a drilled hole. So the hole has to be smaller than the screw. The metal between the hole and the screw size is what the tap turns into thread.
For a 1/4-20 screw, the standard drill is a #7 (0.2010 in). The screw is 0.2500 in across, so the hole is 0.0490 in narrower. The tap cuts 0.0245 in into each side.
Drill the hole too big and the thread is shallow and weak. Drill it too small and the tap works hard and can break.
How to read a thread name
- 1/4-20 means 1/4 inch across and 20 threads per inch.
- #10-32 means screw gauge 10, which is 0.190 in across, with 32 threads per inch.
- M6 x 1.0 means 6 mm across and 1.0 mm from one thread to the next.
Two threads can share a diameter and differ in pitch. 1/4-20 is the coarse thread and takes a #7 drill. 1/4-28 is the fine thread and takes a #3. Never swap them.
How much thread to leave
Charts print one drill, usually the one that leaves about 75% of a full thread. That suits aluminum, brass and many through holes.
Harder metals do better with less. Texas Metalworks suggests 60 to 65% for mild steel and about 50% for stainless and titanium. ToolGrit gives similar ranges and cites studies showing about 90% of a thread's shear strength at 60% thread. OSG, which makes taps, says a thread of 55 to 65% is enough for most requirements.
On 1/4-20, the steel drill is #4 and it leaves 63%. Pick your material in the calculator and it chooses for you. See how we work it out.
Before you tap
- Put cutting fluid on the tap and do not tap dry (ToolGrit, Wikipedia).
- Hold the tap in line with the hole. A tap that is not in line is one of the reasons taps break (Wikipedia, ToolGrit).
- Turn the tap forward, then back it off a little to break the chip (Wikipedia).
- In a blind hole, drill at least 4 threads deeper than the thread you need (ToolGrit).
- For expensive work, check your tap maker's own chart as well.
Every thread page has a step-by-step guide with the depths worked out for that size.
What this site covers
45 common threads: numbered screws from #0 to #12, inch UNC and UNF from 1/4 to 7/8, and metric coarse from M2 to M30. Each has its own page with drills by material and a worked check.
Pipe threads (NPT and BSP), forming taps and clearance holes are not covered yet. Published pipe-thread drill sizes disagree, so we left them out until we can verify them.
FAQ
What is a tap drill size?
It is the diameter of the hole you drill before cutting an internal thread with a tap. The hole is smaller than the screw, so the tap has metal left to cut a thread into.
Why is the metric tap drill just the thread size minus the pitch?
Drilling the major diameter minus the pitch leaves about 77% of the full thread depth, the usual chart target. M6 x 1.0 takes a 5.0 mm drill and leaves about 77% by our calculation.
Does a bigger tap drill make the thread weaker?
Yes, but not by much at first. ToolGrit cites studies showing about 90% of thread shear strength at 60% engagement, and Texas Metalworks says going from 75% to 60% costs only about 5% of pull-out strength.
Which drill should I use for aluminum?
Use #7 or #4, depending on the guide. Texas Metalworks says 70 to 75% thread for soft aluminum, which is #7 on 1/4-20. ToolGrit says 60 to 65% for aluminum alloys, which is #4. Both are normal choices.
What if I only have metric drills?
Use the closest one. For 1/4-20 that is 5.1 mm, which leaves about 76% thread. The calculator shows the closest drill in the other system for every thread.
Last reviewed October 7, 2026. How the numbers are calculated.