Tap drill size calculator
Pick a thread and how much thread engagement you want. You get the closest drill from every set, fractional, number, letter and metric, with the % thread each one actually cuts.
Target thread engagement
75% is the chart standard. Go lower for stainless, hard steel or deep holes; it is easier on the tap and loses little strength.
| Set | Drill | in | mm | Thread |
|---|---|---|---|---|
| Fractional | 13/64" | 0.2031 | 5.16 | 72% |
| Number | #7 | 0.2010 | 5.11 | 75% |
| Letter | no letter drill fits this thread | |||
| Metric | 5.1 mm | 0.2008 | 5.10 | 76% |
Tap drill for 1/4-20
#7
0.2010" · 5.11 mm · 75% thread
Ideal size
0.2013 in
5.11 mm
Chart standard
#7 75%
Basic minor Ø
0.1959 in
Good for
general use
1/4-20 tap drill: #7 (0.2010 in / 5.11 mm), 75% threadInch tap drill chart (UNC / UNF)
Standard tap drills at about 75% thread. Open a size to see every drill that works with it, the thread specs and the metric equivalent. Looking for metric? See the metric tap drill chart.
| Thread | Tap drill | in | mm | Thread |
|---|---|---|---|---|
| 4-40 UNC | #43 | 0.0890 | 2.26 | 71% |
| 6-32 UNC | #36 | 0.1065 | 2.71 | 78% |
| 8-32 UNC | #29 | 0.1360 | 3.45 | 69% |
| 10-24 UNC | #25 | 0.1495 | 3.80 | 75% |
| 10-32 UNF | #21 | 0.1590 | 4.04 | 76% |
| 1/4-20 UNC | #7 | 0.2010 | 5.11 | 75% |
| 1/4-28 UNF | #3 | 0.2130 | 5.41 | 80% |
| 5/16-18 UNC | F | 0.2570 | 6.53 | 77% |
| 3/8-16 UNC | 5/16" | 0.3125 | 7.94 | 77% |
| 3/8-24 UNF | Q | 0.3320 | 8.43 | 79% |
| 1/2-13 UNC | 27/64" | 0.4219 | 10.72 | 78% |
| 1/2-20 UNF | 29/64" | 0.4531 | 11.51 | 72% |
| 5/8-11 UNC | 17/32" | 0.5313 | 13.49 | 79% |
| 3/4-10 UNC | 21/32" | 0.6563 | 16.67 | 72% |
Guides
Percent threadhow much thread you actually need3D printed holeswhy they shrink, how to size themMetric chartM2 to M16 coarse
Frequently asked questions
What is a tap drill?
The drill you use before tapping a thread. It sets the hole's minor diameter, and with it how much thread the tap cuts: a smaller drill leaves more thread but makes the tap work harder.
What percent thread should I use?
75% is the chart standard and right for most work. Use 60–70% in stainless, hard steel or deep holes. Above about 75% the thread barely gets stronger, while the torque on the tap rises fast.
Why do some charts list a different drill for the same thread?
Charts round the ideal diameter to the drills people own, and not all use the same rule. M12 × 1.75 is the common case: 10.2 mm in ISO-based charts (about 79% thread) and 10.3 mm in 75% charts. Both work.
Can I use a metric drill for an inch tap?
Yes, if it gives a sensible percent thread. The calculator lists the closest drill from every set, so you can see what a metric drill does to an inch thread before you drill.
% thread = (major diameter − drill) ÷ (1.299 × pitch) × 100. Drill for a target: major − (% ÷ 100) × 1.299 × pitch. Standard drills are taken from published charts and checked against this formula on every build.