Find the right drill for tapping a thread. Pick a metric coarse or fine thread, or a UNC or UNF size, set the thread percentage, and get the exact drill diameter, the nearest standard drill (fractional, number, letter or metric) and the resulting thread engagement.
Last updated:
Thread system
%
75% is standard; 60–65% for hard or tough materials, up to 85% for soft or thin material
Enter your values to see the result.
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How to use the Tap Drill Size Calculator
Choose the thread system: metric, UNC or UNF.
Pick the thread size.
Set the thread percentage (75% is the usual default).
Read the tap drill, its real thread % and the nearest drill in the other system.
Use the chart for every size at the same thread percentage.
Tap drill formula
Drill diameter = D − 0.01299 × %thread × P, where D is the major (nominal) diameter and P the pitch (mm for metric, 1 ÷ TPI inches for UNC/UNF).
Thread % for a given drill = (D − drill) ÷ (0.01299 × P). Example: 1/2-13 UNC with a 27/64 in (0.4219) drill: (0.5 − 0.4219) ÷ (0.01299 × 0.07692) = 78%.
Common metric tap drill sizes (coarse thread)
Thread
Tap drill
Thread
Tap drill
M2 × 0.4
1.6 mm
M10 × 1.5
8.5 mm
M2.5 × 0.45
2.05 mm (2.1)
M12 × 1.75
10.2 mm
M3 × 0.5
2.5 mm
M14 × 2
12 mm
M4 × 0.7
3.3 mm
M16 × 2
14 mm
M5 × 0.8
4.2 mm
M20 × 2.5
17.5 mm
M6 × 1
5 mm
M24 × 3
21 mm
M8 × 1.25
6.8 mm
M30 × 3.5
26.5 mm
Common UNC and UNF tap drill sizes (about 75% thread)
UNC
Tap drill
UNF
Tap drill
#4-40
#43 (0.089)
#4-48
#42 (0.0935)
#6-32
#36 (0.1065)
#6-40
#33 (0.113)
#8-32
#29 (0.136)
#8-36
#29 (0.136)
#10-24
#25 (0.1495)
#10-32
#21 (0.159)
1/4-20
#7 (0.201)
1/4-28
#3 (0.213)
5/16-18
F (0.257)
5/16-24
I (0.272)
3/8-16
5/16 (0.3125)
3/8-24
Q (0.332)
7/16-14
U (0.368)
7/16-20
25/64 (0.3906)
1/2-13
27/64 (0.4219)
1/2-20
29/64 (0.4531)
5/8-11
17/32 (0.5313)
5/8-18
37/64 (0.5781)
3/4-10
21/32 (0.6563)
3/4-16
11/16 (0.6875)
These are the sizes found on most published tap drill charts. The calculator may pick a neighbouring drill when it is closer to your exact target percentage.
Frequently asked questions
How do I calculate tap drill size?
Tap drill = major diameter − 0.01299 × thread % × pitch (Machinery's Handbook). For metric at about 75% this is close to the simple rule drill = diameter − pitch, so an M6 × 1 thread uses a 5.0 mm drill. For inch threads the pitch is 1 ÷ TPI: 1/4-20 gives 0.2013 in, a #7 drill.
What is thread percentage?
It is how much of the full basic thread depth is actually cut. 100% would be a full-depth thread, but 75% thread has only about 5% less strength than 100% while needing much less tapping torque, so it is the standard choice.
What drill size for an M8 tap?
M8 × 1.25 coarse uses a 6.8 mm drill (about 75% thread). M8 × 1 fine uses a 7.0 mm drill.
What drill size for a 1/4-20 tap?
A #7 drill (0.201 in) gives about 75% thread. A 13/64 in drill (0.2031 in) gives about 72% and is a common substitute.
Should I use a smaller thread percentage for hard materials?
Yes. In stainless steel, titanium and other tough materials, 60–65% thread reduces tap breakage with little loss of strength. In soft or thin materials like aluminum sheet you can go up to about 85% for more engagement.
Is the drill size different for forming (roll) taps?
Yes. Thread-forming taps displace material instead of cutting it, so they need a larger hole, roughly diameter − half the pitch. Use the tap maker's chart for forming taps; this calculator is for cutting taps.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.