A tap drill size calculator and chart for metric and inch threads
This tap drill size calculator gives the drill to use before tapping a thread, for ISO metric coarse and fine threads, UNC and UNF, or any diameter and pitch you type. Set the percentage of thread you want and it works out the exact hole size, the nearest standard drill (number, letter, fractional or metric), the thread percentage that drill really gives, and a full tap drill size chart for every common size.
Most charts are printed for 75% thread and one drill series. Here you can change the percentage for tough materials, see the metric drill that suits an inch thread and the other way round, and copy or download the chart. It runs in your browser.
How to use the tap drill calculator
- Choose Metric (ISO), UNC / UNF or Other thread.
- Pick the thread, for example M8 × 1.25 or 1/4-20 UNC; for other threads enter the major diameter and the pitch in mm or threads per inch.
- Set the thread engagement: 75% for general work, 60–65% for stainless or tool steel, or for deep holes.
- Read the recommended drill, the exact size and the thread percentage it gives; the chart below lists every size in the same system.
- Use Copy chart or CSV to keep a printed tap drill chart by the machine.
Tap Drill Size Calculator & Chart features
Any percentage of thread
The Machinery’s Handbook formula, not a fixed 75% table, so you can tap stainless at 60%.
Every drill series
Number #1–#80, letter A–Z, fractional to 1-1/2 in and metric drills in 0.05, 0.1 and 0.25 mm steps.
Real thread percentage
Shows what the nearest standard drill actually gives, and warns when it falls below the minor diameter.
Charts for mm and inch
ISO metric coarse and fine, UNC and UNF, with a cross-reference to the other system’s drill.
Thread profile sketch
A drawing of the 60° thread with the major diameter, the basic minor diameter and your drilled hole.
Tap drill size formula
For cut taps the usual formula is drill = D − (0.01299 × % thread) ÷ TPI, with D the major diameter in inches and TPI the threads per inch. In millimetres it is drill = D − 0.01299 × % × P with P the pitch. At 75% that is very close to D − P, the metric rule of thumb: M8 × 1.25 needs about 8 − 1.25 = 6.75 mm, so a 6.8 mm drill.
Example: tap drill size for 1/4-20. 0.01299 × 75 ÷ 20 = 0.0487 in, and 0.250 − 0.0487 = 0.2013 in. The nearest drill is a #7 at 0.201 in, which gives 75.4% thread. Turning the formula round, % thread = TPI × (D − drill) ÷ 0.01299.
Here 100% thread means a thread depth of 0.6495 × pitch, three quarters of the sharp 60° V. The ISO basic internal minor diameter D1 = D − 1.0825 P sits at about 83%; a hole smaller than D1 makes the tap cut the minor diameter.
Common tap drill sizes (75% thread)
| Thread | Tap drill | Thread | Tap drill |
|---|---|---|---|
| M3 × 0.5 | 2.5 mm | #4-40 UNC | #43 (0.089 in) |
| M4 × 0.7 | 3.3 mm | #6-32 UNC | #36 (0.1065 in) |
| M5 × 0.8 | 4.2 mm | #8-32 UNC | #29 (0.136 in) |
| M6 × 1 | 5.0 mm | #10-24 UNC | #25 (0.1495 in) |
| M8 × 1.25 | 6.8 mm | #10-32 UNF | #21 (0.159 in) |
| M10 × 1.5 | 8.5 mm | 1/4-20 UNC | #7 (0.201 in) |
| M12 × 1.75 | 10.3 mm (10.2 for ~80%) | 5/16-18 UNC | F (0.257 in) |
| M16 × 2 | 14.0 mm | 3/8-16 UNC | 5/16 in |
| M20 × 2.5 | 17.5 mm | 1/2-13 UNC | 27/64 in |
Choosing the thread percentage
- A 75% thread has about 95% of the strength of a full thread but needs far less torque, which is why it is the standard.
- Hard or tough materials (stainless, tool steel, nickel alloys): 55–65% to save taps.
- Soft metals and thin sheet: 75–85% for more engagement.
- Form (roll) taps displace metal instead of cutting it and need a larger hole — use the tap maker’s chart.
- Drills cut slightly oversize, so measure a test hole on critical work and check the minor-diameter limits for your thread class (6H, 2B).
Tap Drill Size Calculator & Chart FAQ
What is the tap drill size for 1/4-20?
A #7 drill (0.201 in, 5.1 mm) for about 75% thread. A 13/64 in drill (0.2031 in) gives about 72% and is a common substitute.
What is the tap drill size formula?
Drill = major diameter − 0.01299 × percentage of thread ÷ threads per inch. For metric threads, drill ≈ diameter − pitch gives about 77% thread.
Where can I find a tap drill size chart in mm?
Choose Metric (ISO) above: the chart lists M1 to M48 coarse and the common fine pitches with the tap drill in mm, its decimal-inch size and the nearest inch drill.
What drill size for an M8 tap?
6.8 mm for M8 × 1.25 coarse thread and 7.0 mm for M8 × 1 fine thread.
Why do some charts say 10.2 mm for M12 and others 10.3 mm?
10.2 mm (D − P rounded down) gives about 79% thread and 10.3 mm about 75%. Both are within the 6H minor-diameter limits; the calculator shows the nearest drill to the percentage you choose.
Does this work for roll (form) taps?
No. Form taps need a bigger hole, roughly the pitch diameter minus a small allowance, so use the tap manufacturer’s drill chart for them.