Shop tools
Tap drill size calculator
Pick a thread and a tap type and it gives you the hole size, plus the percentage of thread engagement that hole actually delivers. A cutting tap and a forming tap need different holes for the same thread, and getting that wrong is how taps break — so the tool asks which you are using rather than assuming.
The reference below is free to read — no account, no signup. The tool itself is inside MechCircle, which is free to start.
Coarse-pitch metric tap drill sizes
| Thread | Pitch | Cutting tap at 75% | Forming tap at 70% | Nearest drill (cutting) |
|---|---|---|---|---|
| M3 × 0.6 | 0.60 mm | 2.42 mm | 2.76 mm | 2.40 mm |
| M4 × 0.75 | 0.75 mm | 3.27 mm | 3.70 mm | 3.30 mm |
| M5 × 0.9 | 0.90 mm | 4.12 mm | 4.64 mm | 4.10 mm |
| M6 × 1 | 1.00 mm | 5.03 mm | 5.60 mm | 5.00 mm |
| M8 × 1.25 | 1.25 mm | 6.78 mm | 7.50 mm | 6.80 mm |
| M10 × 1.5 | 1.50 mm | 8.54 mm | 9.40 mm | 8.50 mm |
| M12 × 1.75 | 1.75 mm | 10.30 mm | 11.30 mm | 10.30 mm |
| M14 × 2 | 2.00 mm | 12.05 mm | 13.21 mm | 12.10 mm |
| M16 × 2 | 2.00 mm | 14.05 mm | 15.21 mm | 14.10 mm |
| M20 × 2.5 | 2.50 mm | 17.56 mm | 19.01 mm | 17.60 mm |
A forming (roll) tap always needs a LARGER hole than a cutting tap for the same thread and engagement, because it displaces material instead of removing it. The tool covers 91 metric size-and-pitch combinations, not only the coarse pitches shown here.
How to use it
Choose cutting or forming, search for the thread, and adjust the engagement if you want something other than the default. Every pitch is listed as its own entry, so M10 × 1.25 can never be mistaken for M10 × 1.5. The arithmetic is shown, so you can check it against the chart on your wall.
What it does and does not cover
- Metric sizes also show the nearest drill using a step rule: 0.05 mm below 3 mm, 0.1 mm above. That is a rule, not a stock-drill index.
- Inch sizes give the exact hole only. Number (#1 to #80) and letter (A to Z) drill designations are not included.
- BSP threads are not covered — they use a different thread form that these formulas do not describe.
- Confirm the final hole with a tapping test in the actual material.
Ready to get started?
Every tool is included, along with tasks, files and direct communication with your customers, suppliers and consultants.