Calculate a recommended tap drill diameter for metric or inch threads using thread size, pitch/TPI and selected thread percentage.
70% is a commonly used starting point for general-purpose tapped holes, but the correct production drill depends on the tap, material, application and applicable standard.
Enter thread information and calculate.
What is a tap drill? A tap drill is the drill used to create the hole before internal threads are cut with a tap.
How is metric tap drill size estimated? A basic approximation is major diameter minus pitch. Selecting a lower thread percentage increases the drill diameter.
How is an inch tap drill estimated? A simplified calculation uses major diameter minus the selected fraction of the thread pitch, where pitch is 1 divided by TPI.
What thread percentage should I use? The appropriate percentage depends on the application, material, tap type and required thread strength. The calculator lets you compare different percentages.
Does a larger tap drill create more or less thread? A larger drill removes more material before tapping, resulting in a lower percentage of theoretical thread.
Does material affect tap-drill selection? Yes. Material, tapping method, lubrication, tap geometry and production requirements can affect the preferred drill size.
Can I use this for CNC machining? Yes, as a calculation aid, but verify the final drill diameter against the tooling manufacturer's recommendations and the required thread tolerance.
Does this replace a standard tap-drill chart? No. It provides a useful mathematical estimate. Standardized tap-drill tables and tooling recommendations should take priority for production work.