Metric tap drill size calculator has been developed to find recommended cutting tap drill sizes to drill a hole prior to tapping ISO metric thread. The calculator finds metric tap drill size according to ISO 2306:1972 standard.

Calculations can be done by selecting a standard metric thread size from the selection list which contains thread ranges from M1.6 to M56 of ISO general purpose metric screw threads having basic profile according to ISO 68-1.  

Metric thread selection list consists of coarse and fine series screw threads so calculations of both series are available.

More information on basic profile and limits of sizes of the ISO metric thread are available in ISO screw thread calculator page.

Metric Tap Drill Size Calculator:

Metric Thread


Parameter Value
Designation --- ---
Nominal Diameter --- mm
Pitch ---
Screw Thread Series --- ---
Drill Diameter --- mm


Drill bit: A cutting tool used in drilling machines to drill a hole.

Tap: A mechanical tool used to make a standard thread through a hole. Different type of taps exist such as straight fluted taps, spiral pointed taps, spiral pointed only taps, spiral fluted taps, fast spiral fluted taps, thread forming taps, pulley taps, nut taps, and pipe taps.

Tap drill size: The required diameter of hole before the tapping operation of the hole.

Threads forming: A process to form threads by displacing the material to form the thread shape. No chips are produced during thread forming.

General Information

The diameter of a hole produced by a drill depends to some extent upon the degree of accuracy to which the drill point is ground, the material being drilled, the lubricant used, the alignment, feed and speed of the operation.

When tapping relatively soft material, there is a tendency for the material to be squeezed down towards the root and in such cases the minor diameter of the tapped hole may become smaller than the diameter of the drill used. The tendency is very much less with harder materials and in some cases is entirely absent.

The larger the drilled hole, within the relevant minor diameter tolerance, the more economical tapping becomes and the risk of tap breakage is reduced.

With the foregoing points in mind, the tables of ISO 2306 ( also results of the calculator) have been prepared as a guide to drilling prior to conventional tapping. However, it is realized that users may find it beneficial to choose their’ own drill diameters for certain applications. Even in these instances, stocked diameter drills should be used whenever possible.


Link Usage
Metric tap drill chart Tabulated form of the metric tap drill size calculator.


  • ISO 68-1:1998(E) - ISO general purpose screw threads — Basic profile — Part 1: Metric screw threads
  • ISO 2306:1972, Drills for use prior to tapping screw threads