Roll Form Tap Drill Chart Calculator for calculation of recommended roll form tap drill sizes for Unified threads. Calculations are done according to roll form tap drill charts which are given in the Machinery's Handbook. Desired thread size and % of full thread are input parameters for the calculator.

Roll tapping is a process which forms a thread in the hole by displacement of the material into the desired size and pitch. Roll tapping eliminates the problem of chip disposal. The threads produced with roll tapping are stronger than cut threads because the grains are continuous and unbroken, and the displaced metal is work hardened. Roll tapping is a method to be applied to relatively ductile metals such as low-carbon steel, leaded steels, austenitic stainless steels, wrought aluminum, magnesium, copper, and ductile copper alloys.

For roll tapping, cutting tap drill sizes should not be used. Roll tapping displaces metal from the hole walls to form the threads so cutting tap drill leaves to much material in the hole.

Nominal Size in Inches
and Threads per Inch
% of Full Thread 1
Include Drill Oversize 2

Note1: Default value is 65 % .

Note2: The diameter of the drilled holes will be oversize in most materials. If the checkbox is ticked, the effect of drill oversize will be taken into account in tap drill bit selection Amount of expected drill oversize is calculated according to table given in page 897 of Machinery's Handbook, 29th.


Parameter Value
Nominal Size --- ---
Decimal Equivalent --- inch
Threads per inch --- ---
Series Designation ---
Recommended Hole Size
 before Cold Forming Tap
--- inch
Recommended Inch Drill Size 1 --- ---
Drill Size Decimal Equivalent --- inch
Expected Hole Size (Including oversize)2 ---
Expected Amount of Drill Oversize 3 ---

Note1: If the "Include Oversize" checkbox is ticked in Input Parameters table, the effect of drill oversize will be taken into account in tap drill bit selection.

Note2: Equals drill size decimal equivalent plus expected amount of drill oversize.

Note3: Value is obtained by linear interpolation/extrapolation of mean values given in page 897 of Machinery's Handbook.


Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

UNEF (Unified National Extra Fine): Used when finer pitches than UNF are needed (Ex: Short engagement length).

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.

Threads per inch: Number of full thread per an inch length.


Theoretical hole size = basic tap O.D. - 0.0068 × per cent of full thread / threads per inch



  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.
  • FED-STD-H28/2B-1991, Screw-Thread Standards for Federal Services - Section 2: Unified Inch Screw Threads - UN and UNR Thread Forms