Tap drill size calculator was developed to calculate appropriate tap drill sizes and recommended cutting tap hole size limits for inch threads. Calculations are done according to the Machinery's Handbook and FED-STD-H28/2B-1991.

Desired thread size, internal thread class and length of engagement parameters shall be specified in "Input Parameters" table of tap drill size calculator.

Desired unified thread size can be selected from a list of thread sizes which are in the diameter range from 0.060 inch to 2 inch. For Internal thread class, selection can be done among the alternatives 1B & 2B and 3B. For general use, select 1B & 2B thread class. For length of engagement, selection can be done among the alternatives L ≤ 0.33D, 0.33D ≤ L ≤ 0.67D, 0.67D ≤ L ≤ 1.5D, 1.5D ≤ L ≤ 3.0D.

Following paragraph from ASME B1.1-2003 has been taken into consideration while developing this tap drill size calculator.

All references to percent of thread engagement have been eliminated from ASME B1.1-2003. Past changes in the thread form designation of the “basic” thread height from 0.750H to 0.625H confused the calculation of percent of thread engagement. This calculation has been used in the past for threaded products users to determine tap drill size selection. It is now recommended that users select a drill bit size that will result in a hole size that lies between the maximum and minimum size of the internal thread’s minor diameter shown in the tables included in ASME B1.1-2003.


Parameter Value
Nominal Size in Inches
and Threads per Inch
Internal Thread Class
Length of Engagement 1
Include Drill Oversize 2

Note1: D is nominal size of thread and L is length of engagement.
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.


Value Parameter Unit
Nominal Size --- ---
Decimal Equivalent --- inch
Threads per inch --- ---
Series Designation ---
Recommended Hole Size Limits
 before Tapping
Min Max 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.


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

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.

UN form screw threads: The UN thread is intended for general purpose fastening operations.

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.

Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.

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.

General Information

The hole diameter required before tapping depends on the length of engagement. For short length of engagement, the hole diameter shall be minimized to have large thread height and thread strength. For long length of engagement, the hole diameter can be increased to have less risk of tap breakage. According to this logic, the following recommendations were given in FED-STD-H28/2B-1991:

Length of Engagement Minimum Hole Size Maximum Hole Size
Up to and including 0.33D Minimum minor diameter Minimum minor diameter plus ½ minor diameter tolerance
Above 0.33D thru 0.67D Minimum minor diameter plus ¼ minor diameter tolerance Minimum minor diameter plus ¾ minor diameter tolerance
Above 0.67D thru 1.5D Minimum minor diameter plus ½ minor diameter tolerance Maximum minor diameter (minimum minor diameter plus tolerance)
Above 1.5D thru 3.0D Minimum minor diameter plus ¾ minor diameter tolerance Maximum minor diameter plus ¼ minor diameter tolerance

For more detailed information on the calculation method and exceptions, FED-STD-H28/2B-1991 standard shall be visited.

Selection of twist drill and reamer can be done according to hole limits calculated for desired thread size. It should be noted that twist drills are generally drill slightly larger hole with high a variation of hole size. If precise hole diameter is required, reaming operation is advised.


Link Usage
Tooling Calculators Cover dimensions of twist drills which are given in the ANSI B94.11M-1993 - "Twist Drills" standard.
Forming Tap Drill Sizes for Unified Threads Calculates recommended hole size and appropriate tap drill size for cold form tapping of Unified threads.
Unified Inch Screw Thread Sizes & Tolerances Calculator Unified inch screw threads and tolerances calculator was developed to calculate basic major, minor and pitch diameters of the external (bolt) and internal (nut) inch thread according to ASME B1.1-2003 standard. In addition to basic size calculations, limits of sizes calculations of different tolerance classes can be done according to same standard.


  • 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. , pp 2015- 2037
  • FED-STD-H28/2B-1991, Screw-Thread Standards for Federal Services - Section 2: Unified Inch Screw Threads - UN and UNR Thread Forms