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Unified Tap Drill Size Calculator
ANSI/ASME Unified inch threads (UNC/UNF) • Drill sizes in inch
Use this calculator to pick a recommended tap drill size for Unified inch threads (UNC/UNF).
Pick a thread size to get a recommended drill instantly, or use the chart for quick reference.
Instant Tap Drill Calculator
Instant lookup for common Unified threads (UNC/UNF). Use the chart below for quick scanning and export.
Unified Tap Drill Chart (Reference)
Filter the chart to quickly find a recommended drill for a Unified thread. Values come from the same reference data used by the instant lookup.
| Thread |
Series |
Recommended drill |
Decimal (in) |
Major dia (in) |
| #1-64 | UNC | 53 | 0.0595 | 0.073 |
| #2-56 | UNC | 50 | 0.07 | 0.086 |
| #3-48 | UNC | 47 | 0.0785 | 0.099 |
| #4-40 | UNC | 43 | 0.089 | 0.112 |
| #5-40 | UNC | 38 | 0.1015 | 0.125 |
| #6-32 | UNC | 36 | 0.1065 | 0.138 |
| #8-32 | UNC | 29 | 0.136 | 0.164 |
| #10-24 | UNC | 25 | 0.1495 | 0.19 |
| #12-24 | UNC | 16 | 0.177 | 0.216 |
| 1/4-20 | UNC | 7 | 0.201 | 0.25 |
| 5/16-18 | UNC | F | 0.257 | 0.3125 |
| 3/8-16 | UNC | 5/16 | 0.3125 | 0.375 |
| 7/16-14 | UNC | U | 0.368 | 0.4375 |
| 1/2-13 | UNC | 27/64 | 0.4219 | 0.5 |
| 9/16-12 | UNC | 31/64 | 0.4844 | 0.5625 |
| 5/8-11 | UNC | 17/32 | 0.5312 | 0.625 |
| 3/4-10 | UNC | 21/32 | 0.6562 | 0.75 |
| 7/8-9 | UNC | 49/64 | 0.7656 | 0.875 |
| 1-8 | UNC | 7/8 | 0.875 | 1 |
| 1-1/8-7 | UNC | 63/64 | 0.9844 | 1.125 |
| 1-1/4-7 | UNC | 1-7/64 | 1.1094 | 1.25 |
| 1-3/8-6 | UNC | 1-7/32 | 1.2187 | 1.375 |
| 1-1/2-6 | UNC | 1-11/32 | 1.3437 | 1.5 |
| 1-3/4-5 | UNC | 1-9/16 | 1.5625 | 1.75 |
| #12-32 | UNEF | 13 | 0.185 | 0.216 |
| 1/4-32 | UNEF | 7/32 | 0.2188 | 0.25 |
| 5/16-32 | UNEF | 9/32 | 0.2812 | 0.3125 |
| 3/8-32 | UNEF | 11/32 | 0.3438 | 0.375 |
| 7/16-28 | UNEF | Y | 0.404 | 0.4375 |
| 1/2-28 | UNEF | 15/32 | 0.4688 | 0.5 |
| 9/16-24 | UNEF | 33/64 | 0.5156 | 0.5625 |
| 5/8-24 | UNEF | 37/64 | 0.5781 | 0.625 |
| 11/16-24 | UNEF | 41/64 | 0.6406 | 0.6875 |
| 3/4-20 | UNEF | 45/64 | 0.7031 | 0.75 |
| 13/16-20 | UNEF | 49/64 | 0.7656 | 0.8125 |
| 7/8-20 | UNEF | 53/64 | 0.8281 | 0.875 |
| 15/16-20 | UNEF | 57/64 | 0.8906 | 0.9375 |
| 1-20 | UNEF | 61/64 | 0.9531 | 1 |
| 1-1/16-18 | UNEF | 1.000 | 1 | 1.0625 |
| 1-1/8-18 | UNEF | 1-1/16 | 1.0625 | 1.125 |
| 1-3/16-18 | UNEF | 1-1/8 | 1.125 | 1.1875 |
| 1-1/4-18 | UNEF | 1-3/16 | 1.1875 | 1.25 |
| 1-5/16-18 | UNEF | 1-1/4 | 1.25 | 1.3125 |
| 1-3/8-18 | UNEF | 1-5/16 | 1.3125 | 1.375 |
| 1-7/16-18 | UNEF | 1-3/8 | 1.375 | 1.4375 |
| 1-1/2-18 | UNEF | 1-7/16 | 1.4375 | 1.5 |
| 1-9/16-18 | UNEF | 1-12 | 1.5 | 1.5625 |
| 1-5/8-18 | UNEF | 1-9/16 | 1.5625 | 1.625 |
| 1-11/16-18 | UNEF | 1-5/8 | 1.625 | 1.6875 |
| #0-80 | UNF | 3/64 | 0.0469 | 0.06 |
| #1-72 | UNF | 53 | 0.0595 | 0.073 |
| #2-64 | UNF | 50 | 0.07 | 0.086 |
| #3-56 | UNF | 45 | 0.082 | 0.099 |
| #4-48 | UNF | 42 | 0.0935 | 0.112 |
| #5-44 | UNF | 37 | 0.104 | 0.125 |
| #6-40 | UNF | 33 | 0.113 | 0.138 |
| #8-36 | UNF | 29 | 0.136 | 0.164 |
| #10-32 | UNF | 21 | 0.159 | 0.19 |
| #12-28 | UNF | 14 | 0.182 | 0.216 |
| 1/4-28 | UNF | 3 | 0.213 | 0.25 |
| 5/16-24 | UNF | I | 0.272 | 0.3125 |
| 3/8-24 | UNF | Q | 0.332 | 0.375 |
| 7/16-20 | UNF | 25/64 | 0.3906 | 0.4375 |
| 1/2-20 | UNF | 29/64 | 0.4531 | 0.5 |
| 9/16-18 | UNF | 33/64 | 0.5156 | 0.5625 |
| 5/8-18 | UNF | 37/64 | 0.5781 | 0.625 |
| 3/4-16 | UNF | 11/16 | 0.6875 | 0.75 |
| 7/8-14 | UNF | 13/16 | 0.8125 | 0.875 |
| 1-12 | UNF | 15/16 | 0.9375 | 1 |
| 1-1/8-12 | UNF | 1-3/64 | 1.0469 | 1.125 |
| 1-1/4-12 | UNF | 1-11/64 | 1.1719 | 1.25 |
| 1-3/8-12 | UNF | 1-19/64 | 1.2969 | 1.375 |
| 1-1/2-12 | UNF | 1-27/64 | 1.4219 | 1.5 |
Not metric. For ISO metric threads (M6, M8, M10…), use the metric tap drill chart & calculator.
About “percent thread” (why this calculator uses minor diameter limits)
Many older rules of thumb use “percent of thread engagement”. Modern guidance prefers selecting a drill that produces a hole
between the minimum and maximum internal thread minor diameters from the standard. This calculator follows that approach.
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.
Definitions:
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.
Supplements:
Reference:
-
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
FAQ
What drill size do I use for a 10-32 tap?
Select 10-32 (or use the quick pick) and click Calculate. The result shows the recommended drill bit and expected drilled hole size.
What drill size do I use for a 1/4-20 tap?
Select 1/4-20 and calculate. If you need a different process (form tapping), use the roll form tap calculator.
Which class of fit should I choose (1B/2B/3B)?
For general use, 1B/2B provides more production tolerance. 3B is tighter and is often used when higher and more consistent locking torque is required (e.g., locking inserts).