Tap Drill Calculator
• Only enter numbers in the fields.
• Do not enter any units of measurement.
• You can enter fractions. (i.e. 1/4)
• If you don't know the %, just enter 70
Example: 1/2 - 20 Thread
• 1/2 = Major diameter
• 20 = Threads / Inch (TPI)
Percentage of Thread and Tap Drill Selection:
Many variables affect the performance of threaded holes and the taps used to
generate them. One very important factor is the drill size used to produce the
hole that will be tapped. Most tap drill charts call out only one tap drill
size, and that will produce an approximate 75 percent thread. In general, tap
tool life can be increased significantly by using a lower percent of thread and
we suggest using values between 60% and 70% for most applications. Thread
strength is not directly proportional to percent of thread. For example a 100%
thread specification is only 5% stronger than a 75% thread specification but
requires 3 times the torque to produce.
Metric threads of the International Standards Organization (ISO) have the same
60-degree thread form as UN threads, though the diameters and pitches are not
interchangeable. Metric tap drill charts have the same drill size problem in
that they use the same formula as that employed for the obsolete AS threads. In
both ISO and UN thread series, having a range of useful drill sizes allows
designers and machinists to tailor effective thread depth for long holes,
various materials and thread strength requirements. It also allows for the
combining of inch and metric taps and drills where convenient to do so.