How to Calculate a Tap Drill


A tap is a cylindrical metal tube with thread on one end that is used to create female threads in a pre-drilled hole. Prior to using the tap, the hole has to be drilled -- either in plastic or metal -- to the desired specifications of the thread. The width of the hole has to be taken into account, as well as the length. It is also important that the hole be drilled at 90 degrees (perpendicular) to the surface that will be drilled, so that the tap can be installed properly.

Things You'll Need

  • Tape measure
  • Drill bit
  • Felt tip pen
  • Electrical tape
  • Estimate the required thickness of the hole that needs to be threaded. This can be done by measuring the diameter of the bolt/implement thread that will inserted into the threaded hole.

  • Choose a drill bit with same diameter thickness, as the required hole. For example, if the thread needs to be 1/2-inch in diameter, use a 1/2-inch metal drill bit to create the hole.

  • Choose the required depth of the hole -- again, measure the length of the bolt/implement that will be inserted into the hole. Add 1/4-inch to this length, as some taps taper off at the end, and the bolt/implement may not be able to reach the very end of the drilled hole.

  • Measure that distance from the tip of the drill bit to the side of the bit, and mark with a felt tip pen. Then wrap electrical adhesive tape around the bit, so that the distance from the edge of the tape to the drill tip equals the required length of the hole. This is done so that when the bit drills out the hole, it will be drilled to the edge of the adhesive tape, creating the correct depth of hole.

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  • Photo Credit drill bit image by Greg Pickens from
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