EngineerBenchChoosing a tap drill for a cut thread
A common starting estimate for a metric cutting tap is major diameter minus pitch. An M6 × 1 thread therefore gives a 5 mm starting drill size. This is a sizing approximation for cutting a thread, not a universal drill recommendation for every material, tap design, or required thread engagement.
Read pitch and diameter separately
Metric pitch is the distance between adjacent threads in millimeters. Threads per inch express the reciprocal spacing in inch threads. Select the exact thread or enter its dimensions; a similar-looking nominal diameter does not establish that two threads are compatible.
Forming taps need different holes
A forming tap displaces material rather than cutting it and normally needs a different predrilled hole. Use its manufacturer's drill recommendation. Also check the required thread depth, blind-hole clearance, and drill's actual hole size before choosing a production drill.
Interpreting the calculated starting hole
For a metric thread, subtracting pitch from nominal diameter approximates a cutting-tap drill size. With an M8 × 1.25 designation, the arithmetic gives 6.75 mm. That result should be compared with the tap supplier's recommended hole and available drills; the equation itself does not decide which nearby catalog size is appropriate. The actual drilled diameter may also differ from the drill's marked size.
A smaller hole generally leaves more material for the tap to cut, but more thread material is not automatically a better process. Tapping torque, chip evacuation, material behavior, and tool breakage risk also matter. For a blind hole, distinguish full thread depth, tap travel, drill point depth, and clearance beneath the tap. The nominal threaded length on a drawing does not identify all of those distances. Check the thread's pitch and form before choosing tooling, and use the correct process for the material. This worksheet is a starting-size calculation, not a complete tapping cycle or a percentage-engagement specification.
Formula
For a conventional cutting tap, drill diameter ≈ major diameter − pitch.