Work out the key depths for tapping a thread: the height of one thread, how much engagement length you need, and how deep to drill a blind hole so the tap reaches full depth with room to spare.
Engagement of about 1.5x diameter suits steel; use 2x or more for aluminum and soft materials. Blind depth adds room for the tap lead-in and chips.
How it works
Pitch is 1 / TPI. The basic 60-degree thread height is 0.6134 × pitch. Engagement length is the thread depth you want, set as a multiple of diameter. For a blind hole the drill must go deeper than the threads — this adds about four pitches for the tap chamfer and chip clearance.
Tips
More engagement does not always mean more strength: past about 1.5x diameter in steel, the fastener fails before the threads strip. Softer materials need more engagement because their threads are weaker.
FAQ
How deep should I tap a blind hole? Deep enough for full-depth threads over the engagement length, plus a few pitches for the tap chamfer and chips — which is what the blind-hole result gives you.
See the Tap Drill Chart and Thread Dimensions Chart.