Easy Tap Drill Calculation

Someone asked me what the tap drill size for an M6 x 1 is.  “Easy 5mm…don’t you know the trick?”, I said.  For a 75% engaged thread (most common) just subtract the pitch from the diameter.  They had never heard of the trick.  I asked around and nobody had heard of it.  “Too bad it doesn’t work for inch sized taps” they said.

Actually it does, but you might need a calculator and you need to know your  sizes.  For #4-40, you need to know what the actual diameter of a #4 is (about 0.112″) then what is 40 TPI (0.025″).  That equals about .087″.  So that means about a #43 drill.  Isn’t the inch system fun!

But a 3/8-16 is pretty easy. 3/8 – 1/16 = 5/16 drill.

Tap and Drill

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