Calculate Tap Drill Sizes

Use these easy formulas to calculate tap drill sizes. I have listed formulas for both English and Metric.

I will start with metric first because it’s the easiest.

All you have to do with a Metric thread is subtract the thread pitch from the tap diameter.

It’s just that easy!

Example: You want to tap an M8 X 1.5 

Subtract the 1.5 from 8 and you get 6.5

Now go get a 6.5 drill and drill baby drill.

Now let’s get on to the Standard thread. It’s a little more to the standard but its steel easy.

First thing you need to know is the magic number .9742

Remember this number “right it down”.

Now divide the magic number by the thread pitch. Next subtract your answer from the tap diameter this will give you the drill size.

I love magic numbers!

Example: You want to tap a ¼-20 hole.

Divide magic number .9742 by the pitch 20 and you get .0487

Now subtract .0487 from the diameter of the tap .250

You get .2013 this is your drill size.

I hope this was helpful

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See ya and happy tapping