How do I know what size hole to drill?

I downloaded a free app on my phone that has tap drills plus decimal sizes for all drills.
 
I use one called drill tap tool. Tons of thread lookups. For Iphone. Don't know if it does android.
 
If you are going to need tap drill sizes often I would look online and buy a wall chart that gives you imperial and metric sizes and the sizes you need for drilling the holes for all the common tap sizes. Some machine tool catalogs also have this info in them. Or you can google tap drill sizes.
 
I use an app called iEngineer on my iPhone. I don't know if it's available for Android though.
 
All my taps, (mostly Craftsman) have drill size written on the shank. Also use Machinery's Handbook if they are unreadable.
 
Like T.J. I use iEngineer on my iPhone and iPod and iPad. When in my shop, however, I just look at the walls that are plastered with
large print charts for conversions, tap/drill sizes etc. On thing that I like about iEngineer is that it also easily shows things like major/minor diameters, drill sizes for different fits (tight-normal etc), strengths of screws in multiple materials and grades (proof load, advised torque, lubricated torques, tensile strengths, clamping force, allowable shear forces through threads or shoulders, head types and sizes) etc. etc. etc.
All this is readily available without having to refer to multiple pages and charts in books ( I have nothing against books, I love them ) etc.
Its one of the few phone apps that I find are easier to use than the charts, tables and books.

CHuck the grumpy old guy
 
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