Chip, The proper tap drill size often falls in between the standard "fractional" drill sizes, so to cover these we have the "letter" drill index for the larger sizes and the "number" drill indexs for the smaller ones. Generally the "number" drills are split into two different indexes, 1 thru 60 is the larges and the smallest 61 thru 80.

Here are just a few charts you might want to print out to keep around the bench.
http://www.carbidedepot.com/formulas-drillsize.htm
http://www.gearhob.com/eng/design/drill_eng.htm
http://sosnick.uchicago.edu/tap_drill_size.html
http://www.engineersedge.com/drill_sizes.htm
http://www.engineersedge.com/tap_drill_chart.htm
http://icrank.com/cgi-bin/pageman/pageout.cgi?path=/tapsizechart.htm&t=2
http://icrank.com/cgi-bin/pageman/pageout.cgi?path=/drillsizechart.htm&t=2

Denny Graham
Sandwich, IL

Denny G
Sandwich, IL