Did you just by an individual tap? The container/sticker should state the drill bit size needed. Or, from
http://www.korit.com/tapndrill.htm:
National Pipe Tapered Tap Chart
(Recommended tap drill to use for 75% depth of thread)
Code:
To Tap This Size Pipe: Use This Drill Bit: (Closest Fractional): Decimal Inches:
1/8-27 NPT Ltr.R - .3390
1/4-18 NPT 7/16" 7/16" .4375
3/8-18 NPT 37/64" 37/64" .5781
1/2-14 NPT 45/64" 45/64" .7031
3/4-14 NPT 59/64" 59/64" .9219
1-11 1/2 NPT 1 5/32" 1 5/32" 1.1562
1 1/4-11 1/2 NPT 1 1/2" 1 1/2" 1.5000
1 1/2-11 1/2 NPT 1 47/64" 1 47/64" 1.7344
2-11 1/2 NPT 2 7/32" 2 7/32" 2.2188