These days it's a dumb idea not to use a ballistic calculator.
KAC Bullet Flight is a fine calculator, and it served me well. It is very fast to set up and convenient (for me) to use. One particular feature I like is its ability to receive input in metric or imperial, allowing you to freely mix and match without having to configure anything or change any setting. You want to enter temperature in F and pressure in hPa? Fine. Distance in meters, or yards? Fine. Etc.
Having said that, Bullet Flight is not cheap. Ballistic AE is not as fast to set up, and not as flexible - but it is choke-full of useful features. I use it often, and am tempted to say "it's a-must to have". I run it on iPhone, iPod and iPad (note that there's a special Ballistic AE for iPad version, but you'd pay another $20 for it, so I chose to just run the iPhone version on iPad too).
Shooter is also nice, and runs on iPad as well.
Finally, iStrelok is free, runs on iPhone and iPad, and should address your needs in the beginning...