There's two parts to calculating tension; string gage and pitch. Here's a classic example that I use all the time - octave pairs. On my Eastman 'cello I use octave pairs of strings on the C, G and D courses. So one of my D strings is .034. Its partner is .017 and is tuned one octave higher. Guess what the tension in each string is the same. You can't compare just string gages and know anything about the comparable tension.
http://stringtensionpro.com
Graham MacDonald
There are many other free string tension calculators out there.