add 2 lbs for saltwater
There is a very simple formula for calculating the amount of weight needed. I've been using it for years personally and helping newbie divers as a Divemaster:
5% of your body weight
+
2 lbs for saltwater
+
4 lbs for an aluminum tank (omit if using a steel tank)
+
1 lb/ mm thickness of wetsuit (1/2 lb/mm for shortie)
then round up to the nearest even lb.
For instance: I weight 195 lbs,
195 x .05 (5%) = 9.75 lbs
+ 2 lbs for saltwater
+ 0 ( I dive steel tanks)
+ 3 lbs ( 3mm wetsuit)
= 14.75 lbs rounded up to an even number = 16 lbs
This formula works very well for about 85% of people, based on age, body density and type (amount of muscle and body fat). The only real kink in this formula is if you're wearing a really big 7mm wetsuit or semidry suit, then add about 2 lbs/mm.