Basic Weighting Guidelines

Basic weight guess for average build individual in salt water — decrease weight somewhat for lean individuals and increase for heavier individuals.

EXPOSURE SUIT TYPE START WITH
Swim suit/dive skin 1–4 lbs
3mm 1piece full wet suit 5% of body weight
5mm two piece wet suit 10% of body weight
7mm wetsuit w/ hood & gloves 10% of body weight, plus 3–5 lbs
Neoprene dry suit 10% of body weight, plus 7–10 lbs
Shell dry suit, light undergarment 10% of body weight, plus 3–5 lbs
Shell dry suit, heavy under garment  10% of body weight, plus 7–14 lbs
THIS IS WHERE YOU START!
If you are diving in the tropics in a full 3mm suit and you are average build and weigh 150 lbs, you start out with 7.5 lbs of lead.  You then add or subtract as you feel your buoyancy.  (At depth of around 20 ft or so, feel your BC and see how much air you are adding to it to be neutral.)
FRESH / SALT WATER CHANGE
(This is what you use if you already have been diving and have established your buoyancy in either fresh or salt water.  You add or subtract these amounts from what you are currently diving.  In other words, if I have been diving in Waldo Lake with 22 lbs, and I go to Hood Canal, I need to add about 6 lbs of lead to compensate for the salt water or vice versa.)
BODY WEIGHT Add (going to salt)
Subtract (going to fresh)
100- 125 4lbs
126- 155 5lbs
156- 186 6lbs
187- 217 7lbs

(Adventures in Diving Manual, PADI, 2001, p.213)