How much weight do I need for scuba diving?
There is no table that answers this, and anyone who gives you a figure without asking what you are wearing is guessing. What works is a method: find the number for one set of kit in one kind of water, write it down, and start from it next time.
Why no table can do it
Lead is there to cancel buoyancy, and a diver in kit gets buoyancy from several places that all change. The exposure suit is the biggest. A 7 mm wetsuit can want several kilos more than a 3 mm one, a drysuit depends on the undergarment and on how much gas you keep inside it, and neoprene loses much of its lift as it is squeezed at depth and as it gets old.
Salt water is about 2.5 percent denser than fresh. For a diver and kit weighing around 100 kg that is roughly 2.5 kg of extra lift, which is why the first dive in the sea after a season in a quarry feels strange. The cylinder matters twice: an empty aluminium 80 floats by a kilo or two while most steels stay heavy, and the air in a full one weighs a little under 3 kg, about 2 of which you breathe before the stop. You finish the dive that much lighter than you started.
Then there is your body, which changes with the season, and whatever you clip on: a camera, a light, a stage. You will have seen “ten percent of your body weight”. As a first guess on a first dive with an instructor beside you, fine. As an answer, no.
The weight check
The check I trust is done at the end of the dive, at the safety stop, when most of the gas is gone. Stop at 3 to 5 metres, depending on who taught you, with about 50 bar left (500 psi if you were taught in the US) and nothing in your BCD. Breathe normally. If you can hover there with an empty BCD, rising a little on a full breath in and sinking a little on a full breath out, you carry about what you need. If you sink even on a held normal breath, you carry too much. If you have to kick or hold on to something to stay down, you carry too little.
Courses teach it differently. Some start with a float test at the surface and add something for the gas you will breathe; others use the end-of-dive check. I prefer the second because it tests the kit as it is when it matters.
What a wrong number costs
Too much lead is the common mistake, and the easy one to excuse. To float, you put gas into the BCD or the suit, so at depth you carry a bubble you would not otherwise need, and it expands as you rise and has to be let out. Your hips sit low, your trim is poor and you use more gas. If the BCD ever fails, you have more to lift.
Too little is rarer, and it bites at the worst moment, the end of the dive. With a nearly empty cylinder you cannot hold the stop, and a diver who is fighting to stay down is the one who reaches the surface faster than planned. A kilo too much is a nuisance. A kilo too little can be an ascent problem.
Adjusting, and writing it down
Change one thing at a time, in small steps, half a kilo to a kilo, and check again at the same point on the next dive. Write it down the same evening: the date, salt or fresh, the suit and its thickness, the cylinder and what it is made of, how much lead and where it sits, the gas you had left at the check, and what it felt like. After a handful of entries for one kit you have stopped guessing. When the suit, the cylinder or the water changes, start from the nearest entry and check again. Where the lead sits counts too: the same weight on a belt or in trim pockets changes how you lie in the water, and what you can drop in an emergency.
I got tired of guessing between my 5 mm and my 7 mm, and wrote Weights Check to keep the record for me. You build a rig from your own gear once and record a check against it, made at 3 m with 50 bar in every bottle, and the lead you finished with. Until you have recorded one, it shows an estimate as a range and says that it is an estimate. After that it shows your own numbers. It does not decide your weight. The check in the water does, and the app remembers it.