dcfreediver:
It depends on what you're doing. If you just like to dive around in a relatively shallow depth, I'd say 4-5 kg should be good. If you intend to do constant ballast dives, increasing depth, you'll want to "under weight" yourself slightly for easier ascent. For starters it's a good rule of thumb that you should be neutral at about 10 meters. As your depths increase, you might want to go to 15 m for neutral. I currently have 2 kg with a 5mm suit, and started with 5 kg. You'll need less as your "jackknife" and early descent gets better with practise. Just make sure that as a total you are positively buoyant. Just in case the worst would happen and you black out, you will at least float to the surface and keep on floating even if you exhale or your lungs are full of water. Since the risk of SWB is the greatest in the last 15 m of the dive, at least for deep dives I weigh myself so that I get the "free ride up" from about 15-10 m.
It takes a bit of experimenting and every individual is slightly different (body fat etc. being factors). I'd suggest you get about 6 kg of weights at different sizes. 2-4 about 1 kg pieces and 2-4 at 0.5 kg. This will give you freedom to adjust your weight at a fairly fine level.
For starters go into water wearing your suit and weights. Strap on, I'd say about 5 kg and do a full exhale. You should not start to sink, but the water level raises at about your eye level or forehead. The right amount is not far from there, but you'll have to try it out by diving. Dive to your intended target depth where you want to be neutral (if you do mainly recreational diving, this would be the depth at which you will spent your "bottom time", you'll want to be neutral to make your dive comfortable). If you start to ascend when you stop moving, add a little (0.5 - 1 kg) and if you descent take some off. Avoid buying big lumps of weight. They are cumbersome to wear and not very adjustable.
As I said, I use 2 kg for constant weight, but I carry around a 2 kg neck ballast just in case I feel like doing some "snorkeling" or having fun at shallow depth. I just throw it on in about 10 seconds and bring the "neutral zone" up to ~8 m.
Hope this helps...
I knew a guy once who was negatively bouyant with a 6mm suit and no weight. So its a matter of trying it out for your self really.
