Great day in the mornin'!
From the replies you can readily see that the "answer" varies depending on who you talk to and how you define "weight" and "mass" and all that - when what we *really* need to know in Scuba is how much lead we need to carry to sink something, or how much lead we can take off if we carry something that sinks.
For that, you need some lead weights of varying value, some line, some water (swimming pool will do nicely), and all the equipment you want to "weigh". I use a mesh bag tied about a foot and a half from the end of a line, with four four ounce fishing weights tied along the line from the end to the mesh bag. For a floating object like a wet suit, I tie the wetsuit to the line and add weights to the mesh bag until the whole thing just sinks, and see how many of the fishing weights end up on the bottom, which gives me the lead needed to sink that particular piece of gear to the nearest quarter pound.
For things that sink, I tie the object to a line attached to one end of a four foot bar suspended in the center from the diving board, and tie the weight line to the other end of the bar, adding weight until I get a balance, which tells me how much weight I can remove for that object.
I keep all these numbers in my log book.
Then, for any set of gear, I just add and subtract according to what I'm wearing and carrying to come up with the precise weighting I need for that set of gear - no guessing, it's always exactly right (for fresh water). For salt water, add 2 pounds for each 100 pounds total weight of you and your system, (for me that's typically 5 pounds), and once again you'll be perfectly weighted every time.
The only thing you can't use my system for is a drysuit, which you'll actually have to put on and just see how much you need to sink it and you.
Rick