I lost 10 more pounds since I last calculated my weight belt, so I had to re-do my calculation. I will post it here, and maybe we can save it in a common place, since this seems like a key question related to weight loss and scuba.
Short formula: take your body weight change (in any unit like lbs, kg, stone), divide it by 9, and that is the change you need to make to your weight belt (in the same units). Examples: I lost 27 lbs, therefore I must remove 3 lbs off my weight belt. I gained 4.5 kg, so I need to add 0.5 kg to my belt.
The Specific weight of body fat is about 900 kg/m3 (http://www.springerlink.com/content/78dlx1566ug9eyjt/
). That of water is about 1000 kg/m3 (1020 kg/m3 for saltwater, but the difference is not significant enough for this rough calculation). So you take the amount of weight you lost, divide by the specific weight of fat (900 kg/m3), and that will give you the volume of mass you lost. Since that volume will no longer displace water, eureka, you can then remove the difference between the amount of weight that that water weights and the fat weights, which is 100 kg/m3, and multiply it by the volume. Since the units kg/m3 cancel each other out, the ratio 1/9 (0.111111...) can be used with any units.
| current weight: 88.0