Lead (kg) = 0.02 x body weight + suit + tank + build, then in freshwater subtract 2.5 percent of body weight plus 8 kg of gear, rounded to 0.5 kg. Suit terms: none 0, shorty 1, 3 mm 2, 5 mm 4, 7 mm 6, semi-dry 7, drysuit 8 kg. Tank terms: aluminum 80 +1, steel HP100 -1.5, HP120 -2.5 kg. Build: lean -1, average 0, more body fat +1 kg. These are rule-of-thumb values calibrated to common instructor guidance, not a lab model. Treat the result as within about 2 kg.