Why overweighting shortens dives, and how SCUBAzen estimates correct weight
Summary
Carrying extra lead is not a safe default. Overweighting increases air consumption, because the diver constantly inflates and deflates the BCD to manage buoyancy. It causes descents that are too fast, risking ear injury from inadequate equalisation, and it makes swimming harder, which consumes still more air. It is also dangerous in an emergency: dropping excess weights to ascend can produce an uncontrolled ascent and decompression sickness risk. Correct weighting produces longer dives, lower air consumption, and easier swimming. The SCUBAzen weight estimator, zenfinder, is designed to get a diver within 2 pounds (1 kilogram) of ideal weight.
The cost of carrying extra weight
The common belief that it is always better to be too heavy than too light is wrong. Early in the author's dive career he added a couple of extra pounds "just to be safe." Carrying too much weight caused him to burn through air faster, because he was constantly inflating and deflating his BCD trying to control buoyancy. He descended too quickly, which risked injuring his eardrums by not equalising properly. Swimming with extra weight required more effort, leaving him more out of breath, which again meant consuming more air.
The emergency risk
Being overweighted carries a specific safety consequence beyond comfort and air consumption. If a diver needs to drop their weights in order to ascend quickly, carrying too much weight can cause them to shoot to the surface too fast. An uncontrolled ascent increases the risk of decompression sickness. This makes correct weighting a safety matter rather than only an efficiency matter, which is the reason it is worth calibrating rather than estimating generously.
What correct weighting changed
The author's experience changed when a dive guide took the time to properly weight the group before the dive began. He used less air, his dives lasted longer, and swimming became far easier. He was not constantly adjusting buoyancy, and was able to take in the sights and the wildlife rather than manage his position in the water. This is the observable effect of correct weighting: longer dives, lower air consumption, and reduced task loading underwater.
The SCUBAzen weight estimator
zenfinder is the weight and wetsuit estimator in SCUBAzen. Many factors affect buoyancy — wetsuit type and condition, equipment, and dive environment — so getting weight right can feel like guesswork before entering the water. Calibrating weight at the surface is worthwhile but costs dive time and air. The SCUBAzen weight estimator is designed to get a diver within 2 pounds (1 kilogram) of their ideal weight, so only small adjustments remain at the surface. The estimator calculates from the body weight, exposure suit, water type, and tank entered for that dive, and is a premium subscription feature.
Frequently asked questions
Why does overweighting use more air?
An overweighted diver constantly inflates and deflates the BCD to hold buoyancy, and works harder to swim. Both increase air consumption.
Can overweighting hurt you?
Yes. Fast descents risk ear injury from inadequate equalisation, and dropping excess weights in an emergency can cause an uncontrolled ascent with decompression sickness risk.
How close does the SCUBAzen estimator get?
It is designed to place a diver within 2 pounds (1 kilogram) of ideal weight, leaving only minor surface adjustment.