Scuba Weight Calculator: How Much Lead Do I Need?

A scuba weight calculator estimates how much lead you need to be neutral at a 3 m (10 ft) safety stop with 50 bar (725 psi) left in the cylinder, from your water, cylinder, exposure suit and body. The result is a figure to start your weight check in the water with, not a replacement for that check.

The formula

B_{net} = B_{cyl} - M_{gas} + B_{suit} (d) + B_{body} - W_{lead}

Each term in kg, positive floats. B_cyl is the empty cylinder, regulator and rigging in the water, M_gas the mass of the gas inside, B_suit the suit's lift at depth d, B_body your own buoyancy and W_lead the lead. The lead estimate is what makes B_net zero at the 3 m stop with 50 bar left.

B_{suit} (d) = B_{surface} \times [(1 - k) + \frac{k}{r (d)}] M_{gas} = \frac{V (P + P_{atm})}{Z \times 1.01325} \times ρ_{gas}

A wetsuit compresses with depth: r(d) is the ambient pressure at depth over the surface pressure, and k = 0.8 is the share of standard neoprene's lift that compresses. For the gas, V is the cylinder's water volume, P the gauge pressure, P_atm 1.01325 bar and Z the gas's compressibility at that pressure: the fraction is the real-gas litres the cylinder holds, times the density of the gas (air about 1.2 g/L at 15 °C).

Worked example

A diver, 180 cm and 85 kg, dives one Steel 12L 232 cylinder (12 L) filled to 232 bar, in a 7 mm full wetsuit, to 25 m in salt water, with no lead yet. Core adds up each part of the rig at the start of the dive (full cylinder at 25 m) and at the end (50 bar left, at the 3 m stop):

Part (kg, + floats)	Start · 25 m	End · 3 m
Cylinder and regulator	−1.3	−1.3
Gas in the cylinder	−3.2	−0.7
Wetsuit lift	+3.8	+7.3
Body (estimate)	+2.6	+2.6
Net buoyancy	+1.9	+7.8

The wetsuit gives 3.8 kg of lift at 25 m and 7.3 kg at 3 m, and the cylinder is 2.5 kg lighter once the gas is breathed down. The end of the dive is the light point, so the estimate targets it: Carry about 8 kg to start the weight check with.

In imperial: a diver 5 ft 10 in and 180 lb, one AL80 at 3000 psi, a 3 mm wetsuit, 60 ft in salt water. The estimate is 14 lb of lead, with the rig at +13.4 lb at the 10 ft stop before any lead is added. An AL80 alone is +2.9 lb at 725 psi in the cylinder dataset, part of what the lead holds down at the stop.

Lead by suit and cylinder

The lead estimate for one reference diver (175 cm, 75 kg, male, 35) on a single cylinder with a standard regulator, in kg and lb, neutral at the 3 m stop with 50 bar (725 psi) left. A 0 means the rig sinks without lead. Your own body moves these figures by kilograms, so enter it in the calculator above.

Suit	AL80, salt	AL80, fresh	Steel 12L 232, salt	Steel 12L 232, fresh
No suit	3 kg · 6 lb	0.5 kg · 1 lb	0.5 kg · 1 lb	0 kg · 0 lb
3 mm wetsuit	5.5 kg · 13 lb	3.5 kg · 7 lb	3 kg · 7 lb	1 kg · 2 lb
5 mm wetsuit	7.5 kg · 17 lb	5.5 kg · 12 lb	5 kg · 11 lb	3 kg · 6 lb
7 mm wetsuit	9.5 kg · 21 lb	7 kg · 16 lb	7 kg · 15 lb	4.5 kg · 10 lb
Drysuit, medium undergarment	11 kg · 24 lb	8.5 kg · 19 lb	8.5 kg · 18 lb	6 kg · 13 lb

How Dive Kit computes this

Dive Kit adds up the rig twice. Start of dive: full cylinders at your target depth, where the wetsuit is most compressed and the gas weighs most. End of dive: 50 bar left, at the 3 m safety stop, where the rig is lightest. The lead estimate targets the end of the dive, because a diver who can hold the last stop with a near-empty cylinder can hold every deeper one. In kg it is rounded to 0.5 kg, up when lead is to be added and down when it is to be removed; in pounds, to whole pounds.

The cylinder comes from the Dive Kit cylinder dataset (its empty buoyancy, scaled to your water), plus a regulator per cylinder (0.8 kg for a standard one, 1.0 kg for the cold-water regulators of a twinset) and the twinset's bands and manifold (2.5 kg). The gas is air. A wetsuit's surface lift is sized from your body surface area (DuBois formula on height and weight) for a full-cut suit of standard neoprene; a drysuit gives a fixed 4, 8 or 12 kg for a light, medium or heavy undergarment.

Your own buoyancy is estimated from body composition: body fat from BMI, age and sex (Deurenberg's 1991 formula), body density from body fat (Siri's formula), and the gas in your lungs at a relaxed breath. Two people of the same weight can differ by several kilograms here, which is the main reason to finish with a weight check. In the app you can replace the estimate with the result of a float test.

This page keeps the rest at the app's defaults: a 16 kg wing, no backplate, lights, fins or hood, and no stage. A steel backplate is about 3 kg of lead in the app's gear list, so subtract it from the estimate if you dive one. The page also shows the app's checks for marginal wing lift, wetsuit corking risk (more than 2.3 kg of lift regained between 3 m and the surface) and overweighting (more than 2 kg to remove).

The app screen for this tool is documented in the user guide: Buoyancy Calculator in the Dive Kit guide.

How much weight do I need for scuba diving?

It depends on your water, cylinder, suit and body, which is why one rule of thumb fits few divers. The calculator adds each of those up and gives the lead that makes you neutral at the safety stop with 50 bar (725 psi) left. Use it as the starting point for a weight check, then adjust in the water.

How much weight do I need with a 3 mm, 5 mm or 7 mm wetsuit?

For the reference diver in the lead table above (175 cm, 75 kg) with an AL80 in salt water, the estimate is about 5.5 kg (13 lb) with a 3 mm wetsuit, 7.5 kg (17 lb) with 5 mm and 9.5 kg (21 lb) with 7 mm. A steel cylinder or fresh water needs less, and your own body moves the figure by kilograms, so enter it in the calculator and confirm in the water.

How do I do a weight check in the water?

At the end of a dive, with about 50 bar (725 psi) left, hover at the safety stop with an empty BCD and a normal breath. If you have to kick down to stay there, add lead; if you sink with the BCD empty, take some off. A check at the surface (floating at eye level with an empty BCD, holding a normal breath) gets close, but the stop with a near-empty cylinder is the one that counts.

Why do I need more weight in salt water than in fresh water?

Salt water is about 2.5% denser than fresh, so everything that displaces water, mostly your body and your suit, floats more. Switch the water type in the calculator to see the difference for your own rig.

Why does an aluminium cylinder need more lead than a steel one?

An aluminium cylinder displaces more water for its weight than a steel one and floats when it is near empty: in the cylinder dataset an AL80 is positive at 50 bar, where a steel 12 L stays negative. Because the estimate targets the end of the dive, that difference goes straight into the lead.

Does a drysuit need more lead than a wetsuit?

Usually, because the undergarment traps air that you keep at the same volume all dive, while a wetsuit loses lift as it compresses. The calculator uses 4, 8 or 12 kg of drysuit lift by undergarment; your real figure depends on loft and how much air you carry, so check it in the water.

Sources

Du Bois D, Du Bois EF. A formula to estimate the approximate surface area if height and weight be known. Archives of Internal Medicine, 1916 : the body surface area from height and weight that sizes a wetsuit's lift
Potočić Matković VM, Salopek Čubrić I, Pavko Čuden A. Dependence of Thermal Comfort of Diving Suit on Neoprene Properties and Diving Depth. Polymers, 2025 : measured neoprene compression under depth-equivalent pressure, the reason a wetsuit loses lift as you descend
rsingler. Optimal Buoyancy Computer (ScubaBoard, 2019) : the community weighting calculator the wing-lift, corking and overweighting checks are adapted from
Dive Kit open cylinder dataset (MIT licence) : each cylinder's water volume and empty buoyancy

Last updated 30 October 2026

Scuba Weight Calculator