Calculator
Dive weight calculator
A first estimate of your weight, or the change if you already know yours and switch suit, cylinder or water. It runs in your browser: what you type is not sent to or stored on any server.
Fill in the form and press “Calculate”. If your browser doesn't run JavaScript, the “Typical values” table below gives you a starting point.
This is a rough estimate, not your weight. What really floats depends on your body, your suit and your cylinder, and no formula can measure it. Before diving, do the buoyancy check with all your gear, with an instructor or guide. Too much lead makes you use more air and puts you at risk if your BCD fails. Too little stops you holding the safety stop and can cause an uncontrolled ascent. Carry your lead in a system you can drop, and make sure your buddy knows how to drop it. If you change suit, cylinder or water, do the check again.
The buoyancy check
Your weight is settled in the water. This is how DAN describes the check:1
- In water too deep to stand, with all your gear on, empty your BCD completely.
- Breathe normally: the water should sit at eye level.
- Breathe out: you sink until the water covers your head.
- Breathe in normally again: you come back to eye level.
- At the end of the dive, confirm it at the safety stop: at 4.5–5 m (15–16 ft), with about 35 bar (≈ 500 psi) in the cylinder and an empty BCD, you should hover without effort.1
With thin suits, the stop decides. With a suit of 3 mm or less, the surface check can fall short for the stop: plan to confirm it there.
With a nearly empty cylinder. Ideally, do it at about 50 bar (≈ 725 psi).3 If you do it with a full cylinder, add about 2 kg (≈ 4 lb): that is the air you are going to use.23
One kilo (or 2 lb) at a time. Adjust in steps of 1 kg; with weights in pounds, 2 lb at a time.1 Write down the weight that worked with its suit, cylinder and water, and repeat the check if you change gear or water, if your body weight changes or after a long break.3
DAN asks for normal breathing and PADI for a full breath. More air in your lungs makes the result come out lighter; here we follow DAN.12
How it works
It adds up what floats and what sinks in you and your gear, for DAN's check at the surface: empty BCD, normal breathing, water at eye level and the cylinder at about 50 bar. Each term has a central value and an uncertainty. The lead needed is that buoyancy divided by (1 − water density / 11.34), because lead displaces water too. The figures in the table are buoyancy, in kilos, except where they say “of lead”.
| Term | How it is worked out |
|---|---|
| Your body | Body fat comes from the Deurenberg formula (body mass index, age and sex), with an error of ±5 points.5 With the Siri equation, fat weighs 0.90 kg/L and the rest of the body 1.10 kg/L; that gives your body's density and how much it floats.6 Without a height it uses a typical one (174 ± 8 cm for men, 162 ± 7 cm for women) and your build moves body fat 5 points up or down; without an age, any age from 20 to 65. “Muscular” puts body fat between 10% (17% for women) and the estimate, which lowers the starting figure and the bottom of the ranges; the top stays where your body mass index puts it. These are our own assumptions. |
| Air in your lungs | Quanjer's functional residual capacity (height, age and sex), cut to 67% because upright in the water the chest is squeezed, plus a normal breath (0.35 to 0.65 L) and 0.05 to 0.15 L of gut gas. Each person is about 0.5 L away from the formula.78 A full suit of 5 mm or more takes 4% off it.15 |
| Head out of the water | In the check, the part of your head above the eyes does not float: 0.9 to 2.4 L are taken off. This is our own assumption. |
| Suit | Your body surface area (Mosteller formula, from weight and height)9 × the share the cut covers (full suit: 88% for men and 80% for women; shorty: 56–68% and 51–62%)11 × the real thickness (a “3 mm” measures 3.1–3.8 mm; a 5 mm, 4.7–5.8; a 7 mm, 6.7–8.3) × 0.72 to 0.84 kg per m² and per mm10 × 0.80 to 1 for wear and seams (our own assumption: a used suit floats less). |
| Hood | A 5 mm hood covers 4.4% of the surface (4.1% for women): depending on your size, about 0.2 to 0.5 kg (0.5 to 1 lb) of lead.1011 |
| Vest or hooded vest | 3 mm over 30 to 40% of the surface: depending on your size, about 1.3 to 2.4 kg (3 to 5.5 lb) of lead. It is our own calculation, not a measurement. |
| Cylinder, at about 50 bar | Aluminium 11–12 L: floats +0.9 to +1.3 kg (2 to 2.9 lb), from Catalina's table for its 11.1 L S80.4 Steel 12 L: −1.4 to −0.7 kg (−3.1 to −1.5 lb). Steel 15 L: −2.1 to −0.8 kg (−4.6 to −1.8 lb). The steel figures are our own calculations: we could not read any manufacturer's table. |
| BCD, regulator and accessories | About −0.9 kg (−2 lb) in all: between −2.2 and +0.4 kg (−4.9 to +0.9 lb). This is our own assumption. |
| Water | Sea: 1.0228 to 1.0245 kg/L, that of warm water. In the cold water of central and southern Peru (14 to 18 °C) the sea is a little denser and needs about 0.2 to 0.3 kg more. Fresh water: 0.998 kg/L. In fresh water everything floats less, your body included.12 |
The starting figure and the ranges. The calculator adds up the central value and the uncertainty of each term. The figure to start the check with is the central value, rounded to the kilo. The “most likely” range runs from the 25th to the 75th percentile: half the people with your details need an amount inside it, and the rest a little less or a little more. Each end is rounded to the nearest kilo, at least 1 kg apart. The “wide range”, folded below it, runs from the 10th to the 90th percentile (8 in 10 people), with the minimum rounded down and the maximum up. In pounds it converts before rounding: the ranges to the whole pound and the starting figure to the nearest 2 lb. The check in the water decides, 1 kg (2 lb) at a time.
If you don't give your sex, it works it out for a man and for a woman with your details: each range runs from the lower of the two to the higher, and the starting figure is their average.
If you already know your weight: the calculator does not estimate your body again. It takes the weight that worked and adds what the suit, hood, vest, cylinder and water terms change for you. The result is the figure to start with, with a margin of 1 kg (2 lb) either way. It is the more reliable mode.
Warnings: if the top of the “most likely” range is above 14 kg (≈ 31 lb), it asks you to check your entries (judged in kilos even if you choose pounds); if that range is wider than 3 kg (also judged in kilos), it reminds you the check is essential. With a suit of 3 mm or less it warns that the surface check can fall short for the safety stop, and gives the model's central value at the 5 m stop when it differs by 1 kg or more. These thresholds are our own, not a published standard.
What this calculation doesn't know
- It has not been validated with real divers. The figures come from formulas and material measurements, not from weights proved in the water.
- The body-fat formula was built on a European population. It is not validated in Peruvians or in minors, and in people with obesity it overestimates body fat.5
- A used suit floats less than a new one, and every brand is different.10
- A deeper breath moves several kilos: each litre of air in your lungs floats a little over 1 kg.
- We have not yet weighed our own cylinders, BCDs or rental suits in the water.
Typical values
What the calculator gives for two 30-year-olds, with an aluminium cylinder, in the sea:
| Suit | Man, 30, 75 kg (165 lb), 175 cm (5 ft 9 in) | Woman, 30, 58 kg (128 lb), 160 cm (5 ft 3 in) |
|---|---|---|
| No suit or Lycra | start with 0 kg (0 lb); most likely 0–1 kg (0–1 lb) | start with 0 kg (0 lb); most likely 0–1 kg (0–3 lb) |
| 3 mm shorty (short arms and legs) | start with 2 kg (6 lb); most likely 1–3 kg (3–7 lb) | start with 3 kg (6 lb); most likely 2–4 kg (4–8 lb) |
| 3 mm full suit | start with 4 kg (8 lb); most likely 3–5 kg (6–10 lb) | start with 4 kg (8 lb); most likely 3–5 kg (7–10 lb) |
| 5 mm full suit | start with 6 kg (14 lb); most likely 5–7 kg (11–15 lb) | start with 5 kg (12 lb); most likely 5–6 kg (10–14 lb) |
| 7 mm full suit | start with 9 kg (20 lb); most likely 8–10 kg (17–22 lb) | start with 8 kg (18 lb); most likely 7–9 kg (15–19 lb) |
With a 3 mm full suit, a hood adds about 0.3 kg (0.7 lb) and a vest about 1.6 kg (3.6 lb); a 12 L steel cylinder takes off about 2.4 kg (5.2 lb).
The aluminium cylinder
ORKA's cylinders are 12-litre aluminium. We have not measured them in the water yet, so the reference is the manufacturer Catalina's table for its aluminium S80, 11.1 L (80 ft³): −0.73 kg (−1.6 lb) full, +1.27 kg (+2.8 lb) at about 35 bar (≈ 500 psi) and +1.86 kg (+4.1 lb) empty. From full to 35 bar it gains 2 kg (4.4 lb) of buoyancy, which is the air you breathe.4
Why getting it right matters
- Too much lead. It forces you to put air in your BCD, makes you swim more upright and increases effort and air use. Many overweighted divers don't know it.1 If your BCD fails or a weight pocket comes loose, you can sink or rise out of control.2
- Too little lead. Staying down is tiring and, as the cylinder empties, you cannot hold the safety stop.1 DAN reported the case of a diver who lost his weight belt, rose from 35 m (≈ 115 ft) in under a minute and needed three hyperbaric chamber treatments.14
- Make it droppable. In a study of 126 diving deaths in Australia between 2001 and 2013, 81 of the 99 victims whose weights were known were still wearing them. The authors call for practising how to drop them.13
Coming to dive in Los Órganos? Our cylinders are 12-litre aluminium, the option the calculator has selected. Book a dive trip.
Sources (15)
- McCafferty, M. and Seery, P. «Weight Up!», Alert Diver (DAN), Q4 2014: dan.org/alert-diver/article/weight-up/
- Denny, M. «Why Diving Overweighted Is a Bad Idea», PADI Blog, 27-08-2024: blog.padi.com/diving-overweighted/
- Knight, M. «Training Tips: Proper Weighting for Diving», Scuba Diver Life, 07-05-2017: scubadiverlife.com/training-tips-proper-weighting-diving
- Catalina Cylinders, scuba cylinder specifications (S80): catalinacylinders.com/markets/scuba/
- Deurenberg, P., Weststrate, J. A. and Seidell, J. C. «Body mass index as a measure of body fatness: age- and sex-specific prediction formulas», British Journal of Nutrition 65(2):105–114, 1991 (abstract): pubmed.ncbi.nlm.nih.gov/2043597/
- Siri, W. E. «Body composition from fluid spaces and density: analysis of methods», in Brožek, J. and Henschel, A. (eds.), Techniques for Measuring Body Composition, National Academy of Sciences, 1961; the equation and the 0.90 / 1.10 kg/L densities, as described with StatLib's «bodyfat» dataset: mldata-static.ml.tu-berlin.de/repository/data/download/arff/statlib-20050214-bodyfat/
- Quanjer, P. H. et al., «Lung volumes and forced ventilatory flows», European Respiratory Journal 6, supl. 16:5–40, 1993; equations taken from their transcription in analysis plan NCT02662582: cdn.clinicaltrials.gov/large-docs/82/NCT02662582/SAP_001.pdf
- Delapille, P. et al., European Journal of Applied Physiology 86:97–103, 2001 (abstract: functional residual capacity drops 31–35% seated in the water and 20–21% face down): pubmed.ncbi.nlm.nih.gov/11820330/
- Mosteller, R. D. «Simplified calculation of body-surface area», New England Journal of Medicine 317(17):1098, 1987.
- Potočić Matković, V. M., Salopek Čubrić, I. and Pavko Čuden, A. «Dependence of Thermal Comfort of Diving Suit on Neoprene Properties and Diving Depth», Polymers 17(21):2820, 2025: pmc.ncbi.nlm.nih.gov/articles/PMC12608526/
- U.S. EPA, Exposure Factors Handbook, chapter 7 (body surface area by part), 2011: epa.gov/sites/production/files/2015-09/documents/efh-chapter07.pdf
- TEOS-10, International Thermodynamic Equation of Seawater (COI/UNESCO, 2009); densities computed with its GSW toolbox for 17–29 °C: teos-10.org
- Lippmann, J., Stevenson, C. and Taylor, D. McD. «Scuba diving fatalities in Australia, 2001 to 2013: Diver demographics and characteristics», Diving and Hyperbaric Medicine 50(2):105–114, 2020: pmc.ncbi.nlm.nih.gov/articles/PMC7481108
- Dovenbarger, J. «Quick Comeback», Alert Diver (DAN), autumn 2009: dan.org/alert-diver/article/quick-comeback/
- Houtkooper, A. et al., «Impact of neoprene wetsuits on lung volumes and work of breathing», Diving and Hyperbaric Medicine 56(3):218–225, 2026 (abstract): pubmed.ncbi.nlm.nih.gov/42743560/
Reviewed 21 October 2026