Buoyancy in diving: why you sink, float or hover
In short
Buoyancy in diving follows Archimedes’ principle: water pushes you up with a force equal to the weight of the water you displace. Seawater weighs about 1.03 kg per liter (64 lb per cu ft), fresh water 1.00 kg (62.4 lb), so you need a little more lead in the sea. Air spaces shrink with depth.
Your weight check was perfect in the lake. On your first sea dive, you kick, dump every bit of air and still can’t get down. Seawater is denser than lake water, so it pushes you up harder, as Archimedes’ principle predicts.
What is Archimedes’ principle?
Archimedes’ principle states that an object in water is pushed upward by a force equal to the weight of the water it displaces. Compare that push with the object’s own weight:
- heavier than the water it displaces: negatively buoyant, it sinks;
- lighter: positively buoyant, it floats;
- equal: neutrally buoyant, it hovers.
HyperPhysics shows it with same-size balls of cork, aluminum and lead: same push, different weights. The diver’s version: apparent weight = weight − weight of the water displaced.
What changes a diver’s buoyancy during a dive?
Your buoyancy changes whenever your volume, your weight or the water changes.
- Your suit: neoprene’s gas bubbles shrink with depth (Boyle’s law). You displace less water and get heavier as you descend.
- Your BCD: air added on the way down expands on the way up. Vent early.
- Your tank: air has weight, so the tank gets lighter as you breathe it. DAN notes that aluminum cylinders often start negatively buoyant but eventually become positively buoyant.
- Your lungs: a full breath lifts you a little, a long exhale lowers you. Never hold your breath.
- The water: as DAN puts it, saltwater is denser than freshwater, which increases the buoyancy of immersed objects.
Try a question from the app
wetsuit compression
Why do you need more air in your BCD at 30 m (100 ft) than at 5 m (15 ft)?
Show the answer
Answer B. Neoprene is full of gas bubbles that shrink with depth (Boyle’s law), so your volume and your lift decrease. Water is almost incompressible, and your cylinder’s weight doesn’t change with depth.
How do you calculate buoyancy?
You calculate buoyancy by comparing an object’s weight with the weight of the water it displaces.
- Lift = volume displaced × water density: 1.03 kg per liter in the sea, 1.00 in fresh water (64 or 62.4 lb per cubic foot).
- Apparent weight = weight − lift. Positive: it sinks. Negative: it floats.
- Air to make it neutral = apparent weight ÷ water density. In the sea, each liter of air lifts about 1.03 kg, each cubic foot about 64 lb.
The same formulas, with the other gas laws, are on the dive theory formulas cheat sheet.
Worked example
From a lake to the sea. You and your gear weigh 75 kg (165 lb) and you’re neutral in a lake.
- Metric: you displace 75 L. In the sea, 75 L weighs 75 × 1.03 = 77.25 kg: about 2.25 kg of extra lift.
- Imperial: you displace 165 ÷ 62.4 ≈ 2.64 cu ft, which weighs 2.64 × 64 ≈ 169.2 lb in the sea: about 4.2 lb of extra lift.
Add roughly that much lead, a little more since the lead displaces water too, then confirm with a weight check.
Try a question from the app
salt water and fresh water
You switch from a lake to the sea with the same gear. What about your weights?
Show the answer
Answer D. Sea water is denser, so it pushes you up harder: you need a little more weight than in fresh water.
Worked example
Lifting with a lift bag. A 50 kg object displaces 10 L of seawater (imperial: 110 lb displacing 0.35 cu ft). How much air makes it neutral?
- Metric: lift = 10 × 1.03 = 10.3 kg; apparent weight = 50 − 10.3 = 39.7 kg; air = 39.7 ÷ 1.03 ≈ 38.5 L.
- Imperial: lift = 0.35 × 64 = 22.4 lb; apparent weight = 110 − 22.4 = 87.6 lb; air = 87.6 ÷ 64 ≈ 1.4 cu ft.
- Add Boyle’s law: filled at 20 m (66 ft) and left unvented, that air grows by half by 10 m (33 ft), to about 58 L lifting about 60 kg (2.1 cu ft lifting about 131 lb), so the bag speeds up as it rises.
The bag’s own weight is ignored. Never attach yourself to a lift bag, and let a runaway bag go.
Worked example
Why your tank gets lighter. Air weighs about 1.2 g per liter (0.075 lb per cubic foot).
- Metric: a 12 L tank breathed from 200 to 50 bar delivers 150 × 12 = 1,800 L: 1,800 × 1.2 g ≈ 2.2 kg lighter.
- Imperial: an aluminum 80 holds about 77 cu ft at 3,000 psi. From 3,000 to 500 psi, it delivers about 64 cu ft: 64 × 0.075 ≈ 5 lb lighter.
Set your weighting for the end of the dive, when the tank is lightest.
How much weight should you carry?
Carry just enough weight to descend at the start and hover at the end. The common check: at the surface, with an empty BCD and a normal breath, you float at about eye level and sink slowly as you breathe out (some instructors use a slightly different check). Repeat it with any new suit, tank or water.
DAN’s end-of-dive test: you should be able to hover comfortably at 5 m (15 ft) with 50 bar (500 psi) in an aluminum tank and no air in your BCD. Extra lead isn’t safer: DAN notes that it pushes more air into the BCD and tips you upright, which adds drag, effort and air use.
What are the common buoyancy mistakes?
- Getting the water backward. Lake to sea means adding weight; sea to lake, removing some.
- Forgetting the object’s own volume. A lift bag only has to cancel the apparent weight.
- Weighting with a full tank only. A tank that floats near empty can cost you your safety stop.
- Swimming to fix buoyancy. If you have to kick up or down to hold your depth, adjust your BCD instead.
- Big BCD corrections. Add or vent in small bursts, then wait a few seconds.
Which dive course covers buoyancy?
In Octo, the Open Water Diver level covers neutral buoyancy and the weight check, and Advanced Open Water adds trim, hovering, lift bags and dry suit air. The Divemaster level works through Archimedes’ principle with full calculations. For the gas side of the same dive, see air consumption and SAC rate, and check the basics with the Open Water practice questions.
Frequently asked questions
Why do I float more in the sea than in a lake?
Because seawater is denser: a liter weighs about 1.03 kg, against 1.00 kg for fresh water (64 versus 62.4 lb per cubic foot). Your body displaces the same volume in both, but the sea’s water weighs more, so it pushes you up harder. That’s why you need a little more lead in the sea.
Why does my aluminum tank make me float at the end of the dive?
Because you’ve breathed a couple of kilograms of air out of it: about 2.2 kg from a 12 L tank emptied from 200 to 50 bar, about 5 lb from an aluminum 80 emptied from 3,000 to 500 psi. DAN notes that aluminum cylinders often become positively buoyant as the gas is used. Steel cylinders stay negatively buoyant.
How much extra weight do I need in salt water?
Roughly 2.5 to 3% of the total weight of you and your gear, with the rounded densities on this page. For 75 kg (165 lb), that’s about 2.25 kg (4.2 lb), a little more since the lead displaces water too. Treat it as a starting point and confirm with a weight check.
Does the water push harder on me at depth?
Barely. Water is almost incompressible, so a liter of it weighs about the same at 30 m (99 ft) as at the surface. Your own volume changes: your wetsuit and the air in your BCD and lungs shrink with depth, so you displace less water and get heavier. That’s why you add air as you descend.
