Physics for Freediving

We like to say that freediving is a unique activity. And in many ways, it is a unique activity. But, nevertheless, Freediving still happens in a materialistic world and Freedivers use the same laws as anyone else. In this chapter, we discuss physical laws important to know for Freedivers, with only a brief mention of how this information can be applied in Freediving.


Majority of the information on this chapter is taken from the open sources, such as Wikipedia and other online resources. We admire your desire to learn more and advise you to use Google search for it. Good luck!


  1. Ambient pressure

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.

On the surface, a Freediver experiences the atmospheric pressure, but while underwater it is going to be a combination of the atmospheric pressure and the hydrostatic pressure (water pressure). Since water is much denser than air, significant pressure change can be experienced underwater during a descent. In saltwater, every 10 meters of water above a Freediver adds pressure equal to the atmospheric pressure (1 ATM or 1 bar).

For example

At 10 meters – the pressure is 2 bar
at 20 meters – the pressure is 3 bar
at 30 meters – the pressure is 4 bar

Application in Freediving

If we know how pressure changes during a descend, we can understand the importance of regular equalization, lungs compression on the descent, shallow water blackout, pretty much everything related for open water Freediving session


2. Boyle’s law.

The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and the amount of gas remain unchanged within a closed system.

P1V1=P2V2 or PV= constant or V2=P1V1/P2

The equation shows us that if the gas pressure increases than its volume decrease in proportion. And the reverse is true – if the pressure decreases, the volume will increase in proportion.

Application in Freediving

It explains why we need to equalize air spaces during a descent phase of the dive.

The deeper we dive the higher the ambient pressure is. According to Boyle’s law, gas’ volume will decrease during a descent. For example, at 10 meters ambient pressure is going to be twice as higher as on the surface (2 atm), means that the volume of air is going to be decreased twice, compared to the surface.

What air spaces we have in our body? The middle ear, sinuses, lungs (as well as an air space inside a mask/goggles). More information about equalization of these air spaces in chapter Equalization.

A second reason to know Boyle’s law – it helps to understand the process of respiration. During the inhalation phase, the diaphragm goes down and it increases the chest volume, as a result, pressure in the lungs becomes less than atmospheric pressure. And since the gas tends to move from an area with the high pressure to an area with lower pressure, the air is moving from the outside to the inside of our lungs.

More about the respiration system in the chapter Physiology of Freediving


3. Dalton’s law of partial pressure

In the mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

The partial pressure is the pressure that each gas would exert it alone occupied the volume of the original mixture at the same temperature.

Gases dissolve, diffuse, and react according to their partial pressure, and not according to their concentration in gas mixture or liquids.

Ptotal= Pgas1+Pgas2+Pgas3

For example, the air has approximately 21% of oxygen and atmospheric pressure is 760 mmHg (at the sea level), the partial pressure of oxygen 0.21×760=160 mmHg

Application in Freediving

Dalton’s law important to know if you want to understand and avoid such Freediving related problems as LMC/BO/SWB. More about it in the chapter Emergency in Freediving


4. Diffusion.

A physical process where molecules move from an area of high concentration to an area of low concentration.

The word diffusion derives from the Latin word, diffundere, which means “to spread way out”.

Diffusion is driven by a gradient in chemical potential of the diffusing species. A gradient is a change in the concentration, pressure or temperature with the change in another variable, usually distance.

Application in Freediving

Help to understand how gas exchange happens between alveoli and blood and between blood and tissues in our body.


5. Archimedes’ principle.

The upward buoyant force that exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the centre of mass of the displaced fluid

And object at a given time can have one of three types of buoyancy – positive, neutral and negative.

Positive buoyant – an object is lighter than the fluid it displaces. In this case, the object will float on the surface because the buoyant force is greater than the object’s weight (or move to the surface if it was previously submerged).

Negative buoyant – an object is denser than the fluid it displaces. The object will sink because its weight is greater than the buoyant force

Neutral buoyancy – an object’s average density is equal to the density of the fluid in which immersed. In this case buoyancy force in the equilibrium with the force of gravity. 

Application in Freediving

Correctly adjusted buoyancy helps Freediver to spend less energy during both pool and open water sessions.

At the pool, we want to be neutrally buoyant approximately at a half depth of the pool (it can vary in different pools though). During OW session we want to be positive buoyant on the surface, neutral at a certain depth (usually around 10 meters) when negative – freefall. On the way up, positive close to the surface.

Buoyancy depends on body structure, lungs’ size and thickness of the wetsuit and can be regulated by adding or removing weights on a weight belt or neck weight. More about it in chapters Freediving pool/open water skills and Freediving equipment.  


6. Bohr effect.

Hemoglobin’s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide.

Increase in the carbon dioxide partial pressure of blood and decrease in blood pH results in the lower affinity of hemoglobin for oxygen, enhancing unloading of bound Oxygen by hemoglobin passing through the metabolically active tissue and thus improving oxygen delivery.

Carbon dioxide is not only one reason to trigger the Bohr effect. When muscles cells aren’t receiving enough oxygen for cellar respiration, then they start working anaerobically, which releases lactate as a by-product. This increase acidity of the blood far more than CO2 alone.

The Bohr Effect allows for enhanced unloading of Oxygen in metabolically active peripheral tissues such as exercising skeletal muscle.

When CO2 below the normal level it will shift blood pH and cause respiration alkalosis and as a result of the higher affinity of hemoglobin for oxygen

Application in Freediving.

Knowing the Bohr Effect will help you to understand why hyperventilation is dangerous (increase the risk of LMC/BO) and hopefully motivate you not doing it.


7. Henry’s law

At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

While the solubility of the gases present in the air is small at atmospheric pressure, it becomes appreciable at high pressure where, in many cases, the solubility of a gas is proportional to its pressure

Application in Freediving

Help to understand why DCS happens as well as why STA with pure O2 is much longer than normal STA. More about DCS in the chapter Freediving emergencies and about STA with 100% O2 in the chapter Freediving pool skills