Boyle's Law and Equalization: Why the First 10 Meters Matter Most
Understanding pressure changes underwater is fundamental to safe freediving. Boyle's Law explains why equalization becomes critical in the first 10 meters, why your lungs compress at depth, and how to avoid painful—and potentially dangerous—squeeze injuries.
Try our interactive Depth Pressure Calculator to visualize these concepts in action.
What is Boyle's Law?
Boyle's Law states that at constant temperature, the pressure and volume of a gas are inversely proportional. The formula is simple:
P₁ × V₁ = P₂ × V₂
In practical terms: when pressure doubles, volume halves. When pressure triples, volume becomes one-third. This has profound implications for freedivers.
Pressure Changes at Depth
At sea level, we experience 1 atmosphere (ATM) of pressure. For every 10 meters of seawater depth, pressure increases by 1 ATM:
Surface (0m): 1 ATM
10 meters: 2 ATM (pressure doubles)
20 meters: 3 ATM
30 meters: 4 ATM
40 meters: 5 ATM
Why the First 10 Meters Are Most Critical
This is the most important concept for new freedivers to understand. The first 10 meters experience the greatest pressure change:
0-10m: Pressure increases 100% (1→2 ATM). Air spaces compress by 50%.
10-20m: Pressure increases 50% (2→3 ATM). Air spaces compress by additional 17%.
20-30m: Pressure increases 33% (3→4 ATM). Air spaces compress by additional 8%.
This is why most ear injuries and sinus squeezes happen in shallow water. The pressure change from 0-10m is greater than from 30-40m, even though both are "10 meters."
How Pressure Affects Your Body
Lungs
Your lungs are flexible air spaces that compress as you descend. With an average total lung capacity of 6 liters:
At 10m: Lungs compress to 3 liters (50% of surface volume)
At 20m: Lungs compress to 2 liters (33% of surface volume)
At 30m: Lungs compress to 1.5 liters (25% of surface volume)
This compression is why you become less buoyant at depth—your body's "flotation device" is shrinking.
Middle Ear
Your middle ear is a rigid air space behind the eardrum. Unlike your flexible lungs, it cannot simply compress. As pressure increases, the eardrum is pushed inward, causing pain and potential injury if not equalized.
Equalization adds air to the middle ear via the Eustachian tube, balancing the pressure and preventing squeeze.
Sinuses
Your sinuses are air-filled cavities in your skull. They usually equalize passively, but congestion can block equalization pathways, leading to painful sinus squeeze. This is why you should never freedive with a cold.
Mask
Your freediving mask creates an air space against your face. Without equalization (exhaling through your nose), the mask will squeeze, potentially causing bruising around the eyes ("mask squeeze"). Use a low-volume freediving mask and exhale gently through your nose as you descend.
Equalization Techniques
Because of Boyle's Law, you must equalize early and often—especially in the first 10 meters where pressure changes are greatest.
Valsalva Maneuver
Pinch your nose and gently blow against closed nostrils. This is the most common technique but uses chest air, making it less effective at depth when lungs are compressed.
Frenzel Maneuver
Uses the tongue as a piston to compress air in the mouth and push it into the Eustachian tubes. This technique works independently of lung volume and is preferred for deeper freediving.
Mouthfill
An advanced technique where you fill your mouth with air at a specific depth and use that air for remaining equalizations. Essential for deep freediving beyond 30m.
Practical Tips for Safe Equalization
Equalize before you feel pressure. Pre-equalize at the surface and continue equalizing every meter in the first 10m.
Never force it. If you can't equalize, stop your descent, ascend slightly, and try again. Forcing can cause barotrauma.
Descend feet-first initially. This helps keep the Eustachian tubes in a favorable position for equalization.
Don't dive congested. A cold or allergies can block your Eustachian tubes, making equalization difficult or impossible.
Practice on land. Master Frenzel technique in dry conditions before applying it underwater.
Visualize It Yourself
Use our Depth Pressure Calculator to see exactly how pressure affects your lungs and air spaces at any depth. Understanding these physics makes you a safer, more confident freediver.
Key Takeaways
The first 10 meters have the greatest pressure change—equalize early and often
Your lungs, ears, sinuses, and mask all contain air that compresses with depth
Never force equalization—ascend slightly and try again if needed
Learn Frenzel technique for more effective equalization at depth
Understanding Boyle's Law makes you a safer, more knowledgeable freediver