Depth & Pressure Calculator
Understand how water pressure affects you and your gas supply at any depth. See ambient pressure, gas volume changes, nitrogen and oxygen partial pressures, and no-decompression limits on air.
Depth
Physics at 20m
At 20m, the pressure is 3.0x surface pressure. A balloon that's 1 litre at the surface would shrink to 0.33 litres. You breathe gas 3.0x faster than at the surface.
Pressure and Diving
Water is roughly 800 times denser than air. For every 10 metres of depth, pressure increases by 1 atmosphere (1 ATA / 1 bar). At the surface you're at 1 ATA; at 10m you're at 2 ATA; at 30m you're at 4 ATA. This simple relationship governs almost everything in diving.
Boyle's Law - Volume and Pressure
As pressure increases, gas volume decreases proportionally. This is why you equalise your ears (air spaces compress), why your BCD needs more air at depth (your wetsuit compresses), and why you must never hold your breath while ascending (expanding air could rupture your lungs).
Dalton's Law - Partial Pressures
Each gas in a mix exerts a partial pressure proportional to its fraction. Air is 21% oxygen and 79% nitrogen. At 30m (4 ATA), the ppO2 is 0.84 bar and ppN2 is 3.16 bar. High ppN2 causes nitrogen narcosis; high ppO2 causes oxygen toxicity.
No-Decompression Limits
No-deco limits (NDLs) represent the maximum time you can spend at a given depth without requiring mandatory decompression stops during ascent. They decrease rapidly with depth: 56 minutes at 18m but only 9 minutes at 40m. Exceeding NDLs requires staged decompression stops, which is the realm of technical diving.