Scuba Gas Volume at Depth Calculator (Boyle Law)
Calculate compressed gas volume at depth using Boyle Law.
Enter surface volume and depth to see how much your air shrinks at 30, 60, or 100 feet underwater.
Boyle’s Law and Diving
Boyle’s Law states that for a fixed temperature, gas volume is inversely proportional to absolute pressure: P₁ × V₁ = P₂ × V₂
In diving, depth determines pressure. Each 33 feet (10 m) of seawater adds 1 atmosphere to ambient pressure.
Salt water or fresh?
Fresh water is less dense, so it takes 34 feet (10.3 m) to add an atmosphere rather than 33. It is a 3% difference and it is genuinely small, but it runs the same direction on every calculation on a dive, and quarry and lake divers are working in fresh water for every dive they do. Pick the right one above and the answer uses it.
Pressure at depth (salt water):
| Depth | Absolute Pressure |
|---|---|
| Surface | 1 ATA |
| 33 ft (10 m) | 2 ATA |
| 66 ft (20 m) | 3 ATA |
| 99 ft (30 m) | 4 ATA |
| 132 ft (40 m) | 5 ATA |
| 165 ft (50 m) | 6 ATA |
Practical implications for divers:
1. Air consumption multiplies with depth: A breath at 33 ft uses 2× the surface gas. At 99 ft, you’re consuming 4× the surface volume per breath. This is why deep dives end on gas, not on time.
2. BCD and drysuit volume changes: Air added at depth must be vented during ascent. Miss it and the expanding gas overinflates the wing, your ascent accelerates, which expands it further, and the whole thing runs away with you.
3. Lung over-expansion: Holding your breath while ascending is the leading cause of fatal diving injuries. A lungful at 33 ft doubles in volume by the surface, and it exceeds lung capacity long before you get there.
Because that is the direction that hurts people, the result below reports it both ways: what a surface volume shrinks to at depth, and what a volume taken at that depth expands to on the way up. The second number is the one worth internalising.
4. Buoyancy compensator pre-dive air: Air added at the surface compresses to half-volume by 33 ft. Divers compensate by adding more air during descent.
5. Gas planning rule of thirds: For non-deco recreational dives, plan gas as: 1/3 outbound, 1/3 return, 1/3 reserve. Depth multiplies real-world gas consumption, so a 4 ATA dive burns 4× the surface-rate volume for exactly the same breathing.
The volume at depth formula: V_depth = V_surface / (depth in ft / 33 + 1) in salt water, using 34 instead of 33 in fresh.
So 12 cubic feet of air on the surface becomes 3 cubic feet at 99 ft (4 ATA).
And the same law the other way, which is the direction the safety rules are about: V_surface = V_depth × (depth in ft / 33 + 1)
So 12 cubic feet breathed in at 99 ft wants to be 48 cubic feet by the time you reach the boat.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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