Dalton Triangle
Dalton's Law says the partial pressure of a gas equals the fraction of that gas times the ambient pressure: PO₂ = FO₂ × P. Underwater, P is the absolute pressure from depth plus 1 bar of surface pressure (adjusted by water type). This tool lets you choose which value to compute (PO₂, FO₂, or Depth), enter the other two, and get the answer instantly.
How to use the Dalton's Law calculator
Choose which value you want to solve for — PO₂, FO₂, or Depth — then fill in the other two fields and hit Compute. For example, pick "Depth", enter your gas's FO₂ and a target PO₂, and the calculator returns the maximum operating depth for that gas at that PO₂ limit.
FAQ
What is Dalton's Law of partial pressures?
Dalton's Law says the total pressure of a gas mixture is the sum of the partial pressures each individual gas would exert on its own, and that each gas's partial pressure equals its fraction of the mix times the total (ambient) pressure. For diving, that gives PO₂ = FO₂ × P, where P is the absolute pressure at depth.
How do I calculate PO2 at depth?
Work out the absolute pressure at your depth (1 bar at the surface, plus roughly 1 bar per 10 m of depth), then multiply it by your gas's oxygen fraction (FO2). This calculator does that for you, and can also work backwards — enter a target PO2 and FO2 to find the depth, or a target PO2 and depth to find the required FO2.
Why does PO2 matter for gas planning?
PO2 is what drives oxygen toxicity risk — both acute CNS toxicity on deeper or high-PO2 dives, and cumulative OTU exposure over a dive or a day of diving. Agencies set maximum PO2 limits for working and deco gases (commonly around 1.4 for bottom gas and 1.6 for deco gas) specifically to keep this in check, which is why this calculator's depth/FO2 solving is useful for checking a gas choice against a target MOD before diving it.