Calculator
MOD calculator — Maximum Operating Depth
The Maximum Operating Depth (MOD) is the deepest depth at which a gas mix is safe to breathe before oxygen toxicity becomes a hazard. Set your O₂ fraction and the partial pressure of oxygen (ppO₂) you want to cap at — the calculator returns the MOD in metres or feet.
How MOD is calculated
MOD comes straight from Dalton's law: oxygen partial pressure equals oxygen fraction times absolute ambient pressure. Solve for depth.
MOD (m) = (ppO₂ ÷ FO₂ − 1) × 10 ppO₂ target oxygen partial pressure (bar) FO₂ oxygen fraction of the gas mix
Worked example: EAN32 (FO₂ = 0.32) at a 1.4 bar working ppO₂. MOD = (1.4 / 0.32 − 1) × 10 ≈ 33 m of seawater. The calculator above also accounts for fresh vs salt water and includes a separate MOD at deco ppO₂ (typically 1.6 bar) for stage cylinders.
Working ppO₂ vs deco ppO₂
Two limits matter. The working ppO₂ (often 1.4 bar) is what you cap at during the bottom phase, where workload and CO₂ raise oxygen-toxicity risk. The deco ppO₂ (often 1.6 bar) applies on stops, where workload is low and you accept a higher pp to accelerate off-gassing. Most agencies treat 1.6 bar as a hard ceiling.
Frequently asked questions
- What's the MOD of EAN32 at 1.4 bar?
- About 33 metres in salt water (≈ 110 ft). The calculator on this page returns the same value, with a separate column for the 1.6 bar deco limit (≈ 40 m / 130 ft).
- Why is 1.4 bar the standard working limit?
- Above ~1.4 bar, the risk of central-nervous-system oxygen toxicity rises sharply, especially under workload, cold, or elevated CO₂. Most recreational and technical agencies set 1.4 bar as the working cap and 1.6 bar as the absolute ceiling reserved for low-workload deco stops.
- Does MOD change in fresh water?
- Yes, slightly. Fresh water is less dense than salt water, so a given ambient pressure corresponds to a deeper depth. The calculator lets you toggle salt and fresh — the difference is on the order of half a metre at recreational depths.
- Is gas density a separate concern?
- Yes. Even when ppO₂ is fine, gas density above ~5.2 g/L is associated with elevated CO₂ retention and accident risk on technical dives — a key reason helium is added to deep mixes. The calculator surfaces gas density at MOD and flags mixes that exceed the recommended density limit.