Calculator
Altitude calculator — Pressure & dive limits at altitude
Diving at altitude is different from diving at sea level: the surface starts at lower atmospheric pressure, so the same depth produces a smaller pressure ratio. The calculator returns atmospheric pressure for any altitude and an altitude-adjusted MOD for the gas you plan to breathe.
How altitude pressure is calculated
The barometric formula gives atmospheric pressure at altitude assuming an isothermal atmosphere at 15 °C — a good approximation for the troposphere where most altitude diving occurs.
P_atm(h) = P₀ × exp(−h ÷ H) P₀ sea-level atmospheric pressure (1.01325 bar) H scale height ≈ 8434 m at 15 °C h altitude (m)
Worked example: Lake Titicaca at 3812 m. P_atm ≈ 0.63 bar (vs. 1.01 bar at sea level). MOD of EAN32 at 1.4 ppO₂ in fresh water at 3812 m ≈ 35 m — slightly deeper than the sea-level MOD because the atmospheric component is smaller, so more of the pressure budget is hydrostatic.
Decompression at altitude
MOD is one piece. Decompression at altitude also requires altitude-corrected ascent ceilings — modern computers and planners (including the DiveTwin planner) handle this by using the local atmospheric pressure as the surface reference. Quick rule of thumb: above ~300 m altitude, treat it as an altitude dive and follow agency procedures (acclimatization, conservative GFs, etc.).
Frequently asked questions
- What's the atmospheric pressure at 2000 m?
- About 0.79 bar — roughly 78% of sea-level pressure. By 3000 m it's down to ~0.69 bar.
- Does MOD change at altitude?
- Yes — and it changes more than people expect. Lower atmospheric pressure means a given MOD ppO₂ corresponds to a deeper depth in fresh water. The calculator uses the altitude-adjusted ambient pressure for the MOD math.
- How long do I need to acclimatize?
- Agencies vary, but 12–24 hours at altitude before diving is common practice for dives above ~1500 m. Without acclimatization, dive planners apply altitude-correction tables that effectively shorten NDLs.
- What about descending from altitude after a dive?
- Descending to lower altitude after a dive is essentially decompression — your body off-gasses faster as ambient pressure rises. Flying or driving back down to sea level needs the same surface-interval rules as flying after diving.