Multi-Gas Scuba Deco Planner
Decompression stops, time to surface and gas needed for a square dive on air, nitrox or trimix, with up to two deco gases.
Quick examples
Load a full example, then change any value.
Results update as you type.
Your plan
Time to surface (TTS)
CautionLeave the bottom at 25 min. The first stop is at 21 m, and you reach the surface at 47 min after 17 min of stops.
Plan details
- First stop
- 21 m
- Time on stops
- 17 min
- Total runtime
- 47 min
- No-deco limit at this depth
- 9 min
- CNS oxygen clock
- 17.8%
- OTU (lungs)
- 41
- Highest ppO₂
- 1.56 bar
- END of bottom gas
- 40 m
Dive profile
Depth over time. The line style shows which gas you breathe.
- Air
- EAN50
Decompression schedule
| Depth (m) | Stop (min) | Runtime (min) | Gas |
|---|---|---|---|
| 40 | Bottom | 25 | Air |
| 21 | 1 | 29 | switchEAN50 |
| 15 | 1 | 30 | EAN50 |
| 12 | 1 | 32 | EAN50 |
| 9 | 1 | 33 | EAN50 |
| 6 | 4 | 37 | EAN50 |
| 3 | 9 | 47 | EAN50 |
| 0 | Surface | 47 | EAN50 |
Stop times are whole minutes at the stop depth. Runtime is the time you leave the stop, rounded up to the minute.
Gas needed
Surface litres from your SAC and the average pressure of each part of the dive. No reserve is included.
- Air2547 L107 bar
- EAN50493 L45 bar
Add a reserve on top: for example the rule of thirds on the bottom gas, and enough deco gas for a team member who loses theirs.
A planning aid, not a dive plan
Decompression diving needs technical diving training, enough gas with a reserve, and a dive computer that tracks the dive you actually do. This page runs a published model on a square profile at sea level. Real dives are not square, and divers get decompression sickness on plans that stay inside the model. Cross-check every plan with your own software and your training, and dive the plan your computer shows.
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About the Bühlmann deco model
The planner runs the Bühlmann ZHL-16C model: 16 tissue compartments that take up and release nitrogen and helium at different speeds. At every moment of the dive it tracks how much gas each compartment holds and how far it may be supersaturated before the model calls for a stop.
Gradient factors (GF) set how much of that allowed supersaturation you use. GF Low places the first stop, GF High limits the last metres to the surface, and the stops in between follow a straight line from one to the other. Deco gases with more oxygen speed up the release of inert gas on the stops.
With your numbers
Enter valid values to see the calculation worked through with your numbers.
Good to know
- Switch depths use a ppO₂ limit of 1.62 bar, so oxygen goes on at 6 m as most divers plan it: with 1.01325 bar at the surface, oxygen at 6 m is 1.61 bar. EAN50 goes on at 21 m (1.56 bar). Follow your training and your agency's limits.
- At a gas switch depth the plan holds at least 1 minute, so there is time to switch and check the gas.
- The planner assumes sea level. For a mountain lake, work out the surface pressure with the altitude calculator first: lower pressure means longer stops.
- Your dive computer will show different stops. It follows the real profile and its own settings. Dive the computer and carry this plan as a backup.
Formula
Loading at constant depth (Haldane)
Pt = P0 + (Pi − P0)(1 − e^−kt)
Loading while depth changes (Schreiner)
Pt = Pi0 + R(t − (1) ÷ (k)) − (Pi0 − P0 − (R) ÷ (k))e^−kt
Inspired gas and rate constant
Pi = (Pamb − 0.0627) × Fgas, k = (ln 2) ÷ (t½)
Tolerated ambient pressure (Bühlmann with GF)
Ptol = (Pt − GF × a) ÷ (GF ÷ b − GF + 1)
GF at a stop
GF(d) = GFhigh − (GFhigh − GFlow) × (d) ÷ (dfirst)
Deco gas switch rule
ppO2 = FO2 × (1.01325 + (d) ÷ (10)) ≤ 1.62
Where
- Pt: nitrogen or helium pressure in a compartment (bar). P0: its value at the start of the step
- Pi, Pi0: inspired pressure of that gas (bar). R: how fast it changes during a descent or ascent (bar/min)
- t: minutes, t½: compartment half-time (min), Fgas: fraction of that gas in the mix
- a, b: Bühlmann coefficients. With helium both are averaged by the pressures: a = (aN2PN2 + aHePHe) ÷ (PN2 + PHe)
- GF: gradient factor as a fraction (85% = 0.85). d: stop depth (m), dfirst: first stop depth (m)
- Pamb: absolute pressure, 1.01325 + (d) ÷ (10) (bar). 0.0627: water vapour in the lungs (bar)
How it's calculated
- Model: Bühlmann ZHL-16C nitrogen coefficients with the ZH-L16 helium set, 16 compartments, with the 5-minute compartment (1b) as the first one. The coefficients were checked against Erik Baker's M-value tables.
- Pressure: 1.01325 bar at the surface plus 1 bar per 10 m of sea water. Our other calculators use depth ÷ 10 + 1; the extra 0.013 bar matters here because it moves switch depths and stop times.
- Inspired gas: (ambient pressure − 0.0627 bar water vapour) × gas fraction. The tissues start saturated with surface air (79.02% nitrogen).
- Profile: descent on the bottom gas at the chosen rate, and the bottom time includes the descent. Constant depth uses the Haldane equation, descents and ascents the Schreiner equation.
- No stop is needed if a direct ascent on the bottom gas reaches the surface without passing GF High. The NDL is the longest whole-minute bottom time for which that holds, so it depends on GF High only.
- Otherwise the ascent moves up the stop grid (every 3 m or 1 m, down to the last stop). The first stop is the grid depth where the next step up would pass the GF Low ceiling. From there, every grid depth down to the last stop is a stop of at least 1 minute.
- GF runs in a straight line from GF Low at the first stop to GF High at the surface. You leave a stop when the ceiling, worked out with the GF of the next stop, is at or above the next stop. No credit is taken for gas released during the move up. Stop times are whole minutes.
- Gas switch: on reaching the deepest grid depth where a deco gas has ppO₂ ≤ 1.62 bar (oxygen at 6 m is 1.61 bar), the plan switches to the richest breathable gas and stays at least 1 minute. It never switches back to a leaner gas. Deco gases are nitrox or oxygen, without helium.
- Gas needed: SAC × average absolute pressure × minutes for each part, with the bottom SAC for descent and bottom and the deco SAC for the ascent and stops. Bar used = litres ÷ cylinder water volume (ideal gas). No reserve.
- Oxygen: CNS is the sum of each part's minutes divided by the NOAA single-exposure limit for its ppO₂ (the next table row up). OTU = minutes × ((ppO₂ − 0.5) ÷ 0.5)^0.83. Descents and ascents are split into 1-minute slices.
- Checks: bottom ppO₂ above 1.4 bar, a hypoxic bottom gas (below 0.18 bar at the surface, no travel gas is planned), END deeper than 30 m with oxygen counted as narcotic, CNS above 80%, and depths beyond 100 m. Altitude is not included.
- Cross-check: with the same settings, NDLs and stop times match DecoTengu 0.14.1 within 1 minute per stop. GasPlanner uses a different gradient factor method, and there a stop can differ by 2 minutes.
These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification. The deco planner is for trained technical divers. It does not replace technical diving training, your own planning software or a dive computer.
How to use the deco planner
- 1
Enter the maximum depth, the bottom time and the bottom gas. For trimix, fill in both oxygen and helium.
- 2
Switch on the deco gases you carry and set their oxygen. The planner finds the switch depth for each one.
- 3
Pick gradient factors, then check the stop table, the gas needed and the CNS. Change one value at a time to see its effect.
What the plan tells you
Time to surface (TTS) is the time from leaving the bottom to reaching the surface, stops and ascent included. Compare it with your gas: if a problem starts at the end of the bottom time, you need gas for the whole TTS.
The stop table lists each stop, how long to stay and the runtime when you leave it. The deep stops are short and come from GF Low. Most of the time is spent at 6 m and 3 m, where the slow compartments release their gas.
If the result says no stop is needed, the dive is inside the no-decompression limit for that depth, gas and GF High. The limit shrinks quickly with depth, so a few extra minutes can change a no-stop dive to a deco dive.