CNS reduction
Your CNS % when you surface and your surface interval: what you will have left at the start of the next dive.
Looking for how long to wait instead? Interval for a target CNS →
CNS when you surface
Read on your computer or worked out for your dive.
Surface interval
Time spent out of the water, breathing air.
Decay speed
Sources and computers do not all agree. 90 min is the most common rule.
Residual CNS
24%
starting from 60 %
after 2 h
Carry over to the next dive
24 % carried over (23.8 % rounded up). To finish under 80 %, it can cost up to 56 %.
And your next dive?
The residual adds to the CNS the next dive will cost. Enter what your plan or computer predicts.
24 %residual +20 %planned =44 %
That works
You would finish at 44 %, under the recommended 80 %. Room: the dive can cost up to 56 %.
What is left, every 1 h 30
The CNS halves every 1 h 30. Tap a bar to pick that interval.
Dotted line: 60 %, the highest start to finish a 20 % next dive under 80 %.
How it is calculated
CNS (Central Nervous System) measures how much of your oxygen tolerance for the nervous system you have already used, as a percentage of the NOAA limit. At the surface, on air, it falls exponentially with a half-life T of 90 minutes, the model most computers use. T = 120 min is offered as a more cautious option.
Residual CNS = end CNS × 2^(−t / T)
t = T × log₂(end CNS ÷ target CNS)
The residual is rounded up to the next %, and the waiting time up to the next minute, to stay on the safe side.
| End CNS | Interval | Residual (T = 90) |
|---|---|---|
| 80 % | 1 h 30 | 40 % |
| 80 % | 3 h | 20 % |
| 60 % | 2 h | 24 % (23.8 rounded) |
| 100 % | 6 h | 7 % (6.25 rounded) |
Safety guidelines
- Keep a dive’s CNS under 80 %, and never go over 100 %.
- The residual adds to the next dive’s CNS: the total must also stay under 80 %.
- Breathing pure oxygen at the surface does not lower CNS the same way: this calculation assumes air.
The NOAA manual (2013 edition) mentions a 120 min half-life; others prefer a 24 h window. 90 min remains the most common value (Shearwater, Erik Baker, most computers); none has a real experimental basis, hence the safety margins.
A learning aid. It does not replace your computer, your training or your club’s rules.
Understanding CNS reduction
CNS falls during the surface interval
During the dive, your CNS % only goes up. Once at the surface, on air, it halves every 90 minutes: 80 % when you surface becomes 40 % after 1 h 30, then 20 % after 3 h. The tool works out this residual, rounded up, for the length of your interval. To get a dive’s CNS from its profile, use the CNS % calculator.
What matters: the total of the next dive
The residual adds to the CNS the next dive will cost. The usual guideline, used by computers and training courses, is to finish each dive under 80 % and never go over 100 %. Enter the planned CNS for the next dive: the tool checks the total and tells you how long to wait if it does not fit. To start from a target CNS, the CNS interval tool does the reverse calculation.
90 or 120 minute half-life?
The 90 minute half-life is a convention used by Erik Baker, Shearwater, Subsurface and nitrox courses. Some authors quote 120 minutes, more cautious. NOAA itself only sets per-dive and 24-hour limits. Some computers do not lower CNS at the surface: always follow what yours shows.
Frequently asked questions
How long does it take for CNS to halve?
90 minutes at the surface, on air, by the most common convention (Shearwater, Erik Baker, Subsurface). Some authors use 120 minutes, more cautious.
What is left of 80 % CNS after 3 hours?
20 %: 80 % becomes 40 % after 1 h 30, then 20 % after 3 h. The next dive can then cost up to 60 % and still finish under 80 %.
Does residual CNS count for the next dive?
Yes. It adds to the next dive’s CNS, and the total must stay under 80 %. Beyond 100 %, the NOAA exposure limit is exceeded.
When is CNS back to zero?
After 6 half-lives, i.e. 9 h with a 90 min half-life, only about 2 % of the surfacing value is left: the load is considered cleared.