No-decompression limit calculator
How long you can stay at depth before you owe the water a decompression stop. Air or nitrox, metric or imperial.
Free to use, no account needed. The calculator loads below.
What a no-stop limit actually is
Your no-decompression limit, or NDL, is the longest you can stay at a given depth and still ascend straight to the surface without owing a mandatory decompression stop. It is not a countdown to injury. It is the point at which the model you are diving stops promising you a direct ascent.
The number falls sharply with depth, because the pressure driving nitrogen into your tissues rises with depth. At 18 metres a diver on air has the better part of an hour. At 30 metres the same diver has around twenty minutes. At 40 metres it is single figures. That collapse is the reason depth is the first thing any plan fixes.
Which model this uses, and why that matters
This calculator runs Buhlmann ZH-L16C, the decompression model used by most modern dive computers, compiled from the same C++ that runs inside the Diving Standard iPhone and Apple Watch apps. Nothing is reimplemented for the browser, so the number here is the number your watch would give you.
Different models and different conservatism settings give different answers, sometimes by several minutes. A no-stop limit is a model's opinion, not a physical constant. Treat any calculator, this one included, as a planning input rather than an authority.
Nitrox buys you time, not depth
Breathing a mix with more oxygen and less nitrogen loads your tissues more slowly, so your no-stop time goes up. What it does not do is let you go deeper: the extra oxygen brings its own ceiling, and the maximum operating depth of the mix arrives before any benefit does.
The equivalent air depth shown alongside the limit is the depth at which air would load you the same way this mix does. It is the honest way to read a nitrox plan, and it is why a diver on EAN32 at 30 metres plans as though they were at about 24.
Common questions
What is a no-decompression limit?
The longest you can stay at a given depth and still ascend directly to the surface, at a safe rate, without a mandatory decompression stop. Past it you owe the water stops on the way up.
Does this calculator account for repetitive dives?
No. It gives the first-dive limit on clean tissues. Residual nitrogen from an earlier dive shortens your next no-stop time considerably, and tracking that across a day needs your actual dive profiles, which is what the app and the watch do.
Why does my dive computer show a different number?
Almost every computer lets you dial in conservatism, usually as gradient factors, and many use a different model or a different set of tissue compartments. A more conservative setting shortens the limit on purpose. Neither number is wrong; they are different opinions from different models.
Is a safety stop the same as a decompression stop?
No. A safety stop is optional, usually three minutes at five metres, and you make it even when you are well inside your no-stop time. A decompression stop is mandatory: skipping it means surfacing with more dissolved gas than the model considers safe.
The rest of it lives in the app
The app tracks your actual profile, so it knows what your last dive left in your tissues and shortens the next limit accordingly. On the watch it runs live, in the water, and counts down as you dive.