What Is a No-Decompression Limit (NDL)? The Definition, in Plain English
The short answer
A no-decompression limit (NDL) is the longest bottom time a diver can spend at a given depth and still ascend directly to the surface — at the prescribed ascent rate — without being required to make scheduled decompression stops. It is a depth-specific number, published depth by depth in dive tables such as the NOAA no-decompression tables, and it gets smaller as depth increases. You will also see it called no-stop time, maximum bottom time, or simply the limit for that depth.
The key word in the definition is required. An NDL does not describe how long a dive is pleasant, or how long the gas supply lasts. It describes the boundary past which a table or computer stops permitting a direct ascent and begins prescribing stops at specified depths for specified times.
Why the limit exists at all
Breathing compressed air under pressure loads the body's tissues with dissolved inert gas, mostly nitrogen. Depth raises the rate of that loading and time extends it, so the gas burden at the end of a dive is a function of both. Ascending reduces the surrounding pressure and the dissolved gas has to come back out. Decompression procedures exist to control how fast that happens, because the condition they are designed to avoid — decompression sickness — is a recognized dive-related injury with a federal medical chapter of its own in the CDC Yellow Book.
A no-decompression limit is the modeled point at which a direct, controlled ascent is still considered an adequate way to offload that gas. Beyond it, the model says the ascent itself has to be staged.
If the underlying idea of leftover dissolved gas is new to you, start with residual nitrogen in diving and then come back to the limit.
Where the number actually comes from
NDLs are not measured off an individual diver. They are outputs of a decompression model — a mathematical description of how gas moves into and out of theoretical tissue compartments — which is then tested, adjusted, and published as a table or programmed into a dive computer's algorithm.
That matters for two practical reasons:
- Different published sets give different numbers. The NOAA Diving Standards and Safety Manual documents the limits, ascent rates, and repetitive-dive procedures that go with NOAA's own tables. A recreational training agency's tables come as their own package. The limits are not interchangeable line by line.
- An NDL is inseparable from its procedures. A limit assumes a particular ascent rate, a particular definition of bottom time, and often a particular safety-stop convention. Read off the number without those, and you are not really using the table.
Organized diving programs formalize which set applies: scientific programs operating under the AAUS standards for scientific diving, for instance, specify dive-planning and depth-authorization requirements rather than leaving the choice of table to the individual.
How depth and repetitive diving change the number
Two things move an NDL:
Depth. Tables list limits against depth increments, and the deeper row always carries the shorter time. Tables are also built around the deepest depth reached, not the average — a brief excursion to the deepest point of a dive is charged as if the whole bottom time happened there. That is one reason the number on a table and the number on a computer rarely match.
Nitrogen you brought with you. On a second or third dive of the day, part of the allowance is already spent. Tables handle this by converting your post-dive state and surface interval into a residual nitrogen time, which is subtracted from the depth's full limit to leave a shorter allowable time. The mechanics of that subtraction are covered in residual nitrogen time explained, and the question of which dives even count is covered in what counts as a repetitive dive.
Related terms people mix up
- Decompression dive — a dive planned to exceed no-decompression limits, with stops built into the plan from the start. Requires specific training, gas planning, and equipment.
- Safety stop — a short, optional-by-convention pause near the surface on a no-decompression dive. It is not a decompression stop, and it does not extend an NDL.
- Bottom time — the interval the table tells you to count, usually from leaving the surface to beginning the final ascent. The definition belongs to the table you are using.
- Omitted decompression — what a table calls it when required stops were not made. Tables publish separate procedures for this case.
Once the vocabulary is straight, the table itself is mostly arithmetic. How to read a no-decompression dive table walks through the grid row by row, and the Divers Alert Network health resources library is the place to look for the decompression-physiology background behind the numbers.