Dive computers and NDL: what the numbers on your wrist mean

You’ve been photographing a turtle, and when you glance at your wrist, the NDL has dropped to single digits. Time to move shallower or start up: that number counts down to mandatory decompression stops, and it says nothing about your air.

What does a dive computer do?

A dive computer measures depth and time throughout the dive. From them, a built-in model estimates how much nitrogen your body has absorbed (Henry’s law) and shows how much time you have left. It also watches your ascent rate, guides your safety stop, counts your surface interval and gives a no-fly time. Before diving, check its battery, units, gas and conservatism settings.

What is the no-decompression limit (NDL)?

The NDL is how long you can stay at your current depth and still ascend straight to the surface without mandatory decompression stops. It says nothing about your air. The deeper you are, the shorter it gets, because you absorb nitrogen faster at depth. Go up a few meters (feet) and it grows again: on a multilevel dive, moving shallower gives you more time than staying at your deepest point.

An NDL isn’t a guarantee. Writing for DAN, hyperbaric physician Nick Bird explains that the risk of decompression sickness rises as you get closer to the limit, and that staying within 50% of the allowed bottom time carries negligible risk. End the dive or move shallower well before the number runs out.

Try a question from the app

what the NDL is

What does your computer’s no-decompression limit (NDL) show?

1. AThe dive time left before your air runs out
2. BTime left at this depth before stops become mandatory
3. CHow long you must wait at the surface before your next dive
4. DThe maximum depth allowed for this dive

Show the answer

Answer B. The NDL is how long you can stay at your current depth and still ascend directly, without mandatory decompression stops. It says nothing about your air.

Why do two computers show different NDLs?

Because computers use different models and settings. DAN notes that unless two divers use the same computer with the same gas and safety settings, they’ll see different NDLs throughout the dive. A buddy pair follows the more conservative of the two.

Your computer is also personal: it only knows the dives it was on. Wear the same one for every dive of the day and of the trip, and never swap or borrow one between dives. On your next dive, it counts the nitrogen left from the last one and gives you shorter limits.

How fast should you ascend, and how long is a safety stop?

Ascend slowly. DAN reports that the US Navy and NOAA use 9 m (30 ft) per minute, and that training agencies recommend between 9 and 18 m (30 and 60 ft) per minute. Slower is better: follow your computer’s ascent alarm, and take the last few meters (feet) most slowly, where your BCD’s air expands fastest.

On every dive, unless an emergency prevents it, stop at 5 m (15 ft) for 3 minutes before surfacing. The MSD Manual recommends 3 to 5 minutes at 15 ft (4.6 m), and DAN calls the safety stop probably the most significant advance in recreational decompression safety in 30 years. Some computers lengthen the stop after a deep dive or close to the limit: follow yours.

What should you do if you overstay your NDL?

If you stay too long or go too deep, your computer leaves no-stop mode and shows a required stop: a depth you must not go above, often called the ceiling, and a time. It’s no longer optional.

1. Stay calm and tell your buddy.
2. Ascend slowly, no faster than your computer allows, to the depth it shows.
3. Stay at that depth, or slightly deeper, until the stop clears. Never above it.
4. Check your gas. If you run low, signal your buddy early and share gas rather than skip the stop.
5. Once out, tell the dive leader, stay out of the water for at least the rest of the day, longer if your computer or dive center says so, and watch for signs of decompression sickness.

At this level, a required stop means the plan went wrong. Displays differ between brands: read your computer’s manual.

Try a question from the app

stop depth

During a required stop, at what depth should you stay?

1. ASlightly above the depth shown, where it’s calmer
2. BAnywhere between the stop depth and the surface
3. CAt the depth shown, or slightly deeper, never above
4. DWell below it, so the stop clears faster

Show the answer

Answer C. The displayed depth is a ceiling: stay at it or just below. Going above it means skipping part of the stop, while staying much deeper slows the release of nitrogen and makes the stop last longer.

What if your computer fails during the dive?

Signal your buddy, end the dive and ascend slowly with a safety stop, using your buddy’s computer as a guide. A replacement computer doesn’t know the nitrogen you’ve absorbed, so ask a professional and follow the maker’s instructions before diving again. Procedures after a failure differ between agencies and makers: follow your training.

Which dive training levels cover dive computers?

Open Water Diver teaches the NDL, ascents, safety stops and flying after diving. Advanced Open Water covers deep dives and overstaying your limit, and Divemaster explains the decompression models inside your computer. Studying for your first exam? The Open Water study plan and practice questions both cover the NDL.

Frequently asked questions

What does NDL mean on a dive computer?

NDL stands for no-decompression limit: the time you can still stay at your current depth and ascend directly to the surface without mandatory decompression stops. It’s also called the no-stop limit. It shrinks as you go deeper and grows when you move shallower, and it says nothing about how much air you have left.

Is it safe to dive right up to the NDL?

Not as safe as it looks. DAN’s Nick Bird explains that the risk of decompression sickness rises the closer you get to the limit, even without crossing it. Treating the NDL as a safe line is the first of the dive theory myths. Keep a margin: start up with time left, and keep a bigger one when the water is cold, the dive is deep or you’re working hard.

Can you borrow someone else’s dive computer?

Not between dives of the same day or trip. A computer only knows the dives it was on, so one that missed your earlier dives thinks you have no extra nitrogen. DAN advises never relying on a friend’s computer and following your own. If yours fails, ask a professional before diving again.

How long should you wait to fly after diving?

DAN’s guidelines call for at least 12 hours after a single no-decompression dive, 18 hours after several dives a day or several days of diving, and substantially longer after dives with required stops. Your computer also shows a no-fly time: follow the longer of the two.

Sources

1. Nick Bird, A Critical Look at No-Decompression Limits, DAN Alert Diver (2025)
2. DAN Alert Diver, Ascent Rates (2012)
3. DAN Alert Diver, Defensive Dive Profile Planning (2016)
4. DAN Alert Diver, Trouble in Galápagos (2022)
5. DAN, Guidelines for Flying After Diving (2002 workshop)
6. MSD Manual Consumer Version, Diving Safety Precautions and Prevention of Diving Injuries