No-Decompression Limit (NDL): What It Means

The no-decompression limit (NDL) is the longest time you can stay at a given depth and still ascend directly to the surface, at a normal ascent rate, without making decompression stops. The deeper you dive, the shorter your NDL.

This guide explains where the limit comes from, how fast it shrinks with depth and what to do if you run past it. It is general information, not a replacement for your course, your instructor or a dive doctor.

What the No-Decompression Limit (NDL) Means

The US Navy defines the NDL as the most time a diver can spend at a depth and still ascend straight to the surface, at the set ascent rate, without decompression stops. That time is bottom time: from leaving the surface until you leave the bottom to head up, not just the minutes at your deepest point.

Inside the limit you are on a no-decompression (no-stop) dive. Go past it and the dive becomes a decompression dive, with compulsory stops at set depths on the way up. Entry-level recreational training is built around no-stop diving.

Why the Limit Exists: Nitrogen

The air in your tank is about 79 per cent nitrogen. Your body does not use it, but under pressure it dissolves into your blood and tissues. Every 10 metres of seawater adds roughly one more atmosphere of pressure, the same pressure you feel in your ears on the way down (see our equalisation guide). The deeper you go and the longer you stay, the more nitrogen your tissues take up.

On the way up it runs in reverse: nitrogen moves from your tissues into your blood and out through your lungs. A slow ascent lets it leave in an orderly way. If the pressure drops faster than the gas can clear, it can form bubbles, which can cause decompression sickness.

Decompression models treat the body as "compartments" that load and unload gas at different speeds: blood is fast, bone is slow. Your NDL is the point where the model says a compartment would hold more gas than it can safely shed on a direct ascent. The video on this page, The Pressure Profile: The Anatomy of a Dive, walks through this, from the pressure on descent to nitrogen soaking into fast and slow tissues.

How the NDL Shrinks With Depth

The US Navy Diving Manual (Revision 7, 2016) is a public-domain document, and its no-decompression table for air dives (Table 9-7) shows how steeply the limit falls. The Navy works in feet of seawater; the metre figures are conversions.

| Depth (feet of seawater) | About (metres) | No-stop limit on air | |---|---|---| | 20 or shallower | 6.1 or shallower | No limit | | 30 | 9.2 | 371 minutes | | 40 | 12.3 | 163 minutes | | 50 | 15.3 | 92 minutes | | 60 | 18.4 | 63 minutes | | 70 | 21.5 | 48 minutes | | 80 | 24.5 | 39 minutes | | 100 | 30.6 | 25 minutes | | 110 | 33.7 | 20 minutes | | 130 | 39.8 | 12 minutes | | 140 | 42.9 | 10 minutes |

Going from about 9 to 12 metres more than halves the limit. The Navy reads the row equal to or next deeper than your maximum depth, so a dive to 19 metres uses the 70-foot row, not the 60.

Treat the table as an illustration, not a dive plan. DAN places the Navy limits at the "liberal" end of the range. Most recreational tables and computers sit between the Navy and the more conservative Canadian (DCIEM) tables, so for the same depth your course table or computer will usually give you less time. Dive to those numbers, not these.

How Deep Can You Dive Without Decompression?

There is no single depth, because the limit combines depth and time. In the Navy table, a single air dive to about 6 metres or shallower has no time limit. Below that, a dive stays no-stop if it is short enough: about an hour at 18 metres, 25 minutes at 30 metres, about 10 minutes at 40 metres. Recreational tables and computers allow less.

For a new diver, training sets the depth, not the NDL. Open Water certification qualifies you to 18 metres, and our guide to the Open Water training dives explains the depth limit on each course dive. Further courses go deeper, and 40 metres is the generally accepted recreational maximum. Depth brings other hazards too, notably nitrogen narcosis.

Dive Computers vs Dive Tables

A table assumes you spent the whole dive at your maximum depth. A computer tracks your depth continuously and recalculates, so on a dive that starts deep and works shallower it gives you more time. DAN notes the trade-off: longer dives add up to more nitrogen overall, and computer algorithms are mathematical models, not limits proven in controlled human trials.

From DAN's guidance:

- Different computers, settings and gases give different NDLs, so buddies can see different numbers on the same dive. Dive with someone whose set-up is similar, or follow the more conservative computer. - A computer only knows the dives it was worn on. Start a series of dives with it and wear it on every one; do not swap partway through. - It cannot tell how cold you are or how hard you are working, so a more conservative setting builds in a margin.

Our dive computer buyer's guide compares the options.

Repetitive Dives and Surface Intervals

You surface from every dive with extra nitrogen in your tissues, which the Navy calls residual nitrogen. It leaves gradually during your surface interval. Dive again before it has gone and the second dive is a repetitive dive: the leftover nitrogen counts against it, so your NDL is shorter than on a fresh dive to the same depth.

Tables track this with a letter group and a surface-interval table that adds "residual nitrogen time" to your next bottom time; a computer does the sums automatically. Either way, a longer surface interval buys more time next dive. DAN warns that a computer can be conservative on a first dive yet more liberal on repeat dives, so on multi-dive days and dive holidays, stay further from the limits.

Residual nitrogen is also why you wait before flying. The DAN guidelines suggest at least 12 hours after a single no-stop dive, 18 hours after several dives in a day or several days of diving, and much longer after any dive that needed decompression stops.

Safety Stops: Why a No-Stop Dive Still Has One

By the Navy's definition, a dive inside the NDL needs no stops: you can ascend straight up at 30 feet (about 9 metres) a minute. Recreational divers still pause near the top. The usual safety stop is three minutes at 5 metres on every dive, and DAN ranks adding a safety stop to every dive among the most important steps for decompression safety.

The stop is a margin for what the model cannot measure, such as cold, effort and individual differences. Some computers stretch it to five minutes or more when you come within five minutes of your NDL or dive deeper than about 30 metres. Holding a steady stop is a buoyancy skill worth practising.

The "rule of halves"

The rule of halves is NAUI's form of a deep stop. As NAUI's course material sets it out, when you end any no-stop dive deeper than 12 metres, you halve the distance from your deepest point to the surface, pause there for one minute, then carry on to 5 metres for the last two minutes of a three-minute safety stop. From 24 metres, you would pause at 12 metres, then at 5. On dives shallower than 12 metres, half your maximum depth is at or near the normal safety stop anyway.

Research on 25 metre dives found a stop at 15 metres, added to the usual shallow stop, reduced bubbles after the dive, though a follow-up study found a one-minute deep stop too short and recommended at least two and a half minutes for those dives. The rule of halves is one agency's practice, so if your course or computer prescribes something different, follow that.

What Happens If You Exceed the NDL

Go past your NDL and you take on a decompression obligation: you must stop at one or more depths on the way up, for set times, before it is safe to surface. Your computer will show the stops you now need. The US Navy's advice is to end the dive straight away, because the obligation builds quickly, and ascend making every stop the computer shows. It also warns that a diver who stays down can run out of air before the decompression is done. Never skip a required stop.

After a decompression dive or missed stops, watch yourself and your buddy for symptoms; our decompression sickness guide covers the warning signs and first aid. In Australia, DAN's 24-hour emergency hotline is 1800 088 200; call 000 in an emergency. Anyone with symptoms needs assessment by a doctor trained in diving medicine.

Planned decompression diving needs its own training: SSI's Decompression Diving specialty, for example, is limited to 40 metres and 15 minutes of total decompression, with anything more left to technical courses. If your NDL is running low, start your ascent while you still have no-stop time left.

Nitrox and Your NDL

Enriched air nitrox swaps some of the nitrogen in your tank for oxygen, so you take up less nitrogen at the same depth and your NDL gets longer. The Navy converts a nitrox dive to an "equivalent air depth": on 32 per cent nitrox at 60 feet (about 18 metres) you use its 50-foot air row, a 92-minute limit instead of 63. DAN notes you can also dive nitrox to air limits for an extra buffer.

The catch is oxygen, which becomes toxic at shallower depths as its share rises. DAN puts the recreational ceiling for 32 per cent nitrox at about 34 metres, and for 36 per cent at about 29 metres. Nitrox needs its own training and certification.

Staying Well Inside the Limit

The NDL is not a cliff edge. DAN describes decompression sickness risk as rising the closer you get to the limit. In a US Navy analysis of 163,400 shallow no-stop dives, decompression sickness struck about 1 in 10,000 dives that used a quarter of the allowed time, and about 4 in 10,000 when divers used more than three-quarters of it. A hyperbaric physician writing for DAN suggests staying within half the allowed time leaves a negligible risk.

So finish with time to spare, ascend slowly, make your safety stop, and widen the margin when the water is cold, you are working hard or it is your third dive of the day. DAN also advises against a hot shower straight after diving, because rapid warming can encourage bubbles.

Sources

- US Navy (NAVSEA): U.S. Navy Diving Manual, Revision 7 (2016), chapters 2, 9 and 10, incl. Table 9-7 and Table 10-1 (public-release copy hosted by Arizona State University) - DAN: A Critical Look at No-Decompression Limits (Nick Bird MD, Alert Diver, 2025) - DAN: Conservative Diving (Petar Denoble MD, Alert Diver, 2013) - DAN: Decompression Sickness, Chapter 1: Introduction to Decompression Sickness - DAN: Decompression Sickness, Chapter 2: Effective Use of Your Dive Computer - DAN: Decompression Sickness, Chapter 5: Factors Contributing to Decompression Stress - DAN World: 24/7 Emergency Hotline numbers - SPUMS Journal: Rogers, Developing the DSAT Dive Computer Model (1994) - Undersea and Hyperbaric Medicine: Bennett et al., Effect of varying deep stop times and shallow stop times on precordial bubbles after dives to 25 msw (2007) - NAUI: Unit 5, Dive Tables and Dive Computers (Nitrox course slides, Rule of Halves, hosted by Sub Sea Services) - SSI: How Deep Can Scuba Divers Go? Scuba Diving Depth Limits Explained - SSI: Decompression Diving, All You Need To Know

Header photo: An iDive DAN dive computer on a diver's wrist underwater, showing its no-decompression (NDL) display at 2.1 metres on a 32 per cent nitrox dive, by Peter Southwood, CC BY-SA 4.0, via Wikimedia Commons.