No-Decompression Limit (NDL) Calculator

The No-Decompression Limit (NDL) is the maximum time you can spend at a given depth on a single dive without needing to make decompression stops on ascent. Exceeding the NDL means you must stop at specific depths to allow nitrogen to safely off-gas.

PADI recreational dive table NDLs (first dive, no prior dives):

• 10 m (33 ft): 219 min
• 12 m (40 ft): 147 min
• 14 m (47 ft): 98 min
• 16 m (53 ft): 72 min
• 18 m (60 ft): 56 min
• 20 m (66 ft): 45 min
• 22 m (72 ft): 37 min
• 25 m (82 ft): 29 min
• 30 m (99 ft): 20 min
• 35 m (115 ft): 14 min
• 40 m (132 ft): 9 min

Safety stop: Always perform a 3-minute safety stop at 5 m (15 ft) before surfacing, even if within NDL. This is not mandatory but reduces risk of decompression sickness significantly.

Repetitive dives: After a dive, residual nitrogen in your tissues means your NDL for the next dive at the same depth is shorter. Dive computers handle this automatically.

Decompression sickness (DCS): Caused by nitrogen bubbles forming in tissues on rapid ascent.
Symptoms: joint pain, skin mottling, numbness, shortness of breath. Treatment: a hyperbaric oxygen chamber.
Never dive alone. A buddy can spot early DCS symptoms that you will explain away in yourself.

What limits the dive is usually your gas, not the table

New divers plan to the NDL and are then surprised to surface long before it. At 18 m the table allows 56 minutes, but a diver breathing 20 liters a minute at the surface is breathing 56 at that depth, and the 1,800 usable liters in a single 12 liter cylinder at 200 bar run out at about 32 minutes.

Work that through at other depths and an uncomfortable pattern appears: on a single 12 liter cylinder the gas runs out first at every recreational depth, shallow ones included. At 10 m the table allows 219 minutes and the cylinder gives you 45. The no-decompression limit only becomes the real constraint if you carry more gas, which is one of the quieter reasons experienced divers move to 15 liter steels and twinsets.

Two numbers decide it, and both are inputs here rather than assumptions. Surface air consumption (SAC) varies more between divers than almost anything else: a relaxed, experienced one may sit near 12 liters a minute, while a nervous one in cold water can double that, so it is worth measuring your own. Cylinder size does the rest. The calculator assumes a 200 bar fill and a 50 bar reserve, so usable gas is 150 bar times the cylinder volume: 1,800 liters from a 12, 2,250 from a 15, 3,600 from twin 12s.

Depths are seawater. The calculator treats 10 m as one bar, the usual planning simplification. Fresh water is about 3% less dense, so the same pressure needs about 3% more depth in a lake or quarry than in the sea.