What the rule says
The 120 rule is simple: add your depth in feet to your planned bottom time in minutes, and keep the total at or under 120. Dive to 60 feet, about 18 metres, and the rule allows roughly 60 minutes of bottom time. Dive to 80 feet, about 24 metres, and it allows roughly 40 minutes. It's a rough guide to staying inside no-decompression limits for a single dive, worked out with simple arithmetic rather than a table or a computer.
- 60 feet (about 18 m): roughly 60 minutes
- 70 feet (about 21 m): roughly 50 minutes
- 80 feet (about 24 m): roughly 40 minutes
Where it came from
The rule predates dive computers, and it was never meant to be exact. It's a memorable approximation of the no-decompression limits set out in the recreational dive tables that were standard before computers took over dive planning, close enough to be useful as a mental sanity check without needing a table in hand. Different agencies' tables give slightly different numbers at any given depth, which is part of why some instructors teach a rule of 110 instead: it's the same idea tuned to a different table.
Before dive computers became standard kit, divers planned every dive against a printed table, working out their nitrogen loading by hand before they even reached the boat. A rule like this one existed to give divers a quick way to check that plan against gut instinct, not to replace the table itself.
What it doesn't account for
This is where the rule matters less than it first sounds. It's built around a single, first dive of the day, at an ordinary temperature, in normal conditions. It says nothing about repetitive dives, where you're carrying residual nitrogen from an earlier dive into your next one, nothing about cold water, which increases decompression sickness risk, and nothing about a change in depth partway through a dive, which is exactly what happens on a drift dive. Modern dive computers track all of that continuously and are generally more conservative and more accurate as a result.
Applying it to Nusa Penida's drift dives
Not well, at least not on its own. Almost every dive around Nusa Lembongan and Nusa Penida is part of a multi-dive day, and most are drift dives where your depth shifts with the reef profile and the current rather than staying constant, often with a cold thermocline at depth during mola mola season. Those are exactly the conditions the 120 rule wasn't designed for. A dive computer, which adjusts continuously for your actual depth, time and repetitive dive history, is the tool that actually fits how diving works here. See our safety and conditions guide for more on the currents and thermoclines involved.
A second dive later the same day compounds this further: your body is already carrying nitrogen from the first dive, so the time you have available on the second dive is shorter than the rule alone would suggest, at any depth.
Why it's still worth knowing
None of that makes the rule pointless. It's a useful way to understand, in plain terms, why deeper dives have to be shorter, and it's a good sanity check if you ever find yourself planning a dive without a computer. Understanding the logic behind it also makes your dive computer's numbers make more sense, since a computer is really doing the same job, tracking depth against time against nitrogen loading, with far more precision and far less guesswork.
Using a computer instead
If you don't own a dive computer, Sober Diving rents them for IDR 150,000 a day, and it's worth using one rather than relying on mental arithmetic, especially on drift dives where your depth can change more than once during a single dive. Your instructor or guide can talk you through reading your specific computer's display during your dive briefing if it's your first time using one.
This is also part of what a briefing covers before every dive here: your guide will ask what you're diving with, computer or otherwise, and plan the group's dive around whoever has the most conservative limit rather than the most optimistic one.
