What Do the Numbers on a Dive Computer Mean?

Look over a diver's shoulder at their wrist and you'll see a small screen crowded with numbers. None of them are complicated individually; the confusion comes from meeting them all at once. Here's what each one is, and why it's there. This page explains terminology — it is not dive planning instruction, which belongs in a course with an instructor.

Depth

Displayed in feet in the US, meters most other places (33 feet is about 10 meters). Depth matters because water is heavy: pressure increases the deeper you go, roughly one additional atmosphere for every 33 feet of seawater. That pressure is the root of almost every other number on the screen. The physiology of pressure and dissolved gas is covered rigorously in the StatPearls chapter on decompression sickness if you want the clinical-grade version.

Tank pressure

Measured in psi (pounds per square inch) in the US, bar elsewhere. A common aluminum tank starts a dive around 3,000 psi, and the number falls as you breathe. It's a fuel gauge: divers are trained to plan turnaround and ascent points around it, and to surface with a reserve rather than an empty tank.

No-decompression limit (NDL)

The number newcomers find most mysterious. While you're underwater, your body absorbs nitrogen from the compressed air you breathe; the deeper and longer the dive, the more you absorb. The NDL is the computer's running estimate of how much longer you can stay at your current depth and still ascend directly to the surface, at a safe rate, without required decompression stops. Go deeper and the NDL shrinks; ascend shallower and it grows. What happens when nitrogen comes out of solution too fast is decompression illness, described in detail in the CDC Yellow Book chapter on dive-related injuries — the entire recreational system of limits, slow ascents, and safety stops exists to keep divers far away from it.

Ascent rate

Computers monitor how fast you're rising and warn if it's too fast, because a controlled ascent gives absorbed nitrogen time to leave your body gradually. Most also prompt a safety stop — a short pause, typically a few minutes around 15 feet, near the end of a dive. It's a routine margin, not an emergency procedure.

Surface interval

The clock that runs between dives. Residual nitrogen from the first dive affects the limits of the second, so the computer tracks your time on the surface and adjusts. Divers Alert Network publishes approachable explainers on how repetitive diving and surface intervals interact if you want to go deeper than a glossary.

None of these numbers requires math on your part — the computer does the modeling. What your course teaches is the judgment: reading them early, often, and conservatively.