How to Read a No-Decompression Dive Table
What a dive table is
A no-decompression dive table is a printed chart that relates three things: how deep a dive goes, how long it lasts, and how much dissolved nitrogen the diver's body is assumed to carry afterward. Divers absorb nitrogen from breathing compressed air at depth, and the deeper and longer the dive, the more accumulates. Tables were the standard planning tool before dive computers, and they are still taught in most entry-level courses because they make the underlying logic visible. The NOAA Diving Program publishes its no-decompression tables for air and nitrox, and they are a good example of what a well-documented table set looks like.
This page explains the vocabulary so the chart is not a wall of numbers the first time you see one. It does not teach you to plan dives — that is what an instructor-led course is for.
The main pieces
Depth rows and bottom time
Tables are organized by depth, usually in ten-foot increments. For each depth there is a maximum time, often called the no-decompression limit or no-stop limit: the longest a diver can stay at that depth, under the table's assumptions, and still ascend directly to the surface at a controlled rate. Deeper rows have shorter limits. Bottom time, in table terms, generally runs from the start of descent to the start of the final ascent — a definition worth confirming for whichever table you are taught, because tables differ in their conventions.
Letter groups
After a dive, the table assigns a letter — often called a repetitive group or pressure group. The letter is shorthand for how much residual nitrogen the model assumes you are carrying. A short, shallow dive earns a letter near the start of the alphabet; a long, deep one earns a letter further along.
Surface interval
The surface interval is the time spent out of the water between dives. During it, the body gradually releases nitrogen, and the table moves you backward through the letter groups. A second table section converts your letter plus your surface interval into a new letter.
Residual nitrogen time
For a repetitive dive, the table converts your current letter group into residual nitrogen time: minutes that count against the next dive's limit before you even enter the water. This is why a second dive to the same depth has a shorter allowable bottom time than the first.
Why the numbers are conservative — and not guarantees
Tables are built from decompression models, which are simplified mathematical descriptions of how gas moves through tissues. They are deliberately conservative, but staying inside table limits does not eliminate the possibility of decompression sickness, and individual factors matter. The Divers Alert Network's health resources cover the background on decompression stress in far more depth, and the CDC Yellow Book chapter on dive-related injuries is the authoritative federal reference on the topic, including considerations such as the interaction between diving and subsequent air travel. Questions about your own situation — including when it is appropriate for you to fly after diving — belong with your training agency's standards and, where relevant, a physician familiar with dive medicine.
Tables versus dive computers
Most divers now wear a computer that tracks depth and time continuously and recalculates limits in real time, which is less conservative than assuming the whole dive happened at maximum depth the way a table does. Tables remain worth understanding for three reasons: they are the backup when a computer floods or dies, they are how courses teach the concepts, and published table sets like NOAA's document their assumptions in a way a computer's firmware does not.
When your course hands you a table, you now know what you are looking at: depth rows, no-stop limits, letter groups, surface-interval credit, and residual nitrogen. The application — planning real dives with real margins — is your instructor's job to teach.