How Do You Read a Dive Table? The Rows, Columns, and Letters Explained
A dive table looks intimidating the first time you see one: a grid of depths, times, and letters that appears to be written in code. It is actually one of the most readable documents in diving once you know what each part represents. This page explains the anatomy of a no-decompression table — it does not teach you to plan dives with one. That is a skill your certification course teaches and drills, for good reason.
If you want an authoritative table to look at while you read, NOAA publishes its no-decompression air and nitrox tables for free, alongside the NOAA Diving Standards & Safety Manual. Agencies' recreational tables differ in details, but the structure below is common to nearly all of them.
The depth rows
Each row of the main table corresponds to a maximum depth, usually in ten-foot or few-meter increments. The convention is simple and conservative: you use the row for your deepest point during the dive, even if you only touched that depth briefly, and if your depth falls between two rows you round down the page to the deeper row. Depths are given in feet or meters of seawater — units that are really pressure measurements in disguise, as explained in our guide to fsw, msw, ATA, and bar.
The no-decompression limits
Running along each depth row are times. The largest time in the row is that depth's no-decompression limit (NDL): the maximum time a diver can spend there and still, according to the model behind the table, ascend directly to the surface at the prescribed rate without required decompression stops. Deeper rows have shorter limits, because the body absorbs nitrogen faster under greater pressure. The physiological reasoning — inert gas dissolving into tissues under pressure and coming back out as pressure drops — is laid out in detail in the National Library of Medicine's peer-reviewed article on decompression sickness.
The pressure-group letters
After a dive, tables assign you a letter — A, B, C, and so on — called a pressure group or repetitive group. It is shorthand for how much nitrogen the model estimates is still dissolved in your body. A short, shallow dive leaves you in an early letter; a long, deep one pushes you toward the end of the alphabet. The letter means nothing by itself; it exists to feed the next part of the table.
Surface intervals and repetitive dives
The second section of a table tracks what happens during your surface interval, the time spent out of the water between dives. As you off-gas at the surface, your letter migrates back toward A. When you plan a second dive, the table converts your current letter into residual nitrogen time — minutes effectively already "spent" at your next depth before you even descend — which shortens the allowable time for the repetitive dive. This bookkeeping is the whole reason the letters exist.
Surface intervals matter beyond the next dive, too. Time before flying or otherwise ascending to altitude after diving is its own topic with its own published guidance; the CDC Yellow Book chapter on scuba diving covers it, and your training agency's materials will state the intervals it teaches. We deliberately do not quote numbers here — get them from those sources and your instructor.
What a table is not
A table is a model of an average diver, not a measurement of you. Dive computers have largely replaced tables for minute-to-minute tracking, but training agencies still teach tables because they make the underlying logic visible in a way a wrist display never does. Read the table as a document; use it as a tool only with the training that comes with it.