What Do the Letters on a Dive Table Mean? Pressure Groups Explained
The short answer
The letter you land on after a dive — A, B, C, and up — is a pressure group (some tables say repetitive group or group designation). It is shorthand for the amount of leftover dissolved nitrogen the table's model credits you with at that moment, with letters further into the alphabet meaning a larger residual load. It is a bookkeeping label, not a measurement of anything in your body, and it is only meaningful inside the table that produced it.
What the letter actually encodes
A dive table does not track nitrogen in units a diver would recognize. It sorts the possible post-dive states into a set of bands and gives each band a letter. When you enter the table with a depth and a bottom time, you come out with the band your profile falls into.
Because the bands are coarse, two fairly different dives can produce the same letter, and a dive one minute longer can push you into the next letter. That is expected — the letters are rungs on a ladder, not a continuous readout. The underlying physical idea, that you surface still carrying dissolved gas from the dive, is covered in residual nitrogen in diving.
How the letter moves through a day of diving
On a classic three-table layout, the letter is the thread connecting all three grids:
- After the dive. Depth and bottom time go into the first table; a letter comes out.
- During the surface interval. The letter goes into the surface-interval table along with the time you've been out of the water. Off-gassing at the surface moves you toward the front of the alphabet — the longer the interval, the earlier the letter. This is the step people mean when they say a surface interval "cleans you up."
- Before the next dive. The new letter plus your planned next depth goes into the residual-nitrogen table, which returns a residual nitrogen time in minutes. That number is charged against the depth's full limit, leaving less time available. The arithmetic is broken down in residual nitrogen time explained.
So a letter is never the end of a calculation. It is a token you hand to the next grid.
Four things that trip people up
Letters are not portable between tables. An E from one publisher's table does not mean an E on another's. Each table's letters are defined by that table's own bands and procedures. The NOAA no-decompression tables come with their own worksheets and repetitive procedures, documented in the NOAA Diving Standards and Safety Manual; mixing that letter into a different publisher's surface-interval grid produces a number that means nothing.
Letters do not translate to a dive computer's display. A computer runs a multi-compartment algorithm and recalculates continuously; it has no reason to show a letter at all. If you dove on a computer and then want to plan on paper, you are not converting a state — you are starting a table calculation from the profile data, with whatever rounding rules the table specifies.
Deepest depth, whole bottom time. Tables charge your entire bottom time at the deepest depth you reached. A short drop to the deepest point of the dive changes which row you use, which can change the letter. It is one of the biggest sources of difference between table letters and computer readouts.
A letter says nothing about an individual. The bands come from a decompression model applied to a theoretical diver. They are not personalized, and the letter is not a health indicator. Decompression physiology, individual susceptibility, and related considerations sit with medical references such as the Divers Alert Network health resources and the CDC Yellow Book dive chapter, not with the alphabet on a slate.
Why the system is built this way
The letter system exists because it makes repetitive-dive planning doable with a plastic card and a pencil, on a moving boat, with cold hands. Instead of recomputing a gas-loading model, you carry one character forward through three lookups. It is a compression scheme for a calculation — deliberately simple, deliberately conservative in its rounding, and only valid when you follow that table's rules for rounding depths up and times up.
To see the grids in context and follow a letter from the first dive to the second, work through how to read a no-decompression dive table. If you are unsure whether your next dive even needs the repetitive grids, what counts as a repetitive dive covers the clock that decides it.