What Is Residual Nitrogen Time (RNT)? Reading the Number on a Dive Table
The short answer
Residual nitrogen time (RNT) is the nitrogen left over from an earlier dive, restated as a number of minutes. On a repetitive dive, those minutes are added to the time you actually spend at depth, and the sum is what gets compared against the depth's no-decompression limit. In other words, RNT is a penalty in the currency the table already speaks: minutes.
This page is about what the numbers mean and how the arithmetic is laid out. It is not a planning procedure — use the tables, computer, and training you dive with.
The three abbreviations
Table-based planning uses a small vocabulary that trips people up mainly because the acronyms look alike:
- RNT — residual nitrogen time. Minutes charged to you before you get wet, based on your pressure group and the planned depth of the next dive.
- ABT — actual bottom time. The minutes you really spend from leaving the surface to beginning your ascent.
- TBT — total bottom time. RNT + ABT. This is the figure the table treats as your exposure.
The consequence is straightforward: on a repetitive dive, the time available to you at a given depth is the limit minus RNT, not the full limit. A diver who surfaces carrying a heavy modeled gas load and drops straight back down can find that most of the allowance is already spent.
Why the same letter gives different RNT values
RNT is not a single number attached to a pressure group. It is read from a grid where one axis is the group letter and the other is the planned depth of the next dive, because the same quantity of dissolved gas translates into different amounts of "time already used" depending on how fast the next dive loads you. A shallower repetitive dive converts a given letter into a larger number of minutes; a deeper one converts it into fewer, because minutes at that depth count for more.
The NOAA Diving Program publishes the air and nitrox tables where this grid appears, and the procedures for using them — including how repetitive dives and residual nitrogen are handled within the program — are set out in the NOAA Diving Standards and Safety Manual. If the three-table layout is unfamiliar, start with how to read a no-decompression dive table.
The surface interval is the other half of the number
RNT falls as surface time accumulates, because the model is offgassing you the whole time you sit on the boat. That is why the surface-interval table sits between the end-of-dive table and the RNT table: you enter with the letter you surfaced with, spend time, and come out with an earlier letter that produces a smaller penalty. Longer intervals buy back minutes; short ones do not.
For the underlying concept of what is actually being tracked, see what residual nitrogen is. For the boundary conditions — when a dive even counts as repetitive — see what counts as a repetitive dive.
Rounding conventions are part of the number
Tables are built to be rounded in one direction only: depth up to the next listed depth, time up to the next listed time. A 47-foot dive is planned as the next deeper row; 23 minutes is taken as the next longer column. Rounding the other way to make a plan fit is not a shortcut, it is a different and unsupported calculation. Every published table also carries its own instructions, and those instructions are part of the tool.
What a computer does with the same idea
Dive computers never show you an RNT value, but they apply the identical concept invisibly. Rather than issuing a lump-sum penalty at the start of a repetitive dive, they carry their modeled compartment loadings forward continuously through the surface interval, so the remaining no-decompression time displayed on the second dive is already net of everything left over from the first. Turning a computer off, swapping units between dives, or handing one to a different diver discards that history — which is precisely the information RNT was invented to preserve.
The medical background on why any of this conservatism exists, including decompression illness and post-dive flying considerations, is covered by Divers Alert Network's health reference library.