Bottom Time

The duration from the start of descent until the start of the final ascent, the time figure used with depth in decompression planning.

Bottom time is the number you plan a dive around: on classic tables it runs from the moment you leave the surface until you begin your final direct ascent, so it includes the descent and everything at depth, but not the ascent or safety stop. Combined with maximum depth, it determines where you land against the no-decompression limit. Plan 22 m for 35 minutes and the table or computer tells you whether that fits inside the no-stop envelope.

The definition has soft edges across contexts, which trips people up. Dive computers typically log total dive time, from descent to surfacing, which is a few minutes longer than table-style bottom time. Technical divers often define bottom time as start of descent to start of ascent from the working depth, treating the staged ascent separately. When comparing dives or filling in a logbook, it matters which definition your numbers use; most digital logbooks simply record the computer's total dive time.

In everyday diving, bottom time is also the thing everyone wants more of. Diving nitrox, planning multilevel profiles, improving your SAC rate, and staying shallow where the reef is brightest are all legitimate ways to buy it. None of them require going deeper, which is worth remembering: depth is the one variable in the equation that only ever shortens the clock, while everything else can be trained, planned, or bought.