Bottom Time — What It Is and Why It Matters
Bottom time is the duration of the underwater portion of a dive. Different agencies and dive computers define the measurement slightly differently — some measure from the start of descent to the start of ascent, others from submersion to surfacing (total dive time). Your dive computer logs bottom time automatically, and its decompression calculations account for the entire time you spend under pressure.
Maximum bottom time is determined by your no-decompression limit at your dive depth (for recreational dives) or your planned decompression schedule (for technical dives). Gas supply is often the practical limiting factor before NDL is reached, especially at shallower depths where NDLs are long.
Frequently Asked Questions
What limits my bottom time?
Two things: your no-decompression limit (time-based, determined by depth and nitrogen absorption) and your air supply (gas-based, determined by tank size, depth, and breathing rate). Whichever runs out first ends your dive. At shallow depths, air typically runs out first; at deeper depths, NDL is usually the limiting factor.
Does my dive computer track bottom time or total dive time?
Most dive computers display and log both. The main display during the dive typically shows elapsed dive time (total time underwater). The dive log may separate bottom time from ascent time. For decompression purposes, the computer tracks all time spent under pressure regardless of which metric is displayed.
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