What This Calculator Does
This Scuba Diving Bottom Time calculator estimates how long a diver can stay underwater before reaching a chosen reserve pressure. It is a pure gas-law (physics) tool and is universal — it is not tied to any country. All inputs use metric units: liters (L), atmospheres/bar (atm), meters (m), and liters per minute (L/min).
How to Use It
Enter your cylinder's internal water volume, the pressure at the start of the dive, the end (reserve) pressure you want to keep when you exit the water, your average dive depth, and your Surface Air Consumption rate (SAC, also called RMV). The result is the available dive time in minutes while still leaving the reserve in your tank. Setting an end pressure above zero is the main safety feature — it ensures the estimate stops before you run the cylinder empty.
The Formula Explained
$$T = \frac{\text{Tank Vol} \cdot \left(\text{Start P} - \text{End P}\right)}{\left(1 + \dfrac{\text{Depth (m)}}{10}\right) \cdot \text{SAC}}$$ The term \(V \times (P_{\text{start}} - P_{\text{end}})\) gives the usable breathing gas measured at the surface, in liters. The factor \(\left(1 + \frac{d}{10}\right)\) is the ambient absolute pressure at depth: each 10 m of seawater adds roughly 1 atm, so at 10 m you breathe gas twice as fast as at the surface. Dividing by this factor and by your SAC rate gives the minutes available.
Worked Example
Tank 10 L, start 200 atm, end 50 atm, depth 15 m, SAC 20 L/min. Usable gas = \(10 \times (200 - 50) = 1500\) L. Depth factor = \(1 + \frac{15}{10} = 2.5\). Gas at depth = \(\frac{1500}{2.5} = 600\) L. Dive time = \(\frac{600}{20} = 30\) minutes.
FAQ
Does this replace a dive computer or NDL tables? No. This is a planning estimate of gas duration only. It ignores no-decompression limits, nitrogen loading, ascent/descent and safety-stop gas, temperature and gas compressibility. Always respect your dive computer and tables.
Why divide by (1 + depth/10)? Because gas is breathed at ambient pressure. At 20 m the ambient pressure is about 3 atm, so a given lungful contains three times the surface mass of gas, draining the tank three times faster.
What if end pressure is higher than start pressure? Then there is no usable gas and the calculator returns 0 minutes. Your end (reserve) pressure must always be lower than your start pressure.
