Scuba Diving Air Consumption Calculator

Tank duration tells you how long your scuba cylinder will last at a given depth before you must begin your ascent. Planning it matters, because running out of air underwater is one of the worst situations a diver can be in.

Formula: Duration (min) = (Usable gas volume) / (SAC rate × Absolute pressure)

Step-by-step breakdown:

1. Usable gas volume (liters) = (Start pressure − Reserve pressure) × Tank volume Example: (200 bar − 50 bar) × 12 L = 1,800 liters

2. Absolute pressure at depth = 1 + (Depth in meters / 10) At 20 m: 1 + 20/10 = 3 bar (absolute) At the surface: 1 bar (absolute)

3. Duration = Usable volume / (SAC × Absolute pressure) Example: 1,800 / (20 × 3) = 30 minutes

SAC rate (Surface Air Consumption): Your SAC rate is how many liters of air you breathe per minute at the surface. It varies by fitness, experience, and stress level.

• Beginner diver: 25–30 L/min
• Recreational diver: 15–20 L/min
• Experienced diver: 10–15 L/min
• Technical diver: 10–12 L/min

Common tank sizes:

Safety note: Always plan to surface with a minimum of 50 bar (500–600 liters) reserve. This gives you air for a safety stop at 5 meters (3 minutes minimum) and margin for emergencies. Never plan a dive that uses your entire tank. Conditions change underwater, and a current on the way back can double the rate you are breathing at exactly the point you have least left.

What this number does not include

The figure here is bottom time at a fixed depth. It does not cost the descent, the ascent, or the time spent shallower on the way home, and it assumes your SAC rate holds steady, which it will not if the water is cold, the viz is poor, or you are working against a current. Treat the answer as an upper bound on a good day, and plan the dive shorter than it says.