Estimate how long your tank will last at any depth based on your SAC or RMV rate. Includes rule-of-thirds turn pressure, depth comparison table, and no-deco limit warnings.
| Depth (m) | ATA | Duration (min) | Turn Press. | NDL Limit |
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One of the most fundamental questions in dive planning is: how much bottom time does my tank give me? The answer depends on three things working together — how much usable gas you have, how fast you breathe it, and how much pressure the water adds to every breath at depth.
A standard AL80 tank filled to 207 bar might give a calm diver 60 minutes at 10 meters, but only 20 minutes at 30 meters. Understanding this relationship before you enter the water is essential for safe gas management.
The rule of thirds is the gold standard of gas planning for recreational and cave/overhead diving. The principle is simple: divide your usable gas into three equal parts.
This calculator automatically computes your turn pressure — the pressure at which you must begin heading back. For example, if you enter with 200 bar and keep 50 bar reserve, your usable gas is 150 bar. One third of 150 bar is 50 bar, so your turn pressure is 200 − 50 = 150 bar.
The rule of thirds is mandatory in cave and cavern diving, and strongly recommended for all diving. It gives you a clear, unambiguous decision point that doesn't require mental arithmetic at depth.
Your SAC rate (Surface Air Consumption) is how many bar of gas you consume per minute breathing at the surface. It's unique to you and varies with your fitness, experience, water temperature, current, and workload.
Your RMV (Respiratory Minute Volume) is the same concept expressed in litres per minute, independent of which tank you're using. A typical RMV is 12–20 L/min at the surface. Because RMV is tank-agnostic, it's the preferred metric for technical gas planning.
The relationship between the two: RMV = SAC (bar/min) × tank internal volume (L). For example, a SAC of 1.8 bar/min on an AL80 (11.1 L) gives an RMV of about 20 L/min. This calculator accepts either format so you can use whichever you have from your dive log.
Pressure is the core reason depth has such a dramatic effect on tank duration. The atmosphere exerts about 1 bar of pressure at sea level. Water adds roughly 1 bar for every 10 meters of depth (10.06 m in seawater, 10.33 m in freshwater).
The total pressure you breathe at depth is expressed in ATA (Atmospheres Absolute):
This means if you have a SAC of 2 bar/min at the surface, you're consuming 8 bar/min at 30 metres. A full AL80 with 157 bar of usable gas that would last 78 minutes at the surface lasts only about 20 minutes at that depth. The depth comparison table in this calculator makes that relationship immediately visible across common recreational depths.
Use the SAC Rate Calculator to compute your SAC from your dive log data. Track it over time — as you gain experience and reduce anxiety, your SAC will naturally drop.
Drag and poor trim significantly increase your workload. A horizontal body position, tucked hoses, and no dangling equipment can reduce air consumption by 15–25%.
Shallow rapid breathing wastes gas. Breathe fully and slowly — inhale for 3–4 seconds, pause briefly, exhale for 4–5 seconds. Never skip a breath cycle, especially on ascent.
Cold increases your metabolic rate and causes you to breathe more. A properly sized wetsuit or drysuit appropriate to the water temperature pays dividends in both comfort and bottom time.
Brief your buddy on your turn pressure and the minimum pressure at which you'll begin a safety stop. If either diver hits their turn pressure first, both divers turn. No arguments underwater.
If you plan to practice out-of-air scenarios or donate air, factor that into your gas plan. Your reserve third is not for practice — it is genuinely for emergencies.
Use these tools together for complete dive planning: