Methodology & honest scope
The core formulas
- Absolute pressure (ATA): depth in metres / 10 + 1, or depth in feet / 33 + 1 (Boyle's law: gas volume scales with absolute pressure).
- SAC from a dive log: (pressure used × cylinder water capacity) / (ATA × time). Imperial inputs are converted to bar first, so the same ideal-gas math applies.
- Available gas: cylinder water capacity × gauge pressure (AV = CV × CP).
- Gas needed: SAC × ATA × bottom time, plus reserve.
- Rule of thirds: turn pressure = ⅔ of start pressure; total planned gas = needed × 1.5.
- Minimum (rock-bottom) gas: stressed SAC × average ATA over the ascent × (depth / ascent rate + safety stop), for two divers.
Cylinder data sources
Cylinder water capacities and working pressures are compiled from published manufacturer specifications (Luxfer aluminum cylinder specification sheet: AL63 = 9.0 L @ 207 bar, AL80 = 11.1 L @ 207 bar) and cross-checked against community references on scubaboard.com (internal volumes 677/787/793 cu in for AL80/HP100/LP85; HP100 = 12.9 L @ 3442 psi). Metric steel 12/15/18 L cylinders use the standard 232 bar working pressure.
What these numbers can and cannot do
Honest scope: free-gas figures are computed with the ideal-gas law, the standard planning method. Real capacity is typically a few percent lower (compressibility), gauges have error, and your SAC varies with exertion, cold, and stress — always plan with your stressed SAC and a real reserve. Square profiles are assumed (conservative for multi-level dives). This tool does not model decompression, nitrogen loading, or oxygen exposure.
Privacy
All computation happens in your browser. No data is sent anywhere — the page works fully offline once loaded, and your SAC log never leaves your device.
Scuba gas planning — frequently asked questions
How do I calculate my SAC rate?
Record your start and end tank pressure, average depth, and dive time from a real dive — that is all a scuba diving air consumption calculation needs. SAC = (pressure used x cylinder water capacity) / (absolute pressure in ATA x time in minutes). Example: 150 bar used from a 12.2 L cylinder at 20 m (3 ATA) over 30 minutes = 150 x 12.2 / 3 / 30 = 20.3 L/min. Use Mode 1 above — this dive air consumption calculator does the unit math for metric and imperial.
What is the difference between SAC rate and RMV?
They describe the same thing: your air consumption expressed as a surface-equivalent volume per minute. SAC (Surface Air Consumption) is the common recreational term; RMV (Respiratory Minute Volume) is the same number used more in technical diving. Both are usually given in L/min or cu ft/min. Use Mode 2 — a respiratory minute volume calculator — to convert between L/min, cu ft/min, and psi/min.
How do I calculate SAC rate in imperial units (psi)?
Convert your pressure drop from psi to bar first, then use the same formula. Example: 1500 psi used from an AL80 (11.1 L water capacity) at 66 ft (3 ATA) over 30 minutes = (1500 / 14.5) x 11.1 / 3 / 30 = 12.8 L/min, or 0.45 cu ft/min. Mode 1 is both a sac rate calculator in psi and a sac rate calculator in bar — whether you searched for the sac rate calculator metric, imperial, psi or bar version, switch the unit system above and it converts automatically.
What is a good SAC rate for a recreational diver?
Most relaxed recreational divers fall between 12 and 25 L/min (0.4–0.9 cu ft/min). New divers often see 25–35 L/min, and heavy exertion, cold water, or stress can push anyone well above their normal rate. Track several dives and use your average for relaxed planning — and your stressed rate for conservative planning.
What is the rule of thirds in gas planning?
Use one third of your gas for the outward leg, one third for the return, and keep one third as reserve. In practice: turn pressure = 2/3 of your starting pressure (e.g. turn at ~133 bar from a 200 bar start). It is the standard reserve rule for overhead and deep diving, and a sensible default for any dive plan.
What is rock bottom gas / minimum gas?
Rock bottom (minimum gas) is the amount of gas two divers need to reach the surface safely from depth, usually calculated at an elevated (stressed) SAC: ascent time at ~10 m/min plus a safety stop, averaged over the ascent pressure. When your remaining gas equals your rock-bottom number, the dive is over — ascend. Mode 5 computes it for your tank and depth.
How much gas do I need for a planned dive?
Gas needed = your SAC x absolute pressure (ATA) at depth x bottom time, plus a reserve. Example: 20 L/min SAC at 30 m (4 ATA) for 25 minutes = 2000 L, and with a rule-of-thirds reserve you plan for 3000 L total. Mode 4 is a scuba gas calculator. As a scuba / diving gas consumption calculator it compares that against your actual cylinder and tells you whether it is enough.
How long will my tank last at depth?
Bottom time = (usable gas) / (SAC x ATA at depth). Usable gas is your starting gas minus your reserve (rule of thirds or a fixed reserve such as 50 bar). Example: a full AL80 (~2298 L) with a 50 bar reserve and 20 L/min SAC at 20 m gives about 29 minutes. Mode 6 is a dive time calculator and scuba dive time calculator that runs this in reverse for any tank and depth — a dive bottom time calculator with your reserve already set aside.
What do AL80, HP100, LP85 and similar tank names mean?
They are shorthand for cylinder type and nominal capacity. AL80 = aluminum, roughly 80 cu ft (11.1 L water capacity at 207 bar / 3000 psi). HP100 = high-pressure steel, 100 cu ft nominal (12.9 L at 3442 psi). LP85 = low-pressure steel, 85 cu ft nominal (13.0 L at 2640 psi). The metric steel 12/15/18 L tanks are named by their actual water capacity at 232 bar. Mode 3 lists the full database with computed free-gas volumes.
How do I convert between bar and psi, or liters and cubic feet?
1 bar = 14.5 psi; 1 cu ft = 28.32 L; 10 m of seawater = 33 ft of seawater = 1 atmosphere. For tank math the key identity is: free gas = cylinder water capacity x pressure (Boyle's law). Mode 2 converts consumption rates; the tank database in Mode 3 shows every cylinder in both unit systems.
How much gas does it take to fill or top up my tank?
Gas to add = water capacity x (target pressure - current pressure). Example: topping a 12 L cylinder from 60 to 200 bar adds 12 x 140 = 1680 L (about 59 cu ft). Mode 7 computes this and can estimate cost if you enter your fill price. Note: a tank filled hot will read lower once it cools — roughly a 10% drop is normal.
Should I plan with my normal SAC or my stressed SAC?
Plan with your stressed SAC — typically 1.5x your relaxed average. Exertion, cold, current, task loading, and anxiety all raise consumption, sometimes dramatically. Gas planning estimates only — always plan with a reserve (rule of thirds / 50 bar minimum) and stay within your training. SAC varies with exertion, cold, and stress: use your stressed SAC for planning. This tool does not replace dive training or a dive computer.
Why does this tool show slightly more gas than my tank's rated capacity?
We compute free gas from water capacity x pressure (Boyle's law, ideal gas) — the same AV = CV x CP method divers use worldwide. Manufacturers' rated capacities (e.g. 77.4 cu ft for an AL80 vs ~81 cu ft computed) run a few percent lower because real gas compresses slightly non-ideally (z-factor). The computed number is the standard planning figure; the rated capacity is shown in the tank database for reference.
How accurate are these gas planning calculations?
The math is exact, but the inputs are estimates: your SAC varies dive to dive, depth profiles are rarely square, and gauges have error. Treat every result as a planning estimate with a safety margin, not a guarantee. Gas planning estimates only — always plan with a reserve (rule of thirds / 50 bar minimum) and stay within your training.
Can this tool replace dive training or my dive computer?
No. This is a planning aid for certified divers. It does not model decompression, nitrogen loading, oxygen exposure, or emergency scenarios. This tool does not replace dive training or a dive computer — always dive within your certification and follow your training agency's procedures.
How does cold water affect air consumption?
Cold increases consumption two ways: your body burns more energy staying warm (raising your breathing rate), and thick wetsuits or drysuits add drag and task loading. It is common to see 20–50% higher SAC in cold water versus tropical diving. Log your SAC per dive in the log below and compare warm vs cold entries.