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Scuba air consumption calculator (SAC rate and time above reserve)

Your surface air consumption (SAC rate) is the number that makes gas planning possible: how many litres you breathe per minute, normalised to surface pressure. Enter the figures from your last dive to work out your actual SAC rate, then see how much time the gas above your reserve gives you at the same depth.

This tool provides an estimate for logging and planning. It is not a substitute for training, your dive computer, or your agency’s procedures. Your reserve is not usable gas: plan to surface without touching it.

Stamped on the cylinder shoulder (e.g. “V 12 L”). For US cylinders rated in cubic feet, use the water capacity in litres.

Read on your SPG with the valve open, just before entering the water.

Read on your SPG when you surface.

Shown by your dive computer at the end of the dive (“avg”). Not the maximum depth.

Your computer’s dive time, from leaving the surface to returning to it.

Whatever your agency or dive plan requires — commonly 50 bar.

SAC rate

0.64cu ft/min

Within the usual range for recreational divers.

Time above reserve
0min

At or into your reserve — on this dive, that was the time to be at the surface.

This tool provides an estimate for logging and planning. It is not a substitute for training, your dive computer, or your agency’s procedures. Your reserve is not usable gas: plan to surface without touching it.

The method

At depth, every breath draws air compressed to the ambient pressure: at 10 metres (2 bar), the same lung volume drains your cylinder twice as fast as at the surface. To compare dives, everything is normalised back to the surface.

What you actually used is on your SPG: the pressure consumed times the cylinder volume gives free litres of gas. Divide by dive time, then by the average absolute pressure, and you get your SAC rate — your surface-equivalent breathing rate in litres per minute. Divers who track volume rather than pressure call the same figure an RMV.

Time above reserve runs the logic backwards: the gas still available above your reserve, divided by what you actually consume at that depth.

In the formula: P₁ and P₂ are starting and ending pressures (bar), V the cylinder volume (L), t the dive time (min), d the average depth (m), R the reserve (bar). The constant “10” is the standard teaching approximation: 10 m of seawater adds 1 bar.

P_abs = 1 + d / 10 · SAC = (P₁ − P₂) × V / (t × P_abs) · t_remaining = (P₂ − R) × V / (SAC × P_abs)

Good to know

  • Use your computer’s average depth, never the maximum: with max depth the SAC rate comes out too low, which makes your planning optimistic — in the wrong direction.
  • A SAC rate is not a personal constant: workload, cold, stress and current can double it. Work it out over several dives and keep a range, not a single number.
  • Enter total immersion time, not bottom time: your computer’s average depth already accounts for the dive profile.
  • Above roughly 230 bar, real gas compressibility means a cylinder holds a few percent less than “pressure × volume”. Negligible for recreational diving, but it explains small discrepancies.
  • The result is volume-based (L/min): unlike a pressure-based figure in bar/min or psi/min, it stays valid whatever cylinder you dive.

FAQ

What is a normal air consumption for scuba diving?

Most recreational divers sit between 15 and 25 L/min in calm conditions. Very experienced divers often reach 10 to 15 L/min. Early in training, being above 25 L/min is common and nothing to worry about: consumption drops as comfort builds.

What is the difference between SAC rate and RMV?

Both describe surface-normalised consumption, but SAC is often quoted in pressure units (psi/min or bar/min, tied to one specific cylinder), while RMV is quoted in volume (cuft/min or L/min) and transfers between cylinders. This tool outputs a volume figure — strictly speaking an RMV, commonly called a SAC rate in Europe.

Should I use maximum or average depth?

Average depth, always. Consumption depends on ambient pressure throughout the dive, and the average shown by your computer is the correct summary of it. Using maximum depth divides by too high a pressure: your SAC rate looks better than it is, and any plan built on it is too optimistic.

Why does my SAC rate change from dive to dive?

Because breathing follows workload and state of mind: finning against current, cold, poor buoyancy control, stress or plain fatigue can double your consumption. That is why a SAC rate should be measured over several comparable dives — and why you plan with the high end of your range.

How can I reduce my air consumption?

The real levers are well known: correct weighting (overweighting forces constant BCD adjustments and effort), stable buoyancy, long slow fin strokes, full relaxed breaths rather than short ones, and experience. Tricks that promise to slash consumption without changing those factors do not hold up.

Does the formula work in fresh water?

Yes. The “10 m = 1 bar” approximation is calibrated for seawater; in fresh water the error is about 3%, far smaller than the natural variability of your own consumption between dives. No correction is worth making.

Why is the displayed remaining time so short?

Because it only counts the gas above your reserve, at the average depth you entered. On a well-run dive you surface close to your reserve, so a small margin at the end is expected. To plan your next dive, put your planned starting pressure into the ending pressure field — or wait for the planning tool in this same category.