SAC / RMV Calculator

SAC / RMV — Key Features

• Calculate SAC rate from tank specs, depth, time, and pressure used
• Results in liters/min (metric) or cubic feet/min (imperial)
• Interpretation guide — from excellent to needs improvement
• Essential for accurate dive gas planning
• Works with any tank size and gas mix
• Support for both metric and imperial units

SAC / RMV — How It Works

Enter your cylinder volume, start and end pressure, average depth, and dive time. The calculator works out how much gas you used in litres, then divides by the absolute pressure at average depth to normalise the figure to surface — that's your SAC rate (Surface Air Consumption) in litres per minute. RMV (Respiratory Minute Volume) is the same idea expressed independently of cylinder geometry, useful when you switch between cylinder sizes or borrow tanks abroad. Your personal SAC is one of the most important inputs for gas planning: it drives Rock Bottom, turn-pressure, bottom-time budget, and cylinder choice for a planned profile. Resting and exercising SAC can differ by a factor of two to three, so log calm dives and stressful ones separately. Track a rolling average over many dives — fitness, water temperature, gear drag, anxiety, and experience all move the number — and use that personal baseline rather than textbook defaults whenever you plan.

Why Know Your SAC Rate?

• Accurately predict gas consumption for future dives
• Plan appropriate tank sizes for different dive profiles
• Track improvement in your air consumption over time
• Essential input for rock bottom and emergency gas calculations

SAC / RMV — Frequently Asked Questions

What is SAC rate?

SAC (Surface Air Consumption) is your gas use rate normalised to surface pressure, expressed in litres per minute. It is calculated by measuring how much gas you used over a dive, then dividing by the absolute pressure at the average depth — at 20 m the ambient pressure is 3 ata, so a real consumption of 30 L/min at depth equals a SAC of 10 L/min. SAC is the most important input for gas planning: it drives Rock Bottom, turn-pressure, bottom-time budget, and cylinder size choice for any planned profile.

What is a good SAC rate?

A typical resting SAC for a fit recreational diver in warm water is 12-15 L/min, with new divers often 18-25 L/min and experienced technical divers under 10 L/min. SAC is highly personal and varies with fitness, breathing technique, cold, exertion, anxiety, gear drag, and depth-of-experience. Track your own number across many dives — your personal SAC is the only useful number for planning your gas, not anyone else's average or a textbook default. Resting and exercising SAC can differ by a factor of two to three.

Why does SAC matter for dive planning?

Every gas-planning number — Rock Bottom, turn pressure, bottom-time budget, cylinder choice — derives directly from SAC. A 1 L/min error in SAC becomes a 60 L error over an hour, which can be several hundred PSI / 10+ bar on a small cylinder. Using a textbook default instead of your personal SAC means systematically over- or under-planning every dive; on technical dives that matters in cubic-foot quantities and can determine whether you reach the surface with adequate reserves. Log it.

How do I measure my SAC rate underwater?

Measure SAC by recording four numbers per dive: cylinder volume in litres, start and end pressure in bar, average depth, and dive time. The calculator does the rest: it computes total gas used (start − end × volume), divides by dive time to get litres per minute at depth, then divides by absolute pressure at average depth to normalise to surface. Use a dive computer that records average depth — or compute it from your profile. Repeat on many dives and use a rolling average rather than a single-dive snapshot.

What is the difference between SAC and RMV?

SAC and RMV measure the same thing — your minute ventilation — but expressed differently. SAC is in litres per minute and depends on cylinder geometry (compresssible gas inside a fixed volume); RMV is in actual minute volume of breath at surface pressure, fully independent of cylinder geometry. RMV travels: a diver's RMV is the same whether they breathe from an Al80 in the US or a 12 L steel in Europe. SAC is easier to compute from gauge readings, RMV is easier to compare across regions and gear setups.

Does my SAC rate change over time?

Yes — SAC drops measurably with experience as breathing technique improves, anxiety decreases, and you learn to move efficiently in the water. New divers' SAC commonly drops from 25 L/min to 15 L/min over their first 50-100 dives. SAC also varies with fitness (cardio improves it), water temperature (cold raises it), gear drag (more drag, more work), and emotional state (stress raises it). Track a rolling average over many dives and recalculate your Rock Bottom whenever your typical SAC shifts by more than 1-2 L/min.

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