Diving guide
SAC Rate vs RMV: What's the Difference and When to Use Each
Understand the difference between SAC rate and RMV, when to use each metric, and how to convert between them.
Terminology varies among agencies, instructors, and dive computers. In this guide and throughout DiveCalc, SAC rate and RMV both measure how much breathing gas you consume at the surface, but they express it in fundamentally different ways. SAC uses tank pressure, RMV uses actual gas volume. Understanding the difference — and knowing when each one matters — makes your gas planning more accurate and flexible.
What SAC and RMV measure
SAC (Surface Air Consumption) measures the pressure drop from your tank per minute, normalized to surface pressure. It's expressed in bar/minPSI/min.
The SAC guide owns the pressure-and-depth formula, measurement-quality advice, and complete worked example. Here the important point is that SAC remains attached to the cylinder that produced it.
RMV (Respiratory Minute Volume) measures the actual volume of gas you consume per minute at surface pressure. It is expressed in liters per minute (L/min)cubic feet per minute (cu ft/min).
Because RMV is expressed in absolute volume, it works across any tank size. Once you know your RMV, you can calculate how long any tank will last at any depth — even tanks you've never used before.
The key difference
| SAC Rate | RMV |
|---|---|---|
| Measures | Pressure drop per minute | Gas volume per minute |
| Units | bar/minPSI/min | L/mincuft/min |
Tank-dependent? | Yes — same pressure drop means different gas volumes in different tanks | No — actual volume consumed is the same regardless of tank |
Needs tank specs? | No — just gauge readings and dive data | Yes — requires tank volume and working pressure |
| Best for | Recreational divers using the same tank | Technical divers, multi-tank planning, comparing across setups |
Think of it this way: a pressure-based SAC rate means different gas volumes in different cylinders. A 1 bar/min SAC on a 12 L cylinder is not the same gas volume as 1 bar/min on a 15 L cylinder.A 20 psi/min SAC on an AL80 is not the same gas volume as 20 psi/min on a larger cylinder. RMV eliminates this ambiguity by telling you the actual gas volume consumed.
Converting between them
A 1.2 bar/min SAC on a 12 L cylinder becomes 14.4 L/min RMV.A 17.9 psi/min SAC on an AL80 at 3,000 psi becomes about 0.48 cu ft/min RMV. The equation is a unit translation, not a second breathing measurement.
Try it: SAC to RMV Converter
Allowed range: 0.5 to 4 bar/min.
Allowed range: 7 to 24 L.
Estimated tank duration
Assumes a full fill with a 50 bar reserve
Which one to use
- Use SAC for a quick same-cylinder check. It is easy to read from pressure and compare when the cylinder specification is unchanged.
- Use RMV for planning. Travel cylinders, stages, doubles, bailout, and team comparisons all require a tank-independent gas volume.
- Keep more than one RMV. Log a calm baseline and choose a higher value for current, work, cold, stress, and emergencies.
Ready to calculate? Use our free SAC Rate & RMV Calculator to determine your SAC rate from dive data and convert it to RMV in one step. Or use the SAC Rate Calculator for a quick SAC-only calculation.
Sources
- Estimating Your Air Consumption · Divers Alert Network
- Scuba Cylinder Rundown · Divers Alert Network
- NOAA Diving Manual: Diving for Science and Technology (6th ed.) · NOAA — Reference text for diving physics, breathing gases, decompression, and oxygen exposure.
Always use current training materials and operational procedures for dive planning.