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 psi/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 and RMV compared
How the two DiveCalc rates differ in what they measure, their units, and the cylinder data each needs.
Property | SAC Rate | RMV |
|---|---|---|
| Measures | Pressure drop per minute | Gas volume per minute |
| Units | psi/min | L/mincu ft/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 the cylinder's water volumeYes — requires rated capacity 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
SAC is in bar/min and is the cylinder's internal water volume in liters, so RMV comes out in L/min. SAC is in psi/min, is the manufacturer's rated capacity in cu ft, and is the working pressure in psi at which that capacity applies, so RMV comes out in cu ft/min.
A 1.2 bar/min SAC on a 12 L cylinder becomes 1.2 × 12 = 14.4 L/min RMV.A 17.9 psi/min SAC on an AL80 rated 77.4 cu ft at 3,000 psi becomes 17.9 × 77.4 ÷ 3,000 = 0.46 cu ft/min RMV. The equation is a unit translation, not a second breathing measurement.
Both forms are approximations
The conversion assumes that each unit of pressure drop releases the same amount of gas, as it would for an ideal gas. Real air compresses less at diving fill pressures, so over a typical sample from a 2,900–3,400 psi fill the pressure drop corresponds to roughly 5–8% less gas than the water-volume shortcut suggests. Rated imperial capacities already include that effect at the full working pressure, which is why the AL80 is rated 77.4 cu ft rather than the roughly 81 cu ft that 11.1 L × 207 bar would suggest. The difference is usually smaller than dive-to-dive variation in breathing, but the real-gas cylinder calculator shows it for a specific fill.
Try it: SAC to RMV Converter
Allowed range: 0.5 to 4 bar/min.
Allowed range: 7 to 24 L.
Estimated tank duration
Constant depth in seawater, a full fill, and an example 50 bar reserve. Deeper dives need a calculated rock bottom gas reserve instead.
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 (6)
- Estimating Your Air Consumption
Divers Alert Network
- Diving Medical Technician Reference and Formulas
NOAA Diving Program
Air requirement formulas: SAC in psi/min normalized by depth in ATA, cylinder constant as rated volume ÷ rated pressure, RMV = SAC × constant, available volume above a reserve, and cylinder duration.
- Luxfer Aluminum Cylinder Specifications
Luxfer / XS Scuba
Source of the AL80 (77.4 cu ft at 3,000 psi; 11.1 L at 207 bar) and AL100 (98.8 cu ft at 3,300 psi; 13.2 L at 228 bar) ratings used in the examples and tables. Comparing 11.1 L × 207 bar (about 81 cu ft) with the 77.4 cu ft rating shows the real-gas shortfall of the water-volume shortcut.
- Supply Pressure Requirements & Tables (section 1.3, Work Rate Expressed as RMV)
Kirby Morgan Dive Systems
Reproduces the U.S. Navy Diving Manual work-rate table: rest 7–10, light work 10–20, and moderate work 20–37 L/min RMV (0.35–0.7 and 0.7–1.3 cu ft/min). Context for the recreational bands, which are informal rather than published by this source.
- Scuba Cylinder Rundown
Divers Alert Network
General overview of cylinder sizes and choosing a larger cylinder for higher consumption. It names the AL80 by its nominal 80 cu ft; the calculations here use Luxfer's 77.4 cu ft rating.
- 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.