Can I convert between SAC and RMV?

Yes. For imperial units: RMV (cuft/min) = SAC (PSI/min) x Tank Volume (cuft) / Working Pressure (PSI). For metric units: RMV (L/min) = SAC (bar/min) x Tank Water Volume (L). The conversion is straightforward because RMV is simply SAC expressed in actual gas volume instead of pressure drop.

Which should I track — SAC or RMV?

Track SAC if you always dive the same tank — it's simpler since you just need pressure readings. Track RMV if you switch between different tank sizes or want to plan gas for a tank you haven't used before. Many experienced divers track RMV because it's more versatile for gas planning.

Why do technical divers prefer RMV?

Technical divers use multiple tanks of different sizes (back gas, stages, bailout). RMV lets them plan gas for any tank configuration using a single personal consumption number. A SAC rate in bar/min only applies to the specific tank it was measured with, making cross-tank planning impractical. (Technical divers use multiple tanks of different sizes (back gas, stages, bailout). RMV lets them plan gas for any tank configuration using a single personal consumption number. A SAC rate in PSI/min only applies to the specific tank it was measured with, making cross-tank planning impractical.)

Does my RMV stay the same across dives?

Your RMV varies with exertion, stress, water temperature, fitness, and conditions — just like SAC. However, under similar conditions your RMV should be relatively consistent. Track it across many dives to establish your personal baseline for calm conditions and a higher planning value for demanding dives.

What is a good RMV for a recreational diver?

A typical recreational diver has an RMV of 15-20 L/min (0.5-0.7 cuft/min). Experienced divers often achieve 10-15 L/min, while beginners may see 20-30 L/min. As with SAC, your RMV naturally improves with experience and better technique.

SAC Rate vs RMV: What's the Difference and When to Use Each

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 is SAC Rate?

SAC (Surface Air Consumption) measures the pressure drop from your tank per minute, normalized to surface pressure. It's expressed in bar/min.

SAC Formula

\text{SAC} = \frac{P_{\text{start}} - P_{\text{end}}}{t \times D_f}

Where $D_f = \frac{\text{depth} + 10}{10}$ . This removes the effect of depth so you get a surface-equivalent rate.

SAC is quick and practical. You only need your gauge readings and dive computer data — no need to know tank specifications. That's why it's the go-to metric for recreational divers who always use the same tank.

What is RMV?

RMV (Respiratory Minute Volume) measures the actual volume of gas you consume per minute at surface pressure. It's expressed in cubic feet per minute (cuft/min) or liters per minute (L/min).

Converting SAC to RMV

\text{RMV} = \text{SAC} \times V_{\text{water}}

RMV converts the pressure-based SAC into a real gas volume. This makes it independent of any specific tank.

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/min	L/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 SAC rate of 20 PSI/min means very different things in an AL80 versus a Steel 120. The AL80 holds less gas per PSI, so 20 PSI/min represents less actual gas consumption in that tank. RMV eliminates this ambiguity by telling you the actual gas volume consumed.

A Practical Example

You dive a 12L tank for 40 minutes at an average depth of 15 meters. Start pressure: 200 bar. End pressure: 80 bar.

Depth Factor = (15 + 10) / 10 = 2.5

SAC = (200 - 80) / (40 x 2.5) = 1.2 bar/min

RMV = 1.2 x 12 = 14.4 L/min

Now you can use that RMV of 0.48 cuft/min to plan gas for any tank — a Steel 100, doubles, or stage bottles — without needing to dive them first to establish a SAC rate.

Try it: SAC to RMV Converter

SAC Rate

bar/min

Tank

RMV

18.0 L/min

0.64 cuft/min

Personal rates vary by conditions, workload, stress, and temperature.

Estimated tank duration (12L)

10m

50 min

18m

36 min

25m

29 min

30m

25 min

Assumes 200 bar fill with 50 bar reserve

When to Use SAC

You always dive the same tank. If you rent the same AL80 every weekend, SAC gives you quick, consistent data without extra math.
Quick post-dive check. After a dive, you can immediately calculate SAC from your console and computer without looking up tank specs.
Comparing dives on the same tank. SAC is a direct apples-to-apples comparison when the tank doesn't change between dives.

When to Use RMV

Planning gas for unfamiliar tanks. Traveling or switching between rental tanks? RMV lets you plan regardless of what's available.
Technical diving with multiple cylinders. When diving doubles, stages, or bailout bottles, RMV is essential for planning each cylinder's duration independently.
Comparing efficiency across different setups. Your RMV should be the same whether measured from an AL80 or a Steel 120, making it a true personal metric.
Gas consumption planning. Tools like gas planners use RMV to calculate total gas requirements for a dive profile across any tank configuration.

The Bottom Line

SAC and RMV are two expressions of the same underlying measurement — how much gas you breathe at the surface. SAC is simpler to calculate and works well for single-tank recreational diving. RMV is more versatile and essential for technical diving or planning across different tank sizes.

If you're serious about gas planning, learn to work with RMV. Calculate your SAC from a few dives, convert it to RMV, and you'll have a universal number you can apply to any dive scenario.

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.