How to Calculate Your SAC Rate — and Actually Use It

Your SAC rate — Surface Air Consumption — is simply how much gas you breathe in a minute, measured as if you were back at the surface so that dives at different depths can be compared fairly. You can work it out from almost any dive with three numbers off your gauge and computer: how much gas you used, how long you were down, and your average depth. Below is the method, what a “good” number actually looks like (with rather less judgement than you might fear), and how to turn it into real gas planning. There's a calculator further down to do the arithmetic for you.

Why bother working it out?

Knowing your air consumption is what turns “I hope this tank lasts” into “I know roughly how long this tank lasts.” It's the foundation of gas planning — the difference between guessing and having a number — and it's quietly one of the most honest progress markers in diving. As you get more relaxed, trim, and efficient in the water, your SAC rate falls, and you can watch it happen dive by dive. (There aren't many corners of this sport where getting better shows up as a tidy shrinking number, so enjoy it.)

SAC vs RMV: the two terms people mix up

Here's the first thing worth untangling, because divers use these words interchangeably and they aren't quite the same.

SAC (Surface Air Consumption) is usually measured in pressure — bar per minute, or psi per minute. It's easy to read straight off your gauge, but it's tied to the specific cylinder you used, because the same breath empties a small tank faster than a big one. Change cylinder size and your SAC number changes, even though your lungs did precisely the same thing.

RMV (Respiratory Minute Volume) measures the same thing in volume — litres per minute, or cubic feet per minute. Because it's a true volume of gas, it doesn't care which tank you were breathing from, which makes it the more useful number for planning across different cylinders. (If you've ever watched two divers debate which term is “correct,” you've witnessed the diving equivalent of a grammar argument — both are useful; just know which one you're quoting.)

For most recreational divers on one familiar tank, SAC in bar/min is perfectly serviceable. The moment you start swapping cylinder sizes, RMV is the one you want.

What you need from your dive

Five numbers, all of which a modern computer and gauge hand you:

• Starting cylinder pressure (bar or psi)
• Ending cylinder pressure
• Total dive time (minutes)
• Average depth (your computer logs this)
• Cylinder size — its volume in litres (metric), or its rated capacity and rated pressure (imperial)

A word on “average depth”: it's an approximation. Your SAC comes out most accurate from a flat, steady dive, or a dedicated few minutes hovering at one depth. A sawtooth profile will still give you a usable ballpark — just don't treat the last decimal as gospel.

The calculation, step by step (metric)

First, convert your average depth to atmospheres absolute (ATA), because pressure is what governs how fast you breathe your gas down:

Then work out the actual volume of gas you used — pressure drop multiplied by cylinder size:

Then bring it back to the surface by dividing out both the time and the pressure:

A worked example. Say you dived a 12-litre cylinder from 200 down to 60 bar, over 40 minutes, at an average depth of 18 metres:

• ATA = 18 ÷ 10 + 1 = 2.8
• Gas used = (200 − 60) × 12 = 1,680 litres
• RMV = 1,680 ÷ (2.8 × 40) = 15 L/min

And if you'd rather have SAC in bar/min for that same tank: (140 bar ÷ 40 min) ÷ 2.8 = 1.25 bar/min.

Imperial works the same way, with one extra step: because tanks are rated a little oddly (an “80” is 77.4 cubic feet at 3,000 psi), you first convert pressure into volume using a cylinder factor — rated cubic feet ÷ rated psi — then follow the identical logic, using feet ÷ 33 + 1 for your ATA. The calculator handles both, so you don't have to keep the conversion in your head.

What counts as a “good” SAC rate?

This is where divers get unnecessarily hard on themselves, so let's be straight about it. As a rough guide for RMV in calm, warm-water conditions:

• Around 12–15 L/min is efficient — typically a relaxed, experienced diver.
• Around 15–20 L/min is a very normal, healthy average.
• Above 20 L/min is common when you're newer, cold, working against current, or simply a larger person — and none of those is a character flaw.

That last point matters. A bigger diver has bigger lungs and more tissue to supply, so a higher figure is often just physics rather than a lack of skill. Workload, cold, stress, and how recently you've been in the water all move the number too. Treat your SAC as a personal baseline to measure against yourself over time — not a leaderboard to come last on. (In cubic feet, very roughly: under ~0.5 is efficient, ~0.5–0.7 average, and above that is on the higher side.)

Turning it into gas planning

Once you know your rate, you can flip it around to plan a dive — which is exactly where it earns its keep on deeper or longer dives, where the margins are thinner and the exit is further away. Multiply your RMV by the pressure at your planned depth to see how fast you'll breathe down there:

So a 15 L/min diver at 25 m (3.5 ATA) is getting through roughly 52 litres a minute. From there, take your usable gas — start pressure minus a sensible reserve, converted to litres — and divide by that rate to estimate your time. And do keep a real reserve: the calculator's estimate is bottom-time-until-empty, not a dive plan. Leave yourself gas for the ascent, the safety stop, and a problem you didn't see coming. The rule of thirds — a third out, a third back, a third held in reserve — is a sound default anywhere you need to return to a fixed exit.

Lowering it (without doing anything daft)

If you'd like the number to come down, the honest levers are the unglamorous ones: better buoyancy and trim so you're not finning just to hold your depth, a slow and relaxed breathing rhythm, staying warm, streamlining your kit, and general fitness. What you should never do is skip-breathe or hold your breath to fake a lower figure — that's a quick route to carbon-dioxide build-up and a genuinely dangerous habit. A good SAC rate is a by-product of diving well, not something to chase directly.

Try it on your last dive

Punch in the numbers off your gauge and computer — it works in metric or imperial, and updates as you type. The “plan a dive” fields are optional; leave them be if you only want your rate.

My take

After a lot of dives, here's what I actually tell people: work your SAC out a few times, across different conditions, and take the average — a single dive tells you very little, but five or six paint an honest picture. Then more or less forget the number, glancing at it only now and then. Its real value isn't bragging rights on the boat; it's walking into a dive already knowing your gas will comfortably see you through the plan. And watching your own figure quietly fall over a season is one of the most satisfying signs there is that you're becoming a calmer, better diver — which is rather the whole point.

The short version

Grab five numbers off your gauge and computer, run them through the method (or the calculator above), repeat across a few dives for an honest average, and use the result to plan your gas with a proper reserve. Don't compare your number to anyone else's — compare it to your own, six months from now.

Getting fluent in your own gas consumption is a big part of diving like someone who plans rather than hopes — which is the kind of diver we most enjoy helping people become. If you'd like to work on gas planning and efficiency properly, that's exactly the sort of thing we do together in the water.