Your surface air consumption rate tells you how much gas you breathe at the surface per minute. Once you know it, you can predict how long any tank will last at any depth — which is how you stop coming up with a third of your air left or, worse, running short.
Fill in the numbers from any logged dive and the calculator does the rest.
| SAC Rate | RMV (Metric) | What It Means |
|---|---|---|
| Under 0.40 cu ft/min | Under 11 L/min | Excellent — highly experienced or very relaxed diver |
| 0.40 – 0.60 cu ft/min | 11 – 17 L/min | Good — comfortable recreational diver |
| 0.60 – 0.80 cu ft/min | 17 – 23 L/min | High — newer diver or mildly elevated effort |
| Over 0.80 cu ft/min | Over 23 L/min | Very high — anxiety, exertion, or inexperience |
Once you have your SAC rate, you can predict how long any tank will last at any depth using the reverse formula. Multiply your SAC by the average ATA (depth in feet divided by 33, plus 1), then divide your available gas volume by that number to get your bottom time.
Most divers use this to set turn pressures before a dive rather than relying on a generic rule of thumb — your number is specific to you, not some average diver.
New divers breathe harder because they're tense. As you log dives and buoyancy becomes automatic, your SAC rate drops — often significantly within the first 50 dives.
Your SAC rate from a calm, flat dive will be lower than one against a current. Calculate your rate from similar dives when planning similar dives.
The calculator already accounts for depth in the formula. Your SAC rate is normalized to the surface, so it stays consistent regardless of how deep the reference dive was.
First calculate the volume of gas used: divide the pressure drop by the tank's rated pressure, then multiply by the tank's volume in cubic feet. Divide that by the average absolute pressure (average depth in feet divided by 33, plus 1) and then by the dive time in minutes.
In metric: volume used equals pressure drop in bar multiplied by tank water volume in liters. Divide by average ATA (average depth in meters divided by 10, plus 1) and dive time to get RMV in liters per minute.
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