Tools
SRMV, SAC
Measure your personal breathing rate from a real dive so you can plan gas accurately for the next one.
Last updated June 3, 2026
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What it is
This calculator measures how fast you actually breathe underwater, worked out from a dive you already did. You enter the cylinder you used and one or more segments of the dive (a depth, a time, and the pressure your gauge read at the start and end of each), and it gives you your breathing rate as two numbers: SRMV (surface respiratory minute volume, also called RMV) and SAC (surface air consumption).
Your breathing rate is the single most important input to any gas plan, and it is personal. A relaxed diver and a nervous one on the same dive can use very different amounts of gas. Measure yours once from a calm dive, and you can plan gas honestly from then on. In the app this screen is titled SRMV, SAC, and the output field is labelled SRMV.
Why measure it
Every gas plan starts with “how fast do I breathe?” Guess too low and you plan a dive you cannot finish on the gas you carry. Guess too high and you cut dives short or haul cylinders you never needed. A measured rate from a real dive replaces the guess.
Once you have it, type the SRMV straight into the Gas Usage planner to size cylinders and find where your gas runs low on the next dive.
How to use it
Open the calculator
From the Plan tab, open SRMV, SAC.
Set up the cylinder you used
In Cylinder Setup, enter the Cylinder Volume and choose Single cylinder or Twin Set. Picking Twin Set doubles the volume you entered and shows a “2× N = total” hint so you can confirm the combined size. Use the same cylinder you dived, because the pressure drop you read off your gauge only means something against that cylinder’s size.
Add a measurement
In Add Measurement, enter the average Depth of the segment, the Time it took, and the Start Pressure and End Pressure your gauge showed. The form prefills sensible defaults (the first segment starts at 180 bar with an end 10 bar lower), and Save Measurement stays greyed out until depth and time are above zero and the start pressure is above the end. Tap Save Measurement when the numbers are valid.
Add more segments if you want
For a multi-level dive, add a segment per level. The calculator weights them by how much gas each used, so a long deep segment counts more than a brief shallow one. Each new segment defaults its start pressure to the previous segment’s end, so a whole dive chains together cleanly. Every saved segment appears under Measurements as its own card showing that segment’s depth, time, pressure drop, and its own breathing rate, while the Summary combines them into the weighted average.
Fix a mistake
Each saved measurement card has a delete control. Tap it to remove a segment you mis-entered, then add it again with the right numbers.
Read your rate
The Summary shows your weighted-average SRMV and SAC. Note the SRMV down and reuse it for planning.
Inputs and outputs
Inputs follow your unit settings: depth in metres or feet, pressure in bar or psi, volume in litres or cubic feet.
| Field | Meaning | Notes |
|---|---|---|
| Cylinder Volume | Water volume of one cylinder | Twin Set doubles it; use the cylinder you actually dived |
| Depth | Average depth of the segment | Use your computer’s average depth if you have it |
| Time | Minutes the segment lasted | Minimum 0.5 minutes |
| Start Pressure | Gauge reading at the start of the segment | |
| End Pressure | Gauge reading at the end | Must be lower than the start |
For each segment the tool works out the gas used (the pressure drop times the cylinder’s effective volume), then divides by the load to get a segment rate. The load is time multiplied by the ambient pressure in ATA (1 + depth/10), which normalises the gas you breathed at depth back to the surface. That is why a deeper segment of the same length carries more weight. The tool then combines all segments into a weighted average.
| Output | Meaning |
|---|---|
| SRMV | Surface respiratory minute volume (also called RMV), your true breathing volume per minute at the surface, in litres or cubic feet per minute |
| SAC | Surface air consumption, the pressure your specific cylinder loses per minute at the surface, in bar or psi per minute |
SRMV and SAC explained
Both numbers describe the same lungs, but in different units, and the difference matters when you plan.
SRMV is a volume of gas per minute, normalised to the surface. Because it is a true volume, it is yours regardless of cylinder. Carry it to any cylinder and it still tells the truth. This is the number to keep.
SAC is a pressure drop per minute for one particular cylinder. It is convenient because it reads in the same units as your gauge, but it is tied to that cylinder’s size. Put the same diver on a bigger cylinder and the SAC falls even though the breathing has not changed. Treat SAC as a cylinder-specific shorthand, not a portable figure.
In short: SRMV travels with you, SAC travels with the cylinder.
Behaviour and limits
- Garbage in, garbage out. An end pressure that must be lower than the start, an honest depth, and an accurate time are what make the result trustworthy. The app rejects an end pressure that is not below the start.
- One dive is a snapshot. Measure a few dives across conditions you actually face, and plan from the highest sensible rate, not the lowest.
- The figure is your at-rest, in-control rate for that dive. Cold, current, and exertion push it higher, so add a margin when you plan demanding dives.
Related
- Gas Usage takes the SRMV you measure here and turns it into a gas plan for the next dive.
Parts of this guide were drafted with AI assistance and may contain mistakes. It's educational, not a substitute for training. Always dive within your certification and verify with your instruments.