SAC rate explained: how to calculate your air consumption
Your SAC rate says how much air you breathe at the surface equivalent, whatever the depth. It is the number that makes gas planning honest and shows you getting better.
· 2 min read
What SAC is
SAC stands for surface air consumption: the volume of gas you use per minute, converted back to surface pressure so that dives at different depths can be compared. It is measured in litres per minute. A diver at 30 m uses four times the gas they would at the surface, so the raw pressure drop in the cylinder says little until you correct for depth.
The sum
SAC = (bar used × cylinder litres) ÷ (minutes × (average depth ÷ 10 + 1))
A worked example. You start at 200 bar and finish at 60 bar, so you used 140 bar. The cylinder is 12 litres, the dive was 45 minutes and the average depth 15 m, which is 2.5 bar of pressure.
140 × 12 = 1,680 litres. 45 × 2.5 = 112.5. 1,680 ÷ 112.5 = 14.9 litres per minute.
If your dive log has the maximum depth but no average, a common shortcut is to take about two thirds of the maximum as the average; Loggerfish assumes 65 %. A computer's real average is better, because most of a dive is spent shallower than its deepest point.
Why it changes
- Effort and stress. Swimming against a current or being anxious raises it. Calm, slow breathing lowers it.
- Cold. Cold water and a drysuit that lets you down both raise it.
- Fitness and experience. New divers often use twice what a relaxed experienced diver does.
- Equipment. An unfamiliar regulator or a badly weighted rig adds effort.
A single dive's number is noisy. A season of them is not.
Using it to plan
Take a SAC from your harder dives, not your best, and multiply it by the pressure at the depth you plan and the minutes you plan. That gives the gas a dive needs. Add a reserve and a stress margin on top. The minimum gas calculator does this for an emergency ascent, and the SAC calculator does the sum above.
Watching the trend
One SAC tells you about one dive. The useful picture is how it moves over a season: a falling line means you are using less gas for the same dive. In Loggerfish the SAC trend is part of Pro, the optional paid plan; logging the dives and their pressures is free.
These calculations are planning aids, not a dive computer. Your training and your computer decide the dive.
Track your SAC over time in Loggerfish
Free to start, no card. Your logbook, and every export of it, stays yours on every plan.
Names such as PADI, SSI, MySSI, Subsurface, Shearwater and Garmin belong to their owners and are used only to say which product we mean. Loggerfish is not affiliated with any of them. What a guide says about them comes from their own public pages, listed beside the statement with the day it was checked. If something is out of date, write to support@loggerfish.com and we will correct it.