Air Consumption Calculator
Work out your SAC rate, see how depth affects it and how long your cylinder lasts. Teaching tool: always keep an eye on your pressure gauge.
Your surface air consumption
No idea? Work it out in . Typical SAC rate: 15 to 20 L/min.
At 20 m, every breath costs
×3more air than at the surface
Pressure
3.0bar
Your consumption
54L/min
12 L · 200 bar cylinder emptied in
44min
Boyle's law: pressure × volume = constant. At 20 m you are at 3 bar: the air is 3 times more compressed, so each breath (same lung volume) holds 3 times more air than at the surface.
The shallow-water trap
From 0 to 10 m, pressure goes from 1 to 2 bar: your consumption doubles. From 30 to 40 m, it goes from 4 to 5 bar: only +25%. Tap a bar.
Pressure underwater: 1 bar at the surface (the weight of the air), then +1 bar every 10 m of water. At 10 m: 2 bar, at 20 m: 3 bar, at 40 m: 5 bar.
My last dive
Your surface air consumption (SAC rate)
13.3L/min
Gas used
1,680L
Avg. pressure
2.8bar
Bottom consumption
37.3L/min
Boyle's law: at depth, each breath holds more air, so your cylinder empties faster. To compare dives, consumption is brought back to the surface: SAC rate = bottom consumption ÷ pressure.
Teaching simulation, not a dive planner
The calculation empties the cylinder down to 0 bar, with no reserve, no ascent and no stops: that is not what a real dive looks like. The goal is to see how gas time changes with depth, cylinder and your SAC rate.
As taught in dive training, air is treated as an ideal gas: in reality, a cylinder at 200–300 bar holds slightly less air.
Depth
20 m
Your gas time at 20 m
Bottom consumption
54L/min
1 bar lasts
13s
Gauge drops by
4.5bar/min
Boyle's law: at 20 m (3 bar), you breathe 3 times more air than at the surface, so your cylinder lasts 3 times less.
The same cylinder at other depths
Minutes to 0 bar. Tap a bar.
My settings
Two depths, same cylinder
A
10 m
66 min
B
30 m
33 min
with 12 L at 200 bar and 18.0 L/min
Boyle's law: gas time is divided by the pressure. At 10 m (2 bar) your cylinder lasts half as long as at the surface; at 30 m (4 bar), a quarter as long.
What about another cylinder?
Minutes to 0 bar table
| 10 L | 12 L | 15 L | 18 L | 2×7 L | 2×10 L | 2×12 L | |
|---|---|---|---|---|---|---|---|
| 0 m | 111 | 133 | 166 | 200 | 155 | 222 | 266 |
| A10 m | 55 | 66 | 83 | 100 | 77 | 111 | 133 |
| 20 m | 37 | 44 | 55 | 66 | 51 | 74 | 88 |
| B30 m | 27 | 33 | 41 | 50 | 38 | 55 | 66 |
| 40 m | 22 | 26 | 33 | 40 | 31 | 44 | 53 |
| 50 m | 18 | 22 | 27 | 33 | 25 | 37 | 44 |
| 60 m | 15 | 19 | 23 | 28 | 22 | 31 | 38 |
Understand your air consumption
These are the rules taught in dive training. Pressure: 1 bar at the surface, +1 bar every 10 m. Boyle's law: pressure × volume = constant, so your bottom consumption = SAC rate × pressure. Air in the cylinder = volume × pressure (12 L at 200 bar = 2,400 L). Simulated gas time = cylinder air ÷ bottom consumption, down to 0 bar, with no reserve or ascent. Indicative values: cold, exertion and stress increase consumption.
How to measure your air consumption
Note your start and end pressure, the dive time and the average depth (your dive computer shows it). The My SAC tab works out the gas breathed (volume × bar used), then divides it by the dive time and the average pressure. Pick a dive with a steady profile and repeat over several dives: a pressure gauge is only accurate to a few bar, and your consumption changes with cold, exertion and stress.
Why your consumption doubles by 10 m
At the surface you already carry 1 bar: the weight of the air. Underwater, every 10 m adds 1 bar: 2 bar at 10 m, 3 bar at 20 m, 5 bar at 40 m. This is Boyle's law: pressure × volume = constant. When pressure doubles, air is twice as compressed. Your lungs keep the same volume, so each breath fills them with twice as much air. From 0 to 10 m pressure goes from 1 to 2 bar and your consumption doubles; from 30 to 40 m it goes from 4 to 5 bar and consumption rises by only 25%. To convert between depth and pressure in fresh or salt water, use the pressure & depth converter.
Reading gas time and your gauge
A cylinder holds its volume multiplied by its pressure: a 12 L filled to 200 bar contains 2,400 L of free air. At 20 m (3 bar) you breathe 3 times more than at the surface: with a 20 L/min SAC rate, you use 60 L/min. The cylinder would last 2,400 ÷ 60 = 40 min down to 0 bar. On your gauge, that is 5 bar per minute, or 1 bar every 12 seconds. Use this figure to compare depths and cylinders, not to set your dive time.
Limits: this is not a dive planner
The calculation breathes the cylinder down to 0 bar, with no reserve, no ascent and no stops. It treats air as an ideal gas: at high pressure, a cylinder holds slightly less air than volume × pressure (about 4% less at 200 bar, nearly 10% at 300 bar), so the gas time shown is slightly optimistic. Plan your dives with your dive computer, your buddy and the rules of your training agency, and keep an eye on your gauge.
Frequently asked questions
How do you calculate your SAC rate in scuba diving?
Multiply the cylinder volume by the bar used to get the litres breathed, then divide by the dive time and by the average absolute pressure (1 + average depth ÷ 10). Example: a 12 L cylinder from 200 to 50 bar = 1,800 L, over 40 min at an average of 15 m (2.5 bar): 1,800 ÷ (40 × 2.5) = 18 L/min at the surface.
What is a normal air consumption rate for divers?
Most recreational divers have a SAC rate of 15 to 20 L/min. Below 12 L/min is very relaxed; above 25 L/min is high, often because of cold, exertion, stress or poor weighting. It varies from dive to dive, so measure it over several dives.
Why do you use twice as much air at 10 m?
At 10 m the absolute pressure is 2 bar: each breath fills your lungs with the same volume, but that volume holds twice as much air (Boyle’s law). Your consumption at depth is your SAC rate × 2. From 0 to 10 m it doubles, while from 30 to 40 m (4 to 5 bar) it only goes up by 25%.
How long does a 12 litre scuba tank last?
A 12 L cylinder at 200 bar holds about 2,400 L of air. With a SAC rate of 20 L/min and breathing it down to 0 bar: 120 min at the surface, 40 min at 20 m, 24 min at 40 m. On a real dive you keep a reserve (often 50 bar) and gas for the ascent: at 20 m, 150 usable bar only gives you 30 min.
What is the difference between SAC rate and RMV?
Both describe how much you breathe. In this tool, the SAC rate is your consumption in litres per minute converted to 1 bar, so it can be compared between dives; some divers call this RMV and keep “SAC” for bar or psi per minute. Your consumption at depth is SAC × absolute pressure: 18 L/min at the surface means 45 L/min at 15 m.