About Rock Bottom (Minimum Gas)

What is Rock Bottom?

Rock Bottom (also known as Minimum Gas) is a way to plan for the worst case scenario: where a diver experiences a catastrophic and complete loss of breathing gas at the deepest part of the dive.

RB is the minimum amount of gas needed for 2 divers to ascend safely to the surface while sharing gas from a single tank, expressed as psi (or bar for metric users) specific to the tank being used.

How do I use Rock Bottom?

A Rock Bottom calculation will provide a psi (bar) reading at which you should begin your ascent from the deepest part of your dive. This doesn’t mean you need to end your dive, just move to a shallower depth and stay within RB limits for that depth. Repeat as needed.

How do you calculate Rock Bottom?

Based on the max depth, RB determines the amount of time needed for you and your buddy to surface safely, as well as a rough average depth used later in the calculations.

Using those results as well as the consumption rates (RMV) you specify, RB then determines how many cubic feet (litres) of gas you and your buddy will consume during that ascent.

The tank size and the tank's rated pressure (imperial claculations only) are then used to convert the number of cubic feet (litres) needed into a psi (bar) reading for the specified tank.

To determine the time needed for the ascent, we base the calculations on the following profile:

For divers using metric:

Here's an example of a profile of an ascent from 120ft (40m):

A Note on Average Depth:

For ease of use and programming, this calculator divides the max depth in half and uses that as the "average depth". Because of this, that figure isn't really a true average depth and is actually a little deeper than it should be. This results in a slightly higher Rock Bottom psi (bar) than if you had calculated it using the true average depth.

A Note on Ascent Times:

This calculator takes the time to the first stop as well as the time to the surface and rounds both up to the next whole minute. This keeps the math simpler to understand and also provides a little bit more padding to the final result.

How do I use your RB Calculator?

Determine what depth(s) you want RB for.

Know your tank size in cubic feet (litres) and it’s rated pressure (not necessary for metric).

If you know your RMV under stress or "working", use that. Otherwise, a minimum of 1 cf/min (30 l/min) is recommended. This is because we’re assuming both divers are stressed and have elevated consumption rates.

Decide how much (if any) of a buffer you’d like to add to compensate for differences in gauge readings, extra gas at the surface, or just a little more peace of mind.

How do I learn more about RB?

One Last Thing

These calculations and results are rough estimates at best. There are no strict rules regarding RB.

Too many variables are involved to accurately predict exact amounts of gas needed to safely ascend. If you choose to use any of these these calculations in your diving, please remember these are guidelines, and not absolutes.

Have fun, and dive safe.