Rock Bottom Gas Calculation: Minimum Gas for Two Divers, Step by Step

What does rock bottom protect against?

Rock bottom covers one scenario: your buddy runs out of gas at the deepest point of the dive and both of you have to reach the surface breathing from your cylinder. GUE, which now calls the same volume minimum gas, requires its divers to always carry enough gas to surface from maximum depth while sharing with an out-of-gas diver. Above rock bottom, that ascent is paid for. Below it, you are betting that nothing goes wrong.

That scenario sits at the top of the fatality data. In a DAN review of 947 open-circuit recreational diving deaths from 1992-2003, insufficient gas was the most frequent trigger (41%), and emergency ascent was the most frequent disabling agent (55%). Arterial gas embolism was the second most common disabling injury, and 96% of those cases involved an emergency ascent. A controlled, shared ascent is what the reserve buys you instead of a bolt for the surface.

Our gas planner gives the number for any depth and tank in its rock bottom tab. The sections below take it apart, so you know what it covers and where it stops.

The assumptions behind our rock bottom number

Every rock bottom figure rests on a scenario. Ours is printed next to the calculator:

• Two divers breathe from one tank, so the volume is doubled.
• Both breathe at a stressed SAC. The calculator's default is 30 L/min per diver, which is twice a relaxed 15 L/min.
• One minute at maximum depth to notice the problem, get together and hand over a regulator.
• An ascent at 9 m/min (30 ft/min) to 5 m (15 ft), with the gas for the climb counted at its average depth.
• A 3-minute safety stop at 5 m.
• The last 5 m (15 ft) from the stop to the surface at the same 9 m/min, counted at 2.5 m.
• The final pressure rounded up to the next 5 bar.

Why a stressed SAC and 9 m/min?

The ascent rate is the easy part: 9 m/min is the 30 ft/min that the US Navy and NOAA use. Recreational agencies have used anywhere between 30 and 60 ft/min. If you ascend slower than the model, the ascent takes longer and uses more gas than the table below shows.

The stressed SAC is the assumption worth arguing about. DAN notes that exertion, stress and anxiety all raise breathing rate, and that task-heavy dives such as spearfishing can use gas five to 10 times faster than usual. Cave instructor Stratis Kas writes that minute ventilation in a panic can double, sometimes triple, within thirty seconds. So treat double your relaxed SAC as the least you plan with, and if your normal rate is already high, start from your own number. Our SAC calculator works it out from a logged dive.

Worked example: rock bottom for a 30 m dive

Take a no-stop boat dive to 30 m (100 ft) with a 12 L tank and a stressed SAC of 30 L/min per diver. Work in bar-minutes first: the time of each phase multiplied by the absolute pressure it happens at, which is depth ÷ 10 + 1.

Two divers at 30 L/min breathe 60 L for every bar-minute, so 16.83 × 60 = 1,010 L. Divide by the tank size: 1,010 ÷ 12 = 84.2 bar, rounded up to 85 bar. At 85 bar the pair must already be leaving the bottom.

The litres depend only on depth and breathing rate. The pressure depends on the cylinder, which is why each diver works it out for their own tank. The same 1,010 L is 70 bar in a 15 L tank (67.3 rounded up) and 105 bar in a 10 L tank (101 rounded up).

Breathing rate moves the answer more than anything else you choose. At 30 m in a 12 L tank, a stressed SAC of 20 L/min gives 60 bar, 30 L/min gives 85 bar and 40 L/min gives 115 bar.

Rock bottom vs the rule of thirds and a fixed 50 bar reserve

At 30 m in a 12 L tank, both a fixed 50 bar reserve and the reserve third of a 200 bar fill come out below rock bottom. At 18 m they are close to it or above it. The shallower the dive, the less the choice matters.

Conventional wisdom, as a DAN Europe gas planning article puts it, holds that the proper reserve is 50 bar, a figure common enough to have inspired the names of scuba-themed bars. The article's author works through a 30 m boat dive at an RMV of 15 L/min, gets 552 L, about 50 bar in an AL80, and calls that an absolute minimum. His advice is to add a 50-100% margin and plan on 80 or 100 bar. Our stressed SAC builds a similar margin in, which is why the same depth gives 85 bar.

The rule of thirds comes from cave diving, where Sheck Exley set it out in Basic Cave Diving: A Blueprint for Survival: a third of the gas to go in, a third to come out, a third in reserve. It assumes steady breathing on the way out. Kas argues that if an out-of-gas emergency happens at the furthest point, a third will almost certainly be too lean for two stressed divers. In open water, where you can go straight up, rock bottom answers the more direct question of how much two people need to get up from here.

The two ideas combine well. Rock bottom sets the floor, and thirds or halves split the gas above it.

How to use rock bottom during the dive: turn pressure

Rock bottom is the latest moment to start up, not the moment to turn around. On a dive that has to end back at the anchor line or the shore entry, subtract rock bottom from your starting pressure to get usable gas, then turn when a share of it is gone.

DAN describes the same logic: a buddy pair can agree to head back when the first diver has used a third or half of their gas, not counting the reserve meant for the end of the dive. On a drift dive, where the boat collects you wherever you come up, there is no turn point, and rock bottom is simply the pressure at which the ascent starts at the latest. For an out-and-back dive from a 200 bar fill:

• Work out rock bottom for the deepest planned depth: 85 bar for 30 m in a 12 L tank.
• Subtract it from the starting pressure: 200 − 85 = 115 bar of usable gas.
• Choose the share for the outward leg. Half suits a return along the same route; a third leaves spare gas for a current on the way back.
• Turn at the starting pressure minus that share, rounded up to 5 bar: half of 115 is 57.5, so turn at 145 bar; a third is 38.3, so turn at 165 bar.
• Whoever reaches their turn pressure first turns the team.

What you surface with if nothing goes wrong

About 65 bar, if you leave the bottom at rock bottom on the 30 m example and the ascent is normal. One diver at a calm 15 L/min uses about 252 L for the same profile (a minute at depth, the climb, the stop and the last 5 m), which is roughly 20 bar from a 12 L tank.

So starting up at rock bottom does not mean surfacing on fumes. The reserve is still in the tank, it just wasn't needed. The reverse also holds: if you routinely surface from a dive like this with far less, you left the bottom below rock bottom, and a gas-sharing ascent from that point would not have fitted in your tank.

What changes on deeper dives?

Rock bottom climbs steeply with depth, because every minute at depth costs more gas and the ascent gets longer. Between 18 m and 40 m the volume more than doubles, from 666 L to 1,370 L.

At 40 m (130 ft), a 12 L tank filled to 200 bar has 85 bar of usable gas left after rock bottom, and a half-usable turn comes at 160 bar (157.5 rounded up). That is a short dive on a single 12 L, and the 15 L column shows what a bigger cylinder buys. Shallow dives go the other way: at 12 m the formula gives 45 bar in a 12 L tank, which is below the 50 bar reserve our planner tells you to keep in every case, so plan on 50.

Three things the simple model leaves out matter more with depth. Nitrogen narcosis usually becomes noticeable around 30 m, which slows exactly the problem solving the model allows one minute for. The model has only the 3-minute safety stop, so it fits no-stop dives; a decompression obligation adds stops it does not count. And a slower ascent than 9 m/min adds time and gas. Decompression dives need minimum gas planned around their real stops and gases, which is outside what this calculator does.