Rock Bottom & Rule of Thirds
Two reserves, two questions. Rock bottom is the minimum gas two divers need to surface safely from the deepest point. Thirds tells you when to turn the dive so that reserve stays intact. Drag the sliders.
Rock bottom (minimum gas)
Dive plan
Minimum gas to bring both divers to the surface, with a safety stop.
Back-gas reserve is calculated only down to here — above it you're on deco gas.
UK HSE / IMCA reference — RMV per diver, surface
Defaults (20 × 2.0 = 40 L/min per diver) match the HSE emergency planning figure.
The donor's cylinder volume. Use the twin figure for doubles.
Ascent assumptions
Defaults follow common recreational practice: a one-minute stress pause, a 9 m/min ascent, and a 3-minute stop at 5 m. Add deco time here for staged dives.
Result
Minimum gas to keep
— bar
— L surface-equivalent. Never let your gauge drop below this.
Problem
—
L
Ascent
—
L
Stop
—
L
Rule of thirds
Gas on hand
Computed on full starting pressure. Stricter fractions leave a larger reserve.
Result
Your gas, split
Turn pressure
— bar
Turn the dive once you've breathed the first third.
Reserve
— bar
— L
Each leg
— bar
— L
Thirds is the standard reserve rule for overhead and decompression diving. Always compare it against your rock bottom from section 1 and dive the more conservative turn pressure.
Common questions
What is rock bottom gas?
Rock bottom (minimum gas) is the smallest amount of gas you can have and still get two divers safely to the surface from the deepest point of the dive, sharing a single supply. It covers a short problem-solving pause at depth, a slow ascent for both divers, and the safety or decompression stops. You never plan to dip below it.
How is rock bottom calculated?
Sum the gas both divers breathe during each phase of an emergency ascent: a stress reaction at maximum depth, the ascent itself (using the average depth of the climb), and the safety stop. Each phase is the combined SAC of both divers, times a stress factor, times the absolute pressure, times the time. This tool takes each diver’s baseline SAC separately so a heavy breather in the team is accounted for.
How does minimum gas work for technical dives?
On a staged decompression dive your back-gas reserve only needs to bring the team from the bottom up to your first decompression gas, not all the way to the surface. Switch the calculator to Technical mode and set your gas switch depth; it computes the back gas for both divers to reach that switch, sharing, at the elevated stressed SAC. Above the switch you are on deco gas, which you carry and plan separately.
What SAC rate should I use for an emergency?
UK HSE and IMCA commercial diving figures put a working diver around 35 L/min and bailout/emergency planning at 40 L/min, with severe work reaching 62.5 L/min and a panicked diver 50–75 L/min (HSE research RR1073). A common recreational approach is to take each diver’s real resting SAC and multiply by a stress factor of around 2, which lands near the HSE 40 L/min emergency figure.
What is the rule of thirds?
Divide your usable gas into three: one third for the way out, one third for the way back, and one third held in reserve for emergencies. You turn the dive once the first third is gone. With a full 200 bar cylinder that means turning at roughly 133 bar. It is the standard reserve rule for overhead and decompression diving.
Should I use thirds or rock bottom?
They answer different questions. Rock bottom sets the floor you must never cross; the rule of thirds sets your turn point so you come back with that floor intact. On open-water dives with a free ascent, minimum gas plus a half-usable turn is common. In overhead environments, thirds (or stricter) is the norm. Use the more conservative of the two.