Gas Planning & Thirds
RMV, depth, time and cylinder in — gas required, turn pressure, reserve and an honest verdict out. If the plan does not fit the cylinder, the tool says so before the water does.
Using your saved RMV — the average of your logged dives.
The maths, explained
Gas is time. A segment of a dive needs litres = RMV × P × minutes, with P = depth ÷ 10 + 1 — at 30 m every minute costs four surface-minutes of gas.
The rule of thirds splits the cylinder into thirds: one third in, one third out, one third untouched. The reserve is not for a longer dive — it is your answer to a team-mate’s failure at the worst moment. So the plan has to fit inside two thirds of what you carry, and the turn pressure is fixed before you get wet:
turn pressure = start − (start ÷ 3) · reserve = start ÷ 3
When a plan does not fit, there are exactly three levers: less time, less depth, or more gas. Nothing else changes the arithmetic.
This Is Exactly What We Teach On Decompression Procedures
Turn pressure is where planning becomes discipline. Decompression Procedures is where you learn to build these plans, dive them, and handle what happens when they change at depth.
Checked, not trusted: the formulas on this page are pinned by 130 automated reference checks that run on every change — cross-validated against Subsurface’s published planner tests, the independent DecoTengu implementation, printed MOD tables and the team’s own Splash apps.
For training and planning purposes only. Analyse every mix before you breathe it and always dive within your certification.