---
title: "Turn pressure"
description: "Working out the gauge reading at which to turn the dive, so an equal return and the reserve are still in the cylinder."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Turn pressure

import { TurnPressureScreen } from "@/components/react/ToolScreens";

Turn pressure answers *at what pressure do I turn the dive?* It finds the gauge
reading that still holds enough gas for a return as long as the outbound leg,
plus the reserve you decide to keep.

## What you enter

- **Outbound leg**: **Depth** (m) and **Time** (min) of the leg before you
  turn, at one constant depth.
- **Rule**: **Thirds** keeps a third of your start pressure as the reserve.
  **Custom** opens **Keep** (% of start), the reserve as a share of the start
  pressure.
- **RMV** (L/min) and **Water**: shared with the other consumption tools.
- **Size, each** (L), **Start** (bar) and **Cylinders**: your cylinder, also
  shared.

## How the turn is chosen

The reserve comes off the start pressure first. Half of what remains may be
used going out, because the other half has to bring you back. That half is
**Allowed out**. With thirds it is a third of the start pressure.

The planner then compares **Allowed out** with the gas your planned leg would
use, and you turn at whichever limit you reach first:

- **The leg ends first.** Your planned time uses less than **Allowed out**.
  Turn at the planned time; **Turn at** is the reading you should see then.
- **The gas runs out first.** The planned leg would use more than
  **Allowed out**. **Turn at** is the reading that leaves an equal return and
  the reserve, and you reach it before the planned time. The figure shows in
  amber and the verdict says so. The screen above is this case: the
  20-minute leg would use 87 bar against 77 bar allowed out, so the turn comes
  at 155 bar.

**Turn at** rounds up to the whole bar, so the turn never comes later than the
rule allows.

## Reading the result

- **Turn at** (bar): the main figure. Its rail runs up to your start pressure,
  with a solid mark at the reserve.
- **Leg would use** (bar): the gas the planned leg uses at your RMV.
- **Allowed out** (bar): the most the outbound leg may use.
- **Reserve** (bar): the pressure you hold back under the rule.

## What the figure does not cover

- **A return that differs from the way out.** The return is assumed to use as
  much gas as the outbound leg. A current, a deeper return or a slower swim
  back needs more.
- **A change of depth.** The leg is one depth for its whole time; a leg that
  climbs or drops uses a different amount.
- **Whether the reserve is enough.** Thirds is a rule, not a calculation. Work
  out what two divers need to reach the surface with
  [Minimum gas](/tools/minimum-gas), and compare it with the **Reserve**.

## Next

- [Minimum gas](/tools/minimum-gas) sizes the reserve from the ascent itself.
- [RMV from a dive](/tools/rmv-from-a-dive) gives the RMV these figures rest
  on.

Source: https://docs.divelogic.ai/tools/turn-pressure/index.mdx