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

# Cross & isolated margin

> The two margin modes on Arcus perps — how cross and isolated margin measure risk differently, how to switch modes, and how to move collateral onto and off an isolated position

Every perp position runs in one of two **margin modes**, which decide what collateral backs the position and how its risk is contained.

* **Cross margin** — positions share your account's collateral, so the whole balance backs every position. Profit on one position can support another; a loss on one draws on the same shared equity.
* **Isolated margin** — collateral is allocated to a single position, capping that position's risk to its own allocation. A loss is contained to the collateral you have posted to that leg and cannot pull down the rest of your account.

**All positions are cross margin by default.** A position is only isolated after you explicitly enable isolated mode for that market.

<Note>
  This page is conceptual. For the underlying account and leverage mechanics see [Margin & leverage](/concepts/perpetuals/margin); read the current mode and effective leverage per market from [`GET /v1/leverages`](/api-reference/public/get-leverage-and-margin-mode-for-an-account-across-all-markets).
</Note>

## How margin is measured in each mode

The `marginUsed` field on a position row reflects the mode, and the two behave differently:

| Mode | How `marginUsed` behaves | | |
| - | - | - | - |
| **Cross** | The initial margin required at the current mark — \` | size | × mark price × initial-margin fraction\`. It **tracks the mark price**, moving up and down as the market moves. |
| **Isolated** | The walled-off collateral posted to that position's leg. It does **not** track the mark; it changes **only** when you move collateral in or out via `adjustIsolatedMargin`. | | |

In cross mode your used margin floats with the mark; in isolated mode it is a fixed pool you control directly, insulated from the rest of the account.

## Switching modes

Set the margin mode with the optional `isolated` flag on [`POST /v1/setLeverage`](/api-reference/exchange/set-leverage-for-a-market):

* `isolated: true` switches the market to isolated mode.
* `isolated: false` switches it back to cross mode.
* Omit the flag to leave the current mode unchanged.

A **cross → isolated** switch is rejected with `HAS_OPEN_POSITION` if you already have an open position in that market — close the position first, then switch to isolated mode. The reverse (**isolated → cross**) and any no-op change are always accepted.

Changing leverage or margin mode does not affect a position's maintenance margin or liquidation price — those depend on the market's maintenance-margin fraction, not the per-account leverage cap.

## Moving collateral in and out

Once a market is in isolated mode with an open position, use [`POST /v1/adjustIsolatedMargin`](/api-reference/exchange/add-or-remove-margin-on-an-isolated-mode-position) to move collateral between your shared cross bucket and the isolated leg:

* `amount` is a **dollar decimal string** (e.g. `"100"` for \$100) — send dollars, not quote quantums.
* **Positive** adds margin to the leg (cross → isolated); **negative** removes it (isolated → cross); zero is rejected.

The move requires the market to be in isolated mode with an open position, otherwise it is rejected with `NOT_ISOLATED` or `NO_OPEN_POSITION`. Adding margin fails with `UNDERCOLLATERALIZED` if the cross bucket lacks the free collateral to fund it; removing margin fails with `UNDERCOLLATERALIZED` if it would leave the isolated leg below its initial-margin requirement.

## Reading the current mode

The margin mode is exposed as `marginMode` (`CROSS` / `ISOLATED`) and its boolean twin `isolated` in three places:

* On **position rows** (snapshot and streaming).
* On [`GET /v1/leverages`](/api-reference/public/get-leverage-and-margin-mode-for-an-account-across-all-markets), alongside the effective leverage for every market.
* On the [`accountAttributeUpdates`](/api-reference/channels#accountattributeupdates) WebSocket channel, which folds the margin-mode outcome into leverage entries.

## What it means for liquidation risk

The mode determines how far a losing position can reach into the rest of your account:

* In **cross** mode, one shared balance backs every position, so a single position's loss draws on the collateral supporting all of them.
* In **isolated** mode, a position's collateral and risk are contained to its own leg. A move against that position can only consume the margin you have posted to it — the rest of your account is insulated.

Liquidation itself is still governed by maintenance margin the same way in both modes. See [Liquidations](/concepts/perpetuals/liquidations) for how a position becomes eligible for liquidation and what happens when it does.