Buying power as the mark price moves
Same spot deposit and perp size, sweeping the mark price ±20% from entry. Watch how fast cross-margin buying power (green when positive) diverges from the flat isolated-margin line — and where it goes negative, meaning the loss has started eating into spot collateral that has nothing to do with the trade.
| Price move | Perp PnL | Cross buying power | Isolated buying power | Extra tradeable notional |
|---|
Two designs, one open perp position
On an isolated-margin perp DEX, the margin posted for a trade and any unrealized PnL on it sit in a walled-off bucket. Your spot balance's usable buying power never changes just because the perp is winning — you have to close the trade first. On Vertex's cross-margin design, every asset (spot and perp) gets a risk weight and feeds one account health number, so a winning perp raises your total buying power in real time, exactly like a deposit would. See multi-collateral health factor for the equivalent weighted-collateral math when it's multiple spot assets instead of a spot+perp mix.
The math behind unified account health
A cross-margin account's usable buying power is spot_collateral × spot_weight + unrealized_perp_PnL − margin_already_locked. The spot weight (often 0.85-0.95 for majors and stables, lower for volatile long-tail assets) is a haircut against price risk, same idea as an Aave collateral factor. The perp term is the part that isolated-margin venues don't give you: PnL = notional × (mark/entry − 1) × (long ? 1 : −1) flows straight into that shared number the instant the mark price ticks, whether it's positive or negative — nothing about it is realized or settled, it's just marked.
Compare that to an isolated-margin account, where buying power for anything new is simply spot_collateral × spot_weight, full stop — the open perp's margin and PnL are invisible to the rest of the account until the position is closed and the funds are withdrawn back to the spot balance. The gap between the two — cross_buying_power − isolated_buying_power = unrealized_PnL — is exactly the capital-efficiency edge cross-margin is built to sell: a winning trade lets you size up elsewhere without waiting to take profit.
That gap cuts both ways once you multiply it by leverage. If a new position uses the same initial margin requirement as the open one, the extra (or reduced) buying power translates into extra_buying_power ÷ (imf/100) of additional (or lost) tradeable notional. At 10% IMF (10x), a $1,000 unrealized gain isn't just $1,000 of extra room — it's $10,000 of extra notional you could open elsewhere, immediately. A $1,000 unrealized loss removes the same $10,000 of room, and if the loss is large enough to push the shared health number negative, the account starts liquidating spot collateral to cover it, not just the losing position itself.