Portfolio Margining

Reg T, the older US margin regime built for stocks, was never designed with multi-leg options strategies in mind — it margins each position with a fixed formula regardless of what else is in the account, so a fully hedged collar can require nearly as much margin as a naked directional bet. Portfolio margining replaces that rule-based approach with a risk-based one: it looks at the whole account's exposure under a range of simulated market moves and requires collateral for the worst realistic outcome, not the sum of each leg's standalone worst case.

The mechanics

A typical portfolio margin engine (similar in spirit to SPAN) shocks the underlying price across a defined range — commonly ±15% for equities — combined with volatility shifts, revalues every position in the account at each point, and sets the requirement to the largest resulting loss:

$$Margin = \max_{p \in [-15\%,\,+15\%]} \left( -\Delta V_{\text{portfolio}}(p) \right)$$

In words: walk the underlying price through a grid of down and up moves, recompute what the entire account would be worth at each point (including the options' changing deltas and gammas), and hold collateral equal to the single worst loss found anywhere on that grid.

Worked example

An account holds a covered call: long 1,000 shares of a $50 stock and short 10 call contracts struck at $55. Under Reg T, the stock alone requires 50% margin ($25,000) and the short calls carry their own separate margin add-on, pushing total requirements well above $25,000. Under portfolio margin, the engine shocks the stock price from -15% ($42.50) to +15% ($57.50). At -15%, the stock loses $7,500 but the short calls, now deep out of the money, gain almost nothing back — a $7,350 net loss, the worst point on the grid. Portfolio margin requires roughly $7,350, less than a third of the Reg T figure, because the covered call structure genuinely caps downside risk that Reg T's formula doesn't recognize.

What this means in practice

Portfolio margining is why sophisticated options traders and hedge funds can run leveraged, hedged strategies with a fraction of the capital a retail Reg T account would need for the same position — the exchange or broker is extending credit against the account's real risk rather than a conservative rule of thumb. It requires minimum account size and approval precisely because the capital efficiency only works if the account genuinely maintains hedged, not directional, exposure.