Margin requirement for stock futures changes on a daily basis?

Today i realized ( after taking a trade ) that the margin requirement for PNB was more than the usual days.

Yes I guess so, based on volatility.

“SPAN evaluates overall portfolio risk by calculating the worst possible loss that a portfolio of derivative and physical instruments might reasonably incur over a specified time period (typically one trading day). This is done by computing the gains and losses the portfolio would incur under different market conditions.
At the core of the methodology is the SPAN risk array, a set of numeric values that indicate how a particular contract will gain or lose value under various conditions. Each condition is called a risk scenario. The numeric value for each risk scenario represents the gain or loss that particular contract will experience for a particular combination of price (or underlying price) change, volatility change, and decrease in time to expiration.”

The crazy SPAN methodology explained here.

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