Risk-based accounting is the "brain" inside a portfolio margin account. Unlike the "rule-of-thumb" approach used in standard margin accounts, risk-based accounting uses complex mathematical models to predict how a portfolio will behave under duress . This section explores the mechanics of these calculations, the concept of "netting," and how stress tests define your purchasing power.

Strategy-Based vs. Risk-Based Logic

In a standard Reg T account, margin is calculated using a "strategy-based" approach. This means the rules are fixed based on the type of trade you make. If you buy $10,000 of Stock A, the rule says you need $5,000 in equity (50%) . If you then sell $10,000 of Stock B short, the rule says you need another $5,000 in equity. Even if Stock A and Stock B are highly correlated (meaning they move together), the Reg T system treats them as two separate risks requiring $10,000 total margin.

Risk-based accounting, however, looks at the correlation and offsetting nature of the positions . If Stock A and Stock B are in the same industry and move in tandem, a long position in one and a short position in the other effectively neutralizes the market risk. The portfolio margin system recognizes this "net" risk and might only require a fraction of the collateral required by Reg T .

The Stress Test: Simulating Market Chaos

The primary tool of risk-based accounting is the "stress test." Brokerages simulate a range of market outcomes to see how they would impact your specific portfolio . For a standard equity portfolio, the system might simulate a 15% move in the underlying price of each security, both up and down .

How the Stress Test Works:

Identify the "Basket": The system groups related securities (e.g., a stock and its associated options) into a single risk unit .
Apply Price Shocks: The model calculates the theoretical value of that basket at various price points (e.g., -15%, -10%, -5%, 0%, +5%, +10%, +15%).
Volatility Adjustments: For options, the model also simulates changes in "implied volatility" to see how the options' prices would react to a "scare" in the market.
Determine the "Worst Case": The margin requirement for that basket is set based on the single largest loss found across all those simulated points .

Netting and Offsetting Positions

The true power of risk-based accounting lies in "netting." This is the process of combining long and short positions to find the "residual" or "remaining" liability .

Example: The Netting Effect
Imagine you own 1,000 shares of a volatile tech stock trading at $100 ($100,000 total value).

Under Reg T: You would need $50,000 in initial margin .
Under Portfolio Margin: The broker simulates a 15% drop. If the stock drops to $85, you lose $15,000. Your margin requirement might be set near that $15,000 mark.

Now, imagine you also buy 10 Put options with a strike price of $95.

Under Reg T: You still need the $50,000 for the stock, plus the full cost of the options.
Under Portfolio Margin: The system sees that if the stock drops to $85, your put options will gain significantly in value, offsetting the loss on the stock. The "worst-case" loss for the combined position is now much smaller—perhaps only $5,000 (the difference between the $100 price and the $95 strike). Consequently, your margin requirement drops from $15,000 to $5,000 .

The TIMS Methodology

Most brokerages use a system called the Theoretical Intermarket Margining System (TIMS), developed by the Options Clearing Corporation (OCC). TIMS is designed to calculate the minimum amount of net capital required to cover the potential loss in a portfolio over a one-day period . It is a "composite-margin" policy that includes swaps, options, and futures .

Data Visualization: Margin Requirement Comparison

The following table compares the estimated margin requirements for various strategies under the two systems:

Strategy	Reg T Requirement	Portfolio Margin Requirement
Long Stock	50% of Market Value	~15% of Market Value
Short Stock	~50% of Market Value	~15% of Market Value
Long Stock + Put	50% Stock + 100% Put	Max loss (often < 10%)
Iron Condor	Width of the spread	Simulated risk (often 70-80% lower)
Uncovered Short Put	~20% of Underlying	Simulated risk (based on 15% move)

The "Concentration" Penalty

While risk-based accounting rewards diversification, it penalizes concentration. If your portfolio consists of only one or two stocks, the broker-dealer is required by FINRA Rule 4210 to increase your margin requirements . This is because a single-stock event (like a bad earnings report) could wipe out the account more easily than a broad market move. In these cases, the "stress test" might be expanded to simulate a 25% or 30% move instead of the standard 15% .

Step-by-Step: How Your Margin is Calculated Daily

Market Close: At the end of the trading day, the broker takes the closing prices of all your positions .
Valuation: The "current market value" is determined .
Simulation: The risk model runs thousands of "what-if" scenarios across your entire portfolio .
Requirement Setting: The highest potential loss across these scenarios becomes your "Maintenance Requirement."
Equity Check: Your "Account Equity" (Total Assets - Total Liabilities) is compared to this requirement .
Excess/Call: If Equity > Requirement, you have "House Surplus." If Equity < Requirement, you receive a "House Call" .

Frequently Asked Questions: The Math

Q: Does portfolio margin use "Greeks" like Delta and Gamma?
A: Yes. The underlying models (like TIMS) use option pricing models that account for Delta (price sensitivity), Gamma (rate of change of Delta), and Vega (volatility sensitivity) to simulate how the portfolio's value will change .

Q: Can the stress test percentages change?
A: Yes. Brokerages have the right to increase "house" requirements at any time without notice, especially during periods of extreme market volatility .

Q: Why is the requirement for an Iron Condor so much lower?
A: An Iron Condor is a "defined risk" strategy. Because you have both long and short options that cap your potential loss, the risk-based model recognizes that your "remaining liability" is very small compared to the total value of the underlying stock .