The distinction between Initial Margin and Maintenance Margin is the difference between getting into a trade and staying in it. Beginners often confuse the two, leading to unexpected margin calls when they believe they still have plenty of "room" in their account. This section explores the mathematical nuances of these two thresholds and how they interact to create your trading boundaries.

Initial Margin: The Entry Requirement

The Initial Margin is the amount of equity required to open a new position. Under Regulation T, this is generally 50% of the purchase price for most stocks . However, the $2,000 minimum equity rule from FINRA is an absolute floor. If you want to buy $3,000 worth of stock, even though 50% is $1,500, you must still have $2,000 in the account to satisfy the FINRA minimum .

The Reg T Payment Period

When you make a trade that exceeds your current cash balance, you are essentially using your "buying power." Under current rules, you have a specific window to meet the initial margin requirement. This is known as the "payment period," which is typically four business days from the trade date (T+4), though this is moving toward a T+3 standard . If you fail to deposit the required funds within this window, the firm is required to liquidate assets in your account to cover the shortfall .

Maintenance Margin: The Ongoing Safety Net

Once the position is established, the Initial Margin requirement (50%) no longer applies. Instead, the Maintenance Margin requirement takes over. This is a lower threshold—FINRA sets it at 25%—designed to give the investor some breathing room for normal market fluctuations .

Why the Thresholds Differ

The gap between the 50% initial requirement and the 25% maintenance requirement exists to prevent every minor price dip from triggering a margin call. It creates a "buffer zone."

Example Scenario: The Buffer Zone

Purchase: You buy $20,000 of Stock A.
Initial Margin (50%): You deposit $10,000. Your loan is $10,000.
Maintenance Requirement (25%): Your equity must stay above 25% of the current market value.
The Math: If the stock stays at $20,000, your required maintenance equity is $5,000 ($20,000 * 0.25). Since you have $10,000 in equity, you have a $5,000 "cushion."

House Requirements: The Broker's Prerogative

It is a common misconception that the 25% FINRA rule is the only one that matters. In reality, most major brokerages use "House Requirements" that are stricter. Fidelity, for instance, notes that many firms require 30%, 35%, or even 40% equity .

Requirement Type	Percentage	Source
Reg T Initial	50%	Federal Reserve
FINRA Maintenance	25%	FINRA Rule 4210
Typical House Maintenance	30% - 40%	Individual Brokerages

Factors Influencing House Requirements

Brokers don't just pick these numbers at random. They increase requirements based on:

Volatility: Stocks with large daily price swings are riskier for the broker to hold as collateral .
Concentration: If your entire portfolio is in one stock, the broker may raise the requirement because a single bad news event could wipe out your equity .
Liquidity: Stocks that are hard to sell (low volume) often have higher requirements because the broker might not be able to liquidate them quickly in a crisis .

Calculating Your "House Surplus"

To know exactly where you stand, you need to look at your "House Surplus." This is the amount of equity you have above your broker's specific requirement .

Formula: House Surplus = Current Equity - (Market Value * House Requirement %)

If your House Surplus is a positive number, you are safe. If it becomes a negative number, it is reflected as a "House Call," meaning you must add funds to bring the equity back up to the house minimum .

The Role of the SMA (Special Memorandum Account)

In some margin accounts, you will see a balance labeled "SMA" or "Fed Call." The SMA is a special account that tracks the excess equity created when your stocks go up in value . Under Reg T, if your stocks increase, you gain additional buying power. However, if the initial requirement is not met, a "Fed Call" is issued . While maintenance calls are about protecting the broker from a falling market, Fed calls are about ensuring you have enough skin in the game when you first enter a trade.

Frequently Asked Questions: Margin Requirements

Can my broker change my maintenance requirement without telling me? Yes. Firms can increase house requirements at any time and are not required to provide advanced notice .
What is the absolute minimum I need to trade on margin? FINRA requires at least $2,000 in account value .
Does the maintenance requirement apply to short selling? Yes, margin accounts are required for shorting, and maintenance rules apply to those positions as well .
What happens if my equity falls below $2,000 due to a market drop? If the decline is purely due to market value and you make no new trades, no immediate deposit is necessary. However, you cannot make new commitments until the equity is restored .
Is interest included in these calculations? Yes, while your loan balance stays the same, it actually grows over time as monthly interest accrues, which slowly eats away at your equity .