Buying on Margin

A margin purchase splits the position into two claims: the broker's loan, which is a fixed dollar amount, and the investor's equity, which is the residual. Equity equals the current market value of the collateral securities minus the outstanding loan balance plus accrued interest. Because the loan does not move with the market, every dollar of price change accrues entirely to equity.

The leverage ratio is one divided by the initial margin percentage. Posting 50 percent equity gives a leverage ratio of two; posting 25 percent gives four. That ratio scales the percentage price move of the security into a percentage move in equity, before financing costs. It also works symmetrically: leverage magnifies losses on exactly the same multiple, and with sufficiently thin equity a modest adverse move can erase the investor's stake entirely.

The holding period return on a leveraged position is the total change in the investor's own capital divided by that capital. Build the numerator step by step: take the proceeds from selling the shares, subtract the loan principal that must be repaid, subtract the interest accrued on that loan, add any dividends received while holding, subtract commissions paid on both the purchase and the sale, then subtract the initial equity outlay. Divide the result by the initial equity, which is the purchase price times the number of shares times the initial margin percentage plus the buying commission.

Interest and dividends are the reason the answer is never simply the unlevered return times the leverage ratio. Borrowing costs drag on the position regardless of direction, while dividends on the full share count accrue to a much smaller equity base and can lift the levered return above the naive multiple.

Maintenance margin is the minimum ratio of equity to market value that the account must sustain continuously. As price falls, equity shrinks faster than market value, so the ratio deteriorates; when it breaches the maintenance level the broker issues a margin call demanding cash or additional collateral. Failure to meet it allows the broker to liquidate positions without further consent.

The trigger price follows directly from setting equity per share equal to the maintenance percentage of price. Equity per share is price minus the loan per share, and the loan per share is the purchase price times one minus the initial margin percentage. Solving gives the familiar relation between purchase price, initial margin and maintenance margin. Candidates should be able to derive it rather than memorise it, and to mirror the logic for a short sale, where the danger lies above the trade price.

In the United States, Regulation T sets the initial margin requirement for equities, exchange and self-regulatory rules impose a lower maintenance floor, and brokers routinely apply stricter house requirements on volatile or concentrated positions. Securities bought on margin are hypothecated to the broker, which may in turn rehypothecate them, linking client leverage to broker funding. Aggregate margin debt is tracked as a sentiment and vulnerability indicator.

Distinguish this from futures margin, which is a performance bond marked to market daily rather than a loan, so no interest is charged on it. The systemic concern is common to both: falling prices force sales, sales push prices lower, and further calls follow. Understanding that loop matters as much as the arithmetic.