What is a margin call, and how does it force a crash to snowball?

What is a margin call in plain terms?

A margin call happens when you borrow from a broker to buy shares and the value of your account drops too far. The shares serve as collateral for the loan, so a falling price weakens the lender’s protection. The broker then asks for more money or more collateral, or tells you to sell some holdings. The call is a demand to restore the cushion, and it arrives when prices are already falling.

The word margin refers to your own share of the position, the equity left after the loan. Regulation T lets a broker lend up to 50% of a purchase price on new purchases (FINRA). A margin account therefore starts with a cushion of equity, and the call arrives when losses eat into that cushion faster than you can replace it. The mechanism is simple, and its effect on a crash is large, as later sections show.

How does a margin call work from purchase to demand?

The sequence has 5 steps. You buy shares with part of your own money and a loan, the shares become collateral, and the broker watches the account against a maintenance level. When prices fall and equity drops below that level, a margin deficiency exists. The firm issues the call, and you must deposit more collateral or liquidate positions (FINRA). Each step uses rules that set a floor for your equity.

Calls follow 3 kinds of events, according to FINRA investor education: a trade that creates a deficit, a drop in the value of the account, or a firm raising its own requirement. The third kind matters in a crash, because brokers tighten terms when volatility rises. A rule change by the lender can therefore trigger calls even before your shares fall further. Terms vary by firm and product, so the account agreement is the reference.

Which price fall triggers a margin call on a 1,000-share purchase?

Take 1,000 shares bought at $50, a purchase value of $50,000. With a 50% loan, you invest $25,000 of your own money and borrow $25,000. Suppose the firm requires equity of at least 25% of the market value, a level used here for illustration, since firms set their own. Equity equals market value minus the loan, so the call arrives when the shares reach $33.33.

That is a fall of 33.3%, and your equity at that moment is about $8,333. The loss on your own money is 67%, since $25,000 became $8,333. A higher firm requirement moves the trigger closer to the purchase price, and a portfolio of more volatile shares reaches it sooner. Running this calculation before you borrow is the simplest protection.

What choices does an investor have after a margin call?

3 responses exist. You can deposit cash, which restores equity and keeps the position, you can deposit other securities that the broker accepts as collateral, or you can sell part of the position and reduce the loan. Each response needs speed, because the call sets a deadline. A deposit made in time keeps the decision in your hands, while a late one hands the decision to the broker.

The broker holds a right that matters in a crash: it may liquidate an account at any time to eliminate a margin deficiency (FINRA). The firm picks the securities and the timing, usually in a falling market, so the sale price is poor. Investors with spare cash held back keep control, and investors who have spent every unit of cash become forced sellers. This is the point where one person’s problem becomes the market’s problem.

How does a margin call make a stock market crash snowball?

One call is a private event, and thousands of calls are a market event. When prices fall, many leveraged accounts cross their thresholds in the same hours. Forced sales from those accounts add supply to a market that already has more sellers than buyers, and prices fall again. The lower prices push other accounts across their thresholds, and the snowball grows. The guide on the margin call spiral follows each turn of the loop.

3 features speed the snowball. Calls arrive together, because accounts share similar leverage. Sales happen at any price, because the broker’s goal is to close the shortfall rather than to find the best price. And the information spreads, since a drop that triggers calls is visible to everyone, which prompts voluntary sellers to join. The loop ends when prices find buyers with cash or when leverage has been cleared.

What do 1929 and 1987 show about margin calls in a crash?

In 1929 buyers typically put down 10% of the price and borrowed the rest (Federal Reserve History), so a 10% fall erased a stake. On 28 and 29 October 1929 the Dow fell nearly 13% and nearly 12%. The guide on margin calls in the 1929 crash tells the story of those weeks and of the loans behind them. A cushion that thin made every call urgent.

In 1987 the Dow fell 508 points, or 22.6%, on 19 October, and margin calls rose to about 10 times their average size and about 3 times their previous highs. The strain on brokerage firms became the main danger after the crash, according to Wikipedia. The 2 episodes sit 58 years apart, and in both the call turned a fall into a chain of sales.

How can you manage margin call risk before a crash?

3 habits cut the risk. Borrow less than the maximum, so that the trigger price sits far below the purchase price. Hold cash that can meet a call without selling anything else. And recompute the trigger price when volatility rises, since firms raise requirements in those weeks. A written trigger price turns an emergency into a routine. Review the loan terms each quarter and record the result with a date.

The EQOS Asia Λ simulator tests a leveraged case in 10 minutes, and a 30-minute pre-diagnostic applies the scoring to a real exposure. This page is general information, not investment advice, as of October 2026. Terms differ by firm and by country, so the account agreement and the regulator’s guidance remain the reference for your own account. Pair the calculation with a stress test of 30% to see how many calls would arrive together.

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About the author

Stéphane Jambu

President, EQOS Forecast Asia

Stéphane Jambu leads EQOS Forecast Asia and works as a consultant in e-commerce, search visibility, and AI. LinkedIn profile