How this instrument works
Buying a stock on margin means paying only part of its price in cash, the initial margin, and borrowing the rest from the broker. That loan is a fixed dollar amount; it does not shrink when the stock falls, so every dollar the price drops comes straight out of the sliver of equity you funded yourself. A margin call arrives at the point where that equity, measured against the position's current market value, drops below the maintenance margin the broker requires on an ongoing basis, a separate figure that has nothing to do with the percentage you originally put down.
The trigger price falls out of one equation: equity divided by current market value must stay at or above the maintenance percentage. Substitute in the fixed dollar loan and solve for price, and the initial margin and the maintenance margin end up on opposite sides of a ratio rather than simply subtracted from each other. That is why a position bought at 50% initial margin with a 25% maintenance requirement calls after roughly a one-third price drop, not a one-quarter drop; the two percentages interact through division, and the gap between them is what actually protects the account.
Traders running a personal margin account, and the risk desks that set house requirements above the exchange minimum, use this number to know how much room a position has before a maintenance shortfall forces a decision: deposit more cash, sell part of the position, or let the broker liquidate it without being asked first. The result assumes a single purchase price and no partial paydowns or added shares; real accounts often blend several lots bought at different prices, and brokerages routinely set house maintenance requirements above the regulatory floor, so a live account's true call price can sit well above what this instrument returns.
- Enter the Purchase price per share, $ — the price you paid before any move up or down.
- Set the Initial margin, % — the share of that price you funded in cash rather than borrowing.
- Set the Maintenance margin, % — the broker's minimum equity cushion once the position is open.
- Read the Price that triggers a margin call — the level below which the broker demands more cash or begins selling.
Worked example — $100 stock, 50% down, 25% maintenance
Buy one share at $100 (P₀ = $100) with 50% initial margin: you put up $50 cash and borrow the other $50 from the broker, and that $50 loan stays fixed no matter what the stock does next. With a 25% maintenance margin, the formula gives P_call = 100 × (1 − 0.50) ⁄ (1 − 0.25), which is 50 ⁄ 0.75, or $66.6666666667, shown here as $66.67.
At $66.67 the equity in the position is $66.67 minus the $50 loan, or $16.67, and $16.67 divided by $66.67 is exactly 25%, the maintenance floor. One cent lower and the account is undercollateralized: the broker issues a margin call, and if more cash or securities do not arrive within its deadline, it can sell the position without further notice to bring the account back into compliance.
Questions
Why does 50% initial margin trigger a call at roughly a 33% price drop, not 50%?
Because the maintenance requirement, not the initial one, sets the trigger. Initial margin only fixes how much you borrowed at purchase; maintenance margin fixes how much equity the broker demands afterward, measured against the current price rather than the purchase price. Solving equity divided by price equals maintenance percentage, for a fixed loan, produces a call price above a simple 50% haircut — in the $100 example here the stock only has to fall to $66.67, a 33.3% drop, well before the original down payment is gone.
What actually happens once the price reaches the call level?
The broker issues a margin call demanding you deposit cash or securities to restore the account to its maintenance requirement, often within one to a few business days. Miss that window and the broker can sell holdings in the account, sometimes without contacting you first, to bring equity back above the floor — and the broker chooses which positions to sell, not you.
Is 25% maintenance margin a fixed rule everywhere?
No. It is a common regulatory floor for many marginable securities, but individual brokerages routinely set house requirements higher, often 30% to 40%, and volatile or low-priced stocks can carry steeper minimums still. Confirm your own broker's maintenance percentage in the account agreement rather than assuming this figure applies to your holdings.
Does raising the initial margin I put down protect me from a call?
It buys a lower call price, not immunity. More cash down means a smaller loan, so equity stays above the maintenance floor through a larger price decline before the same equation triggers. Putting down 100% removes leverage and margin risk entirely, since there is no loan for a falling price to erode against a fixed maintenance percentage.
Why is the purchase price part of this formula at all?
Because the dollar amount borrowed, which stays fixed regardless of where the price moves, is set once at purchase as a percentage of that price. Double the purchase price and the loan doubles too, so the call price scales up in direct proportion — this instrument shows that by design, not as a coincidence of the numbers chosen.
Can the call price ever equal the purchase price itself?
Yes, if the initial margin percentage equals the maintenance percentage: the formula then reduces to P_call = P₀, meaning any price drop at all, however small, breaches the requirement. Brokers set initial margin comfortably above maintenance margin specifically to avoid this, giving a real price cushion before a call can occur.
References
- Federal Reserve — Regulation T, Credit by Brokers and Dealers
- SEC Investor.gov — Investor Bulletin: Understanding Margin Accounts
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.