What Is a Margin Account? How It Works, the Rules, and 33 Years of SPY at Twice Your Cash

Michael Whitman · Trading since 2017, full-time since 2020 · YouTube creator, 137K subscribers · Published 2026-09-06 · Updated 2026-11-03 · Reviewed 2026-11-03

A margin account is a brokerage account that lends you money against the securities in it, so you can hold more than your cash buys, and the broker can sell those securities if your equity falls below its requirement. Bought at twice your cash, SPY’s history since 1993 produced a call at FINRA’s minimum within a year from 3.42% of starting days, and from 19.14% at a 40% house requirement.

How it works

A margin account is a loan facility attached to a brokerage account. You put up cash, the broker lends against it, and you hold a larger position than your own money would buy. A cash account, by contrast, must pay in full; the SEC’s investor bulletin notes that some account applications make margin the default type, so check which one you are signing for.

If you are new to investing, leave margin switched off. A cash account can lose what you put in; a margin account can lose more.

The collateral is the position itself, so the thing backing the debt and the thing losing value are the same. Leverage is applied to the outcome, not to the probability.

Two numbers govern it. Regulation T’s initial requirement (12 CFR 220.12) is 50% for a margin stock, so $10,000 of cash buys $20,000 of stock. FINRA Rule 4210 adds a floor: at least $2,000 of equity before the broker extends credit, though you never have to deposit more than the cost of what you bought.

The maintenance requirement is the fraction your equity must stay above once the position is on. FINRA Rule 4210’s floor is 25% of the market value of stocks held long. The same rule tells every firm to set its own house requirements, which can be higher on any stock, and Rule 2264’s disclosure says the firm can raise them “at any time” without advance written notice.

The margin maintenance arithmetic

Equity is position value minus the loan; the call comes when equity ÷ position value falls under the maintenance fraction. Call price = loan ÷ (shares × (1 − maintenance fraction)). Bought with half your own money at a 25% requirement, that is a fall to two-thirds of entry, a 33.33% fall; on a purchase at $75 the call level is exactly $50.

Borrowing three dollars for every one of your own removes the cushion. At four dollars of stock per dollar of equity the call price as a fraction of entry is 3 ÷ (4 × 0.75) = 1.0 — the maintenance line sits at your entry price. That was the most a pattern day trader could extend intraday under FINRA’s old rules, which Regulatory Notice 26-10 replaced, $25,000 minimum equity included, with intraday standards sized to actual exposure, effective 4 June 2026. Firms that need more time may phase them in until 20 October 2027, so a broker may still be running the old rules; the day trading buying power page covers the change.

A worked example

Take $15,000 and buy $30,000 of stock: 400 shares at $75. You owe $15,000. At $60, a 20% fall, the position is worth $24,000 and equity is $9,000, 37.5% of value: no call, and you have lost 40% of your money on a 20% move.

At $50, a fall of one third, the position is worth $20,000 and equity is $5,000, exactly 25%. That is the call at FINRA’s minimum. A house requirement moves it, on the same 400 shares and $15,000 loan:

A 40% house requirement halves the room.

What arrives is a demand, and any deadline is a courtesy. The disclosure every US broker must hand you under FINRA Rule 2264 says the firm “can sell your securities or other assets without contacting you”, that you do not choose what is sold, and that you are “not entitled to an extension of time on a margin call.” An extension may be available under certain conditions, never as a right.

The same lines on a real crash: SPY in 2020

Put $10,000 with $10,000 of margin into SPY at the close on 19 February 2020, $338.34, the high before the pandemic sell-off. The fractions above turn into four prices: $281.95 at a 40% requirement, $260.26 at 35%, $241.67 at 30% and $225.56 at FINRA’s 25%.

• 40% line: crossed on 9 March, 13 sessions in, at a close of $274.23.
• 35% line: crossed on 12 March at $248.11.
• 30% line: crossed on 16 March at $239.85.
• 25% line: crossed on 23 March at $222.95, a fall of 34.10%, and the lowest close of the whole decline. SPY closed at $243.15 the next day.

At that 23 March close the position was worth $13,179.05 against the $10,000 loan, so equity was $3,179.05, or 24.12%. FINRA’s 25% asked for $3,294.76: a shortfall of $115.71, met with $115.71 of cash or by selling $462.85 of SPY at the low. Even on intraday prices the line held until that day; the lowest trade before it was $228.02 on 18 March. A house at 40% would have been calling for two weeks by then.

How often a buyer at twice their cash was called

Repeat that purchase at every daily close and watch the next 252 sessions, about a year. The test uses closing prices, as a daily margin check does, and leaves out loan interest (which pulls the line closer) and dividends (which push it away).

The calls bunch into a few bad years. Every SPY start that met a 25% call within the year began in 2000, 2001, 2002, 2007, 2008 or 2020. At 40% the list grows to 15 calendar years, including 2018, 2019, 2021, 2022, 2024 and 2025. For SPY, moving from 25% to 40% multiplied the called starts by more than five; for QQQ, whose falls ran deeper, by less than two (856 to 1,463). This is one broad fund and one technology fund; single stocks were not tested.

What came after the call. A forced sale locks in the loss, so the next question is how long holding on would have taken. Of the 282 SPY start dates called at 25%, SPY closed at or above the purchase price one year (252 sessions) after the purchase on only 9, and two years after on 12. Every one of them did get back eventually: counting from the first close under the call line to the first close at or above the purchase price, the median wait was 640 sessions, about two and a half years, the shortest 97 and the longest 1,408. At a 40% house line, 484 of the 1,578 called starts were at or above the purchase price a year later and the median wait was 375 sessions. These are closing prices, dividends left out. A buyer with no margin could have waited; a margin buyer without the cash to meet the call did not get the choice. Each called start, with its call date and recovery date, is in the called-starts table.

Meeting a margin call: cash against sales

Back in the $75 example, let the price fall to $45. The position is worth $18,000 against the $15,000 loan, so equity is $3,000; 25% of $18,000 is $4,500, a $1,500 shortfall. Depositing $1,500 fixes it. Selling takes $6,000 of stock, because a sale repays loan and shrinks the position equally, so equity stays at $3,000 while the requirement falls 25 cents per dollar sold: $3,000 = 25% × ($18,000 − $6,000). At a 25% requirement selling takes four times the cash amount; at 40%, two and a half times.

Two kinds of call behave differently. For a maintenance call, Rule 4210 gives the firm at most 15 business days to collect unless FINRA grants more time. A Regulation T call, on a new purchase, must be met within one payment period under 12 CFR 220.4, extendable only by the firm’s examining authority on the firm’s application; if unmet the firm must sell enough to cover it (shortfalls of $1,000 or less excepted). The loan also carries interest, whether the position works or not.

The short side of a margin account

Every short sale lives in a margin account. Regulation T sets the short’s initial requirement at 150% of the stock’s value: the proceeds count as 100% and you deposit 50%. Short 100 shares at $50 and the credit is $7,500. FINRA Rule 4210’s maintenance for a short stock at $5 or more is the greater of $5 a share or 30% of value, so the call comes when $7,500 − 100 × price = 0.30 × 100 × price: $7,500 ÷ 130 = $57.69, a rise of 15.38% (30% there is $17.31 a share, above the $5 floor). The long had a 33.33% cushion; the short has less than half, and a rise has no ceiling.

The original data

The SPY and QQQ test uses Yahoo Finance daily closes downloaded 1 November 2026 (split-adjusted, not dividend-adjusted), start dates from 29 January 1993 (SPY) and 10 March 1999 (QQQ) to 29 October 2025, the last with 252 sessions after it. A start counts as called if any of the next 252 closes is below the call price; a close exactly on the line is not a call. Every start and its lowest close is in the start-date table, and the 2020 prices in the crash table.

How much is borrowed across the industry. Under FINRA Rule 4521, member firms carrying margin accounts report customers’ total debit balances as of each month’s last business day. FINRA’s margin statistics page, read on 1 November 2026, still ends at $1,480,054 million for September 2026, against $1,126,494 million a year earlier in FINRA’s workbook: up 31.39%, while the S&P 500 rose 14.40%, from 6,688.46 to 7,651.54 (Yahoo Finance month-end closes, price only). The record is June 2026 at $1,502,072 million. Customers also held $433,909 million of free credit balances in cash and margin accounts. The workbook downloaded that day is byte-for-byte the one read on 27 October.

Over twelve months, margin debt swings harder than the market. Comparing each month-end with the same month a year earlier, February 2011 to September 2026, gives 188 twelve-month changes. Margin debt’s change was the larger of the two in 118 (62.77%); the two went the same direction in 152 (80.85%), with a correlation of 0.84. In the 23 spans where both fell, margin debt fell further in 21. The largest rise was the year to March 2021, +71.62% against the index’s +53.71%; the largest fall the year to December 2022, -33.34% against -19.44%.

A second source agrees on direction. The Federal Reserve’s Z.1 accounts carry broker-dealer receivables from customers, a different measure that includes other receivables and is a smaller number ($742,321 million in June 2026). On the 62 quarter-ends from March 2011 to June 2026 its twelve-month change went the same direction as FINRA’s 54 times (87.10%), correlation 0.85. These are totals across all customers, not any one account, and say nothing about what the index does next. Data: the twelve-month table and the monthly table.

When it fails

A stop-loss does not cap a margin loss. If the market opens through the stop, you are filled where it opened, and a large enough gap can leave the account owing more than it holds.

A margin account can go below zero, because the debt is fixed while the collateral is not; the deposit is not the maximum loss.

The failure that catches people is timing, not direction. In 2020 the 25% line was touched on the one close that turned out to be the low; a buyer forced to sell there, or at a 40% house line two weeks earlier, was right about SPY and still paid for it.

And a call met by selling shrinks the position faster than it feels: four dollars of stock per dollar of shortfall at the FINRA minimum, at the worst prices of the move. The guide to avoiding a margin call sizes the buffer before the trade.

Questions people ask

Should a beginner use a margin account?

The SEC’s investor bulletin says margin accounts “are not appropriate for everyone”. The difference is the size of the worst case: a cash account caps the loss at what you put in, and a margin account does not.

How much money do you need for a margin account?

FINRA Rule 4210 requires $2,000 of equity before the firm extends credit, or the full cost if the purchase is smaller; firms may ask for more. Regulation T then lends up to half of a stock purchase.

Do you have to pay back a margin account?

Yes. It is a loan with interest, repaid by deposits or out of sale proceeds, and the margin agreement sets how; firms generally give at least 30 days’ written notice before changing how interest is computed.

Can a broker sell my stock without telling me?

Yes. The FINRA Rule 2264 disclosure says the firm can sell your securities without contacting you, and that you are not entitled to an extension of time on a margin call.

Related

Margin is the account; leverage is what you do with it. Short selling is a margin position whether you asked for one or not. The pattern day trader rule governed intraday buying power until June 2026 (phase-in runs to October 2027), settlement explains why a cash account behaves so differently, and cash account vs margin account lays the two sets of rules side by side.

This page is educational, not financial advice. Test every idea on your own charts before risking money.