Day Trading Buying Power: The Old Formula and What Replaced It
Day trading buying power (DTBP) was the FINRA Rule 4210 limit on a pattern day trader's trading in one session: four times the maintenance margin excess at the previous close. FINRA replaced it with an intraday margin standard on 4 June 2026, and brokers may phase the change in until 20 October 2027.
For about 25 years, a US margin account flagged as a pattern day trader had a second, separate limit sitting on top of ordinary margin: day trading buying power. It decided how much stock could be bought and sold inside one session, and crossing it produced its own kind of margin call. FINRA removed it in 2026. This page sets out the formula as it stood, what took its place, one hypothetical account run through both, and measured SPY and large-cap data on how quickly leverage near the limit gets tested.
How it works
The formula used yesterday’s numbers. Under the former Rule 4210(f)(8)(B)(iii), day-trading buying power was the account’s equity at the close of business on the previous day, minus the maintenance margin required under paragraph (c), multiplied by four for equity securities. The part inside the brackets is the maintenance margin excess: the equity above the 25% minimum that long stock needs.
It applied only to pattern day traders. The limit came with the designation described on the pattern day trader page, along with its $25,000 minimum equity. An account that was not flagged never had a DTBP figure in the rule’s sense.
What crossing the limit set off
Going over the limit set off a chain of steps. The account was margined on the cost of all the day trades made that day, at 25% for margin-eligible stock and 100% for stock that was not. The shortfall became a day-trading margin call, to be met within five business days. The multiplier also dropped from four to two, and the “time and tick” method of counting only the largest open position could not be used.
An unmet call froze leverage for 90 days. If the call was still open after five business days, the account could trade only on a cash-available basis for 90 days or until the call was met. Money deposited to meet it had to stay in the account for at least two business days.
What replaced it on 4 June 2026
The dates. The SEC approved FINRA’s amendments on 14 April 2026, FINRA published Regulatory Notice 26-10 on 20 April 2026, and the change took effect on 4 June 2026. Firms that need longer may phase it in until 20 October 2027, and FINRA’s investor guidance says a firm may keep running the old day trading requirements during that window. The pattern day trader flag, the day-trade count and the $25,000 minimum all went with it.
The new measure is the intraday margin level, or IML. Under new paragraph (d)(2) of Rule 4210, the IML is the cash a customer could withdraw while still meeting maintenance margin, or, as a negative number, the cash they would need to deposit to meet it. It is measured after every IML-reducing transaction: any purchase or sale that lowers it, a short sale included, any withdrawal, or a long option expiring in a way that lowers it.
A negative reading becomes a deficit. The intraday margin deficit for a day is at least the largest negative IML after any of those transactions. It must be met as promptly as possible, by net deposits or by anything else that raises the IML after the day ends, and an unpaid deficit lapses after the fifteenth business day.
The 90-day freeze now needs a pattern. A firm must block new or larger short positions and debit balances for 90 calendar days, or until the deficit is met, only if the customer makes a practice of paying late and leaves a deficit unmet at the close of the fifth business day. Deficits no larger than the lesser of 5% of the account’s equity or $1,000 do not count toward that practice.
The term itself is gone from the rule. “Day-trading buying power” no longer appears in Rule 4210. A broker may still show a buying power figure on screen, but that is the firm’s own calculation, and FINRA lets firms either block trades in real time or compute each account’s deficit once at the end of the day. Separately, FINRA’s investor guidance says $2,000 of equity is the minimum for using leverage at all; below it, a margin account trades only with the cash it holds.
A worked example
Take a hypothetical margin account at Thursday’s close: $30,000 of equity, holding $20,000 of margin-eligible stock. Maintenance margin is 25% of $20,000 = $5,000, so the maintenance margin excess is $30,000 − $5,000 = $25,000.
Under the old rule, DTBP for Friday was $25,000 x 4 = $100,000. On Friday the account buys $120,000 of stock at 10:00 and sells it at 14:00 at the same price, to keep the arithmetic clean: a single day trade $20,000 over the limit. The special maintenance margin is 25% of $120,000 = $30,000. Take away the $25,000 excess and the day-trading call is $5,000, the same as dividing the $20,000 overage by four. The rule then cut the multiplier to two, so on unchanged equity the limit became $25,000 x 2 = $50,000.
Under the new rule, the same trade is measured at the moment it fills. Before the purchase the IML is $30,000 − $5,000 = $25,000. After buying $120,000 at the execution price the account holds $140,000 of stock, maintenance is 25% of $140,000 = $35,000, equity is still $30,000, and the IML is $30,000 − $35,000 = −$5,000. The intraday margin deficit is $5,000.
The size test decides whether it counts toward a freeze. 5% of $30,000 is $1,500, and the lesser of $1,500 and $1,000 is $1,000. A $5,000 deficit is larger, so leaving it unpaid past the fifth business day would count toward a practice. Had the account bought $100,000 instead, maintenance would be 25% of $120,000 = $30,000, the IML would be exactly $0, and there would be no deficit under either rule.
The difference shows up in a smaller account. A second hypothetical account holds $20,000, all in cash. Once flagged as a pattern day trader under the old rule, it could not day trade until it had $25,000. Under the new rule its IML starts at $20,000, and $20,000 / 0.25 = $80,000 of stock bought during the day would take the IML to exactly $0, before any higher requirement the broker sets. Anything still held at the close also faces Regulation T’s 50% initial margin.
The original data
How far does price travel against a position inside one session? SPY’s daily bars from Yahoo Finance cover 8,472 sessions from 29 January 1993 to 25 September 2026. The median fall from the open to the day’s low was 0.46%. It was 1% or more on 1,806 sessions (21.3%), 2% or more on 422 (5.0%), 3% or more on 128 (1.5%) and 5% or more on 22 (0.3%). The largest was 9.69%, on 6 May 2010.
Single stocks travel further. Across 30 large caps (AAPL, MSFT, NVDA, AMZN, GOOGL, META, TSLA, BRK-B, JPM, V, MA, JNJ, WMT, XOM, CVX, PG, KO, PEP, HD, COST, MRK, ABBV, BAC, DIS, CSCO, INTC, NFLX, AMD, BA and NKE) from 4 January 2010, 124,765 stock-days, the median open-to-low fall was 0.73%. It reached 2% on 17,163 stock-days (13.8%) and 5% on 1,768 (1.4%). The threshold counts are published in full.
Leverage multiplies those falls into equity. A position of four times equity turns SPY’s median 0.46% dip into a 1.84% hit to the account. More to the point, a cash-only account that buys four times its equity sits exactly at the 25% maintenance line on the fill, so any fall at all leaves it short.
The cushion grows quickly as leverage comes down. For a position of L times equity bought at the open, the maintenance line is reached after a fall of (1 − 0.25L) / (0.75L). At 3.5 times that is 4.76%, which SPY’s low reached on 24 of 8,472 sessions (0.28%) and the large caps on 2,048 of 124,765 stock-days (1.64%). At three times it is 11.11%: SPY never, the large caps 72 times. At two times, 33.33%: SPY never, the large caps once. The leverage table holds all four rows.
What the numbers do not say. An open-to-low fall assumes the worst entry, bought exactly at the open and marked at the exact low, and a daily bar cannot show whether the low came before or after any other trade. Under the new rule a price move alone does not create an intraday deficit; the deficit is read at the next IML-reducing transaction, or as an ordinary maintenance call at the close.
When it fails
The old number ignored the day itself. Because DTBP was fixed at the prior close, a losing morning left the same limit in place for the afternoon, even though the equity behind it had shrunk. The new IML is computed on current values, which closes that gap.
A full allowance is not a position size. At four times equity the account is at the maintenance line on the fill. The limit tells you where the lender stops, not how much the trade can safely carry; that is a job for leverage arithmetic and the stop.
House rules sit on top. FINRA sets 25% as the floor for long stock, and firms may require more. Short sales carry higher minimums under Rule 4210(c): the greater of $5.00 a share or 30% at $5 and above, and the greater of $2.50 a share or 100% below $5. Any of these shrinks what a platform will let you trade.
The transition hides which system you are on. Until 20 October 2027 two accounts at two brokers can run on different rules. A screen still labeled DTBP, or a message about a day-trading call, usually means the old requirements are still being applied to that account.
Related
The pattern day trader page covers the designation this limit belonged to and the 2026 change that ended it. The margin account page explains the maintenance requirement every calculation here starts from. And leverage shows why the largest position a lender will fund is rarely the one worth holding.
I treat any buying power figure on a platform as the lender’s ceiling, not a size to aim for. I size a trade from the stop and the account first, then check that it sits well inside whatever the broker allows.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.