Buying power is the total dollar value of securities a trader can purchase at any given moment, combining cash equity with broker-extended margin. It is not a single number — it splits into two distinct limits that operate simultaneously on the same account: overnight buying power governed by Reg T (2:1), and intraday day-trading buying power for PDT accounts (4:1). Misreading which limit applies — and when it expires — is the most common source of unexpected margin calls for active traders.
Key Takeaways
- Intraday buying power (4:1) is a same-session privilege that resets to overnight limits (2:1) at 4 PM ET — holding an oversized position past the close triggers a margin call.
- The PDT rule requires $25,000 minimum equity to access 4:1 intraday margin; accounts below this threshold are capped at 2:1 and face a 90-day cash restriction after violating the rule.
- Futures and forex buying power operate under entirely different margin regimes (SPAN and CFTC caps) — leverage ratios that look similar on paper carry very different risk profiles than Reg T equity margin.
How Buying Power Works
Buying power is calculated from account net liquidation value multiplied by the applicable leverage ratio. Two separate calculations run in parallel:
Overnight Buying Power = Account Equity × 2 (Reg T, 50% initial margin)
Intraday Buying Power = Account Equity × 4 (PDT accounts, FINRA Rule 4210)
Federal Reserve Regulation T, in effect since 1974, requires 50% initial margin for equity purchases — meaning a $30,000 account can hold up to $60,000 in overnight positions. FINRA Rule 4210(f)(8)(B)(iv) extends this to 4:1 for intraday trades in PDT-designated accounts with at least $25,000 in equity.
Buying power depletes as positions are opened and is restored when positions close. The critical mechanic: intraday buying power does not carry over. At 4 PM ET, any open position is evaluated against the lower overnight limit. If the position exceeds overnight buying power, the broker issues a margin call.
Maintenance margin — typically 25% at Schwab and Fidelity, 30% at IBKR for concentrated positions — determines when forced liquidation begins during the trading day. This is a separate threshold from initial margin and is breached when unrealized losses erode equity below the maintenance floor.
Non-PDT accounts face a harder constraint. Without PDT status, same-day round trips are restricted, and accounts under $25,000 have no access to intraday leverage. Executing four or more round-trip day trades within a rolling five-business-day window with under $25,000 equity triggers a 90-day cash-only restriction at most brokers.
Futures and forex operate under different frameworks entirely. ES futures (CME) carry roughly $250,000 notional value at SPX 5000 but require only $12,000–$15,000 in initial margin — approximately 17–20:1 leverage under SPAN margin, which resets daily based on portfolio risk. US retail forex is capped at 50:1 on major pairs and 20:1 on minors per CFTC rules. Neither system uses Reg T.
Prop firm accounts add another layer of confusion. Funded accounts advertise large buying power figures, but static or trailing drawdown limits function as a shadow equity floor — often stricter than broker maintenance margin. A $100,000 funded account with a $3,000 trailing drawdown limit has effective buying power constrained by a $3,000 loss tolerance, not a $100,000 equity base.
Practical Example
A trader holds $30,000 in a margin account with PDT status. Intraday buying power: $120,000 (4:1). At 10 AM, they buy 200 shares of SPY at $575 — a $115,000 position, within the intraday limit.
SPY drops 2% to $563.50 by 3:30 PM. The trader holds, expecting a bounce. At the 4 PM close, the position is still open and worth $112,700.
The overnight buying power is $60,000 (2:1 on $30,000 equity). The position exceeds this by $52,700. The broker issues a margin call: deposit $52,700 or liquidate sufficient shares by noon the next business day. If ignored, the broker force-liquidates at the next open — potentially at a worse price.
The trader did not violate intraday rules. The margin call arose solely because they held a position sized for intraday limits past market close. The 4:1 intraday advantage is a same-session privilege, not a persistent one.
Buying power is the total value of securities a trader can buy using their account balance plus margin. Intraday accounts get four times their equity, but that limit drops to two times at the close. Holding an oversized position overnight triggers a margin call.
Common Mistakes
- Treating intraday buying power as permanent. The 4:1 limit resets to 2:1 at 4 PM ET. Traders who size positions to the intraday limit and hold overnight will receive a margin call — even if the trade is profitable.
- Ignoring the PDT equity floor. Accounts that dip below $25,000 lose access to 4:1 intraday margin immediately. A string of losses that pushes equity under this threshold mid-session can strand a trader in a position they can no longer legally hold under the rules they traded on.
- Applying Reg T logic to futures. ES futures at 17–20:1 look aggressive compared to equity margin, but SPAN margin adjusts daily to portfolio risk and does not work like Reg T. Traders who switch asset classes without understanding the margin regime often size incorrectly.
- Confusing prop firm buying power with real equity. The leverage is real, but the drawdown limits are not the same as maintenance margin. A $200,000 funded account with a $5,000 max drawdown is a $5,000 risk vehicle wearing a $200,000 label.
How JournalPlus Tracks Buying Power
JournalPlus logs each trade’s notional size alongside account equity at entry, making it straightforward to identify when a position was sized against intraday versus overnight limits. The day trading journal dashboard flags trades held past the intraday session so traders can review whether overnight margin exposure was intentional. For prop firm traders, JournalPlus supports custom equity floor inputs so drawdown-based buying power constraints are reflected accurately in position sizing analysis alongside leverage and margin metrics.
