How Does Margin Buying Power Work?
When you open a margin account, the broker extends credit secured by the market value of securities in the account. Federal Regulation T initially allows up to 50% margin on new stock purchases — meaning you can control roughly twice the equity you deposit, subject to individual stock marginability and firm policy. Intraday buying power can be higher for qualified accounts, but overnight positions face standard maintenance margin requirements.
Buying power fluctuates with market prices. A drawdown in your portfolio reduces equity and can shrink available margin instantly. Active traders must monitor buying power throughout the session, not just at the open, because a mid-day slide can turn an otherwise manageable position into a margin call scenario.
What Triggers a Margin Call?
A margin call occurs when account equity falls below the broker's maintenance requirement — often 25% of total market value for equities, though firms may set higher house minimums. You must deposit additional funds or liquidate positions promptly to restore required equity. Failure to meet a call allows the broker to liquidate holdings without your consent to protect the loan.
Margin calls are not theoretical edge cases for leveraged traders; they follow volatile sessions and concentrated positions. Risk management — position sizing, stop discipline, and diversification — is how you stay distant from maintenance thresholds, not optimism about recovery bounces.
How Do Pattern Day Trader Rules Apply?
If you execute four or more day trades within five business days in a margin account, and those day trades represent more than 6% of your total trades in that period, FINRA classifies you as a pattern day trader (PDT). PDT status requires maintaining at least $25,000 in account equity to continue day trading without restriction. Accounts below that threshold face limited day-trade capacity until equity is restored.
These rules apply to margin accounts, not cash accounts, which is why account selection and trade counting matter for intraday strategies. Track round-trip trades explicitly if you are near the threshold — brokers flag PDT status automatically, and appeals are rare.
What Are the Ongoing Costs of Margin?
Margin loans accrue interest on borrowed balances, typically calculated daily and charged monthly. Rates vary by broker and debit size. Even short-term intraday margin use can generate interest if positions are held overnight. Compare your broker's margin schedule against expected holding periods before assuming leverage is cost-free beyond commissions.
Used deliberately, margin is a tool for capital efficiency. Used casually, it converts manageable losses into account-threatening drawdowns. Treat margin as a loan with a daily price tag and a hard equity floor — because that is exactly what it is.