How Margin Works
When you open a margin account rather than a cash account, the broker extends a line of credit secured by the securities you hold. The flow is straightforward.
- You deposit cash or securities. This is your equity — the margin that backs the loan.
- The broker matches your funds. Under Regulation T, the Federal Reserve rule, you must put up at least 50% of the purchase price. The broker lends you the other 50%. With $10,000 in cash, you can buy $20,000 worth of stock.
- You pay interest on the loan. Margin rates vary by broker but typically run 6-10% annually. That cost erodes returns the longer the position stays open.
- You keep or lose the profits. If your $20,000 position rises to $24,000, you sell, repay the $10,000 loan plus interest, and walk away with about $14,000 — a 40% return on your $10,000 (versus 20% without margin). If the position falls to $16,000, you sell, repay the $10,000 loan, and have $6,000 left — a 40% loss on your $10,000 (versus 20% without margin).
Margin Requirements
| Requirement | Rule | What It Means |
|---|---|---|
| Initial margin | 50% (Regulation T) | You must put up at least 50% of the purchase price from your own funds. |
| Maintenance margin | 25% (FINRA minimum) | Your equity must stay above 25% of the position's current value. Most brokers set the floor at 30-40%. |
| Pattern day trader | $25,000 minimum equity | Four or more day trades in five business days requires a $25,000 minimum balance in the account. |
Buying Power
Your buying power is the maximum amount you can deploy, including the broker's margin loan. With 50% initial margin:
- $10,000 cash = $20,000 buying power (2:1 leverage)
- $25,000 cash = $50,000 buying power (2:1 leverage)
- For day trades (positions opened and closed the same session), some brokers extend 4:1 leverage — $25,000 cash = $100,000 buying power. The day-trade buying power vanishes at the close, and the risk profile is severe.
The Margin Call
A margin call is the event every leveraged trader spends their career trying to avoid. It triggers when account equity falls below the maintenance margin requirement. The broker demands additional cash deposits or the sale of positions to restore compliance. If the trader does not act fast enough, the broker will sell positions at the prevailing market price, without the trader's permission, to bring the account back into line. (I've eaten margin calls myself — they always arrive on the worst possible morning, never on a quiet Tuesday.)
Example. You have $10,000 in equity and buy $20,000 of stock on margin, borrowing the other $10,000. The stock drops 30% and the position is now worth $14,000. Your equity is $4,000 ($14,000 minus the $10,000 loan), which is 4,000/14,000 = 28.6% of position value. If the broker's maintenance margin is 30%, you are below the threshold and a call is issued. You must deposit roughly $200 in cash or sell enough stock to lift the equity ratio above 30%.
The compounding problem is timing. Margin calls arrive when markets are falling — the moment when raising cash is hardest and selling stock is the worst available choice. Forced selling at market lows is the mechanism by which margin destroys portfolios.
When Margin Makes Sense
Margin is a tool. The outcome depends entirely on how it is sized and managed.
- Short-term trades with defined risk. A swing trade with a tight stop-loss can use modest margin (1.2x to 1.5x rather than the full 2x) to enhance returns while keeping risk capped. The stop limits the downside before a margin call can develop.
- Portfolio margin for diversification. Some traders use a small margin allocation to add positions to a diversified book without selling existing holdings to free up cash. As long as total margin usage stays below 30-40% of the account, the structure is manageable.
- Hedging. Margin used to short sell an index ETF against a long book can actually reduce portfolio risk, even though leverage is technically in play.
When Margin Destroys Accounts
- Concentrated positions on full margin. Loading the full margin buying power into a single name is the fastest path to a blow-up. One bad earnings print, one 20% gap down, and the call arrives with no room to maneuver.
- Holding through drawdowns. “It will come back” is the line every margin-called trader has used at least once. Margin amplifies losses. A 25% decline on 2:1 leverage wipes out 50% of equity. The stock may recover. The account may not be there to see it.
- Ignoring interest costs. Margin interest compounds daily. A $10,000 loan at 8% costs $800 a year. If the trade is not generating at least that much, the trader is paying the broker for the privilege of losing money.
- Margin on volatile stocks. Full margin on penny stocks or micro-caps is structurally unsound. These names move 20-50% in a single session. A short intraday swing can trigger a margin call on a fully leveraged position before the trader has a chance to react.
Margin Interest: The Hidden Cost
Unlike a mortgage or auto loan, margin interest has no fixed term. The trader pays it for the entire duration the loan is outstanding, and it accrues against the position's return.
| Margin Balance | Rate | Annual Cost | Monthly Cost |
|---|---|---|---|
| $5,000 | 8.0% | $400 | $33 |
| $10,000 | 8.0% | $800 | $67 |
| $25,000 | 7.5% | $1,875 | $156 |
| $50,000 | 7.0% | $3,500 | $292 |
For a position carried on margin across several months, the interest charge can erode — or entirely eliminate — the profit. Margin interest belongs in the expected-return calculation before the trade is entered, not after.
Practical Guidelines
- Cap leverage well below the maximum. Using 30% margin (1.3:1) instead of the full 2:1 preserves the benefit of leverage while creating a much larger buffer before a maintenance call. The position can absorb roughly a 46% decline before triggering a call at 1.3x, against about 37% at 2x.
- Set a stop-loss on every margin position. The stop should fire well before a margin call could develop. With a 30% maintenance requirement, the stop should limit losses to less than 20% of the position's value.
- Hold a cash reserve. Keep at least 20-30% of the account in cash or money-market funds. The reserve absorbs margin calls without forcing sales at the worst possible moment.
- Monitor positions daily. Margin accounts require active oversight. Set price alerts. Know the exact level at which each position would trigger a maintenance call.
- Build the track record before adding leverage. A new trader should run a cash account until producing at least a year of profitable results. Margin amplifies skill and mistakes equally; the skill needs to be there first.
See also: Short Selling Explained · Position Sizing and Risk Management · Day Trading · Order Types · Trading Psychology: Fear and Greed
