Margin Interest

Margin interest is the fee your broker charges when you borrow money to trade. In a margin account, your broker lends you funds to buy securities or cover short positions, and you pay interest on the borrowed amount for as long as you hold the position. Margin interest rates vary by broker from roughly 5% to 13% annually, and they directly reduce your trading profits. For options traders, margin interest matters primarily when assignment or exercise creates a stock position funded by borrowed money.

Why It Matters

Most options strategies do not directly involve borrowing — buying a call costs the premium in cash, and defined-risk spreads use buying power but do not incur interest. However, margin interest becomes relevant in several common scenarios: when you are assigned on a short put and do not have enough cash to buy the shares, when you exercise a call using margin, or when you hold a short stock position resulting from a call assignment.

For traders who sell cash-secured puts, the distinction between "cash-secured" and "margin-secured" matters. If your put is secured by cash, assignment incurs no interest. If it is secured by margin, assignment creates a margin loan and interest starts accruing immediately. Over days or weeks, this interest can meaningfully reduce the premium you collected.

How It Works

How margin interest is calculated:

• Interest = Borrowed Amount x Annual Rate / 360 (most brokers use 360-day year)
• Interest accrues daily and is typically charged monthly
• Rates are usually tiered — larger loan balances get lower rates

Margin rates by broker (approximate, rates change with Fed funds rate):

• Interactive Brokers: Among the lowest — benchmark rate + 1.5% for larger balances
• tastytrade: Competitive, varies by balance
• thinkorswim (Schwab): Higher end — often 10%+ for smaller balances
• Fidelity: Moderate — tiered from about 8% to 12%
• Robinhood Gold: Competitive flat rate for Gold members

When options traders pay margin interest:

• After put assignment: You sell a put, get assigned, and do not have cash to cover the stock purchase. The shortfall is borrowed on margin.
• After call exercise: You exercise a call and borrow on margin to fund the stock purchase.
• Short stock from call assignment: You are assigned on a short call you do not have shares for. You now have a short stock position. While short sellers do not pay margin interest in the traditional sense, they may pay stock borrow fees.
• Naked or uncovered options: Some positions require margin that, if used to its limit, may result in interest charges.

Reducing margin interest costs:

• Keep enough cash in your account to cover potential assignments
• Close in-the-money options before expiration instead of accepting assignment
• Choose a broker with competitive margin rates if you anticipate using margin
• Roll or close short options that are deep in the money to avoid assignment
• Monitor your margin balance daily — interest accrues from day one

Margin interest and your P&L: Margin interest is a real cost that reduces your net trading profit. If you sell a put for $2.00, get assigned, hold the stock on margin for 30 days at 10% annual interest on $10,000 borrowed: $10,000 x 0.10 / 360 x 30 = $83.33 in interest. That is $83 out of your $200 premium — 42% of your profit consumed by interest.

Quick Example

You sell a $100 put for $2.00 on a stock. The stock drops to $97 and you are assigned — buying 100 shares at $100 ($10,000). You have $5,000 cash in the account, so you borrow $5,000 on margin. Your broker charges 10% annually. You hold for two weeks before selling the shares. Interest: $5,000 x 0.10 / 360 x 14 = $19.44. Your net profit on the trade: $200 premium - $300 stock loss + $0 assignment fee - $19.44 interest = -$119.44. The margin interest made a bad trade slightly worse.