How the margin calculator works
Buying "on margin" means paying for part of a stock purchase with cash you borrow from your broker, using your account as collateral. This calculator covers the standard Reg T mechanics for a long stock position — not portfolio margin, futures margin, day-trading rules, or short selling, which all work differently.
Your total position value is Stock Price × Shares. Your broker's
initial margin requirement sets the minimum percentage of that value you must
cover with your own cash — the rest is the amount borrowed:
Amount Borrowed = Position Value × (1 − Initial Margin % / 100), and
Cash Required = Position Value × (Initial Margin % / 100). The standard Reg T
minimum initial margin is 50%, meaning you can borrow up to half the position value.
Interest cost accrues on the amount borrowed, not the full position:
Interest = Amount Borrowed × (Annual Rate / 100) × (Days Held / 365).
The maintenance margin call price is the stock price at which your equity (position
value minus what you owe) falls to your broker's minimum maintenance percentage — commonly 25%.
Below that price, your broker can force you to deposit more cash or sell shares. It's calculated as
Margin Call Price = Amount Borrowed / (Shares × (1 − Maintenance Margin % / 100)).
Finally, the breakeven stock price is simply the price at which your gain exactly
offsets the interest you've paid: Breakeven Price = Stock Price + Interest Cost / Shares.
Below this price, you're paying to hold a losing (or merely flat) position on top of any price
decline.
Worked example
Suppose you buy 200 shares at $50 per share (a $10,000 position), with your broker's standard 50% initial margin and 25% maintenance margin, borrowing at 8% annual interest, and plan to hold for 90 days.
Amount borrowed = 10,000 × 50% = $5,000.00, and that's also your cash required. Interest cost = 5,000 × 8% × (90/365) ≈ $98.63.
Your margin call price = 5,000 / (200 × 75%) = 5,000 / 150 = $33.33 — the stock would have to fall 33.3% from your $50 purchase price before you'd face a margin call. Your breakeven price = 50 + (98.63 / 200) ≈ $50.49 — just under a 1% gain needed to cover the interest cost of borrowing.
Common mistakes to avoid
1. Confusing the margin call price with your purchase price minus your cash
It's tempting to assume the stock can fall by roughly your initial margin percentage before trouble starts, but the margin call trigger is based on the lower maintenance margin, not your initial margin — which is exactly why the margin call price in the example above ($33.33) is much further from the $50 purchase price than the 50% initial margin might suggest.
2. Forgetting that interest accrues whether the stock moves or not
Margin interest is a fixed, ongoing cost tied to time and the amount borrowed — it accrues regardless of whether the stock goes up, down, or sideways. A flat stock price over a long holding period still leaves you behind by the full interest cost.
3. Not accounting for how leverage magnifies both gains and losses
Because you only put up part of the position's value, both percentage gains and percentage losses on your own cash are magnified relative to the stock's actual move — the same leverage that can boost returns on the way up accelerates losses (and the risk of a margin call) on the way down.