Buying on Margin

Buying on margin means paying for part of a stock purchase with money borrowed from your broker, using the securities in your account as collateral. It is the stock market's version of a down payment: you put up a fraction, the broker finances the rest, and you own the whole position — its gains, its losses, and the interest bill — from day one.

The Ground Rules

• You need a margin account. Standard cash accounts can't borrow; upgrading involves a margin agreement whose fine print — the broker's right to liquidate your positions without notice — is the most consequential document most investors never read. FINRA rules require at least $2,000 in equity to open and maintain margin borrowing.
• Regulation T caps the initial loan at 50%. The Federal Reserve's Reg T lets brokers lend up to half the purchase price of most marginable stocks. Deposit $10,000 and your theoretical buying power is $20,000. (Brokers may lend less on volatile names, and some securities aren't marginable at all.)
• Interest starts immediately. Margin loans accrue interest daily and post monthly. Rates vary enormously between brokers — from near benchmark rates at the cheapest to double digits at some large retail firms — and the rate matters more than most users realize, because it's a guaranteed cost set against uncertain returns.
• There's no repayment schedule. The loan persists as long as your collateral supports it. You repay by depositing cash or selling positions — or the broker repays it for you, by force, which is the next lesson.

A Worked Example

You deposit $10,000 and buy $20,000 of a stock at $100 per share — 200 shares, half yours, half borrowed. Ignore interest for a moment:

Stock moves to	Position value	Loan	Your equity	Your return
$120 (+20%)	$24,000	$10,000	$14,000	+40%
$100 (flat)	$20,000	$10,000	$10,000	0% minus interest
$80 (−20%)	$16,000	$10,000	$6,000	−40%
$50 (−50%)	$10,000	$10,000	$0	−100%

The symmetry is the whole product: 2:1 margin doubles every move, in both directions. The stock falling by half wipes out your entire deposit — the loan is owed in full regardless. And the flat case isn't free: at, say, a 10% margin rate, a year of flat prices costs you $1,000 of interest on the $10,000 loan — a 10% loss on your equity for the crime of nothing happening.

What Margin Is Actually Used For

• Short-term opportunity — adding size to a high-conviction, defined-duration trade without selling long-term holdings (and triggering taxes).
• Liquidity bridging — covering a purchase for days rather than waiting on transfers or settlement.
• Strategy requirements — short selling and most options-writing strategies are only possible in margin accounts.
• What it's worst at: permanent buy-and-hold leverage. Interest compounds continuously while returns arrive irregularly, and the drawdowns along the way threaten forced liquidation — an asymmetry the final lesson makes concrete.

Details That Bite

• Buying power ≠ a target. The 50% maximum is a ceiling, not a suggestion; experienced margin users routinely borrow a fraction of what's offered, precisely to keep the maintenance math (next lesson) far away.
• The pattern day trader rule. Margin accounts making four or more day trades within five business days need $25,000 minimum equity or face restrictions — relevant if your margin use trends toward active trading.
• Your shares can be lent out. Positions held on margin can be rehypothecated and lent to short sellers; among the side effects, some dividends can arrive as differently-taxed "payments in lieu."
• Rates are negotiable at scale — and checking them before choosing a broker matters, because moving a margined account later is awkward.

Next — the mechanism that enforces all of this: the margin call, what triggers it, and what happens when you can't answer it.