Margin and Short Selling

Why this matters

Most investors never need margin or short selling. But margin accounts are often offered by default, borrowing is advertised as a way to "boost" returns, and short selling makes headlines. Knowing how both work, and how they fail, protects you from using them by accident or by impulse.

Margin: investing with borrowed money

In a margin account, you can borrow from your brokerage firm to buy securities, using the investments in your account as collateral. Under the Federal Reserve's Regulation T, you can borrow up to 50% of the purchase price, and some firms require you to put up more of your own money.¹ You pay interest on what you borrow.

Borrowing to invest is a form of leverage. It doesn't change how much the investment moves. It changes how much your money moves.

Jamie has $10,000, borrows another $10,000 on margin, and buys 200 shares of a fictional stock at $100. The figures ignore interest and fees, which would make the results worse.

Share price	Value of shares	Owed to the firm	Jamie's equity	Change in Jamie's money
$100 (start)	$20,000	$10,000	$10,000	0%
$120	$24,000	$10,000	$14,000	+40%
$75	$15,000	$10,000	$5,000	−50%
$50	$10,000	$10,000	$0	−100%

A 20% rise becomes a 40% gain on Jamie's money, and a 25% fall becomes a 50% loss. Without margin, a 25% fall would have cost Jamie 25%. If the price halves, Jamie's entire $10,000 is gone, and any further fall means owing the firm money.

Margin calls

Your account equity is the value of your investments minus what you owe. FINRA rules require equity of at least 25% of the value of the securities bought on margin, a level called the maintenance requirement. Many firms set their own, higher requirements, often 30% to 40%.¹

If your equity falls below the requirement, you face a margin call: you must add money or securities. In the example, a 25% requirement is breached if the price falls below about $66.67, and a 30% requirement if it falls below about $71.43.

The details are harsher than many people expect. A firm can sell your securities without contacting you first, it chooses which holdings to sell, and it can sell enough to pay off the loan, not just to meet the call.² That means a temporary drop can become a permanent loss at the worst possible moment.

Short selling: betting on a fall

Short selling reverses the usual order. Instead of buying first and selling later, a short seller borrows shares, sells them now, and hopes to buy them back later at a lower price to return to the lender.³

Price when bought back	Result
$40	Gain of $1,000
$50	Break even, before costs
$80	Loss of $3,000
$150	Loss of $10,000

The most a short seller can gain is the full sale price, if the stock falls to zero. But there's no limit to how high a price can rise, so there's no limit to the possible loss.³ When you buy a stock, the most you can lose is what you paid.

Short selling also involves borrowing, so it can carry ongoing costs, and it's usually done through a margin account, which brings margin calls into play too.

Key takeaways

• Margin lets you borrow to invest, which magnifies gains and losses on your own money and costs interest.
• If equity falls below the maintenance requirement, the firm can sell your holdings, without contacting you and choosing what to sell.
• Short selling profits if a price falls, but losses have no ceiling if it rises.
• Both are optional tools with serious risks. A cash account avoids them entirely.

Check your understanding

You invest $5,000 of your own money plus $5,000 borrowed on margin. The investment falls 20%. Ignoring interest, roughly what happened to your own money?

1. It fell 20%.
2. It fell 40%.
3. It fell 10%, because the loss is shared with the firm.

Why can short selling lose more than the amount of the original sale?

1. Because short sellers must pay the company a fee.
2. Because short positions can't be closed once they're opened.
3. Because there's no upper limit to how high a price can rise.

What's next

That completes the lessons in Part 1. You now understand what investing is, how markets work, and how investing happens in practice. Before moving on, put it all together in Capstone: Follow the Trade. Then Part 2 moves on to what you can actually own, starting with What a Stock Is.