Options guide · Reviewed 28 August 2026

Margin buying power is a limit, not a target.

Two separate percentages govern a margin account: how much you can borrow to open a position, and how much equity you must maintain afterward. Confusing the two is how margin calls catch people by surprise.

Initial margin sets what you can buy; maintenance margin sets what you must keep

When you open a margin position, the initial margin requirement determines the minimum share of the purchase you must fund yourself — under U.S. Regulation T, that's commonly 50% for many securities, meaning your equity effectively doubles your buying power (equity ÷ 50% = 2× buying power). This requirement only applies at the moment of purchase; it's a one-time gate, not an ongoing balance you must maintain at that exact level.

After the position is open, a different, usually lower threshold takes over: the maintenance margin requirement, often around 25%, sets the minimum percentage of equity you must hold in the account going forward. If the position moves against you enough that your equity share falls below this maintenance level, the broker issues a margin call — and the account value at which this happens can be calculated directly from your existing loan balance and the maintenance percentage.

Why margin amplifies losses more than it feels like it should

Margin's core mechanic is straightforward but easy to underestimate: gains and losses are calculated on the full position size, not just the equity you put in. With 50% initial margin, a 10% drop in the underlying security produces roughly a 20% loss on your equity, since your equity is only half the position's size. This same amplification means a losing position can erode your equity percentage quickly, pushing it toward the maintenance threshold faster than the price move alone might suggest.

This is also why margin calls tend to arrive during volatile periods rather than steady ones — a fast, sharp decline can push equity below the maintenance threshold before there's time to react, which is precisely when a broker is most likely to force a sale.

What a buying power calculation does and doesn't tell you

A margin buying power calculation tells you the maximum position size the margin rules permit and the account value that would trigger a call — useful, mechanical facts about the account rules. It does not evaluate whether using that full buying power fits your risk tolerance, time horizon or overall portfolio, and it does not predict how likely a margin call is for any specific position. Those require separate judgment about volatility and your own capacity to add funds on short notice if needed.

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Frequently asked questions

What does 'buying on margin' actually mean?

Buying on margin means borrowing money from a broker to purchase securities, using your existing cash and securities as collateral for the loan. It lets you control a larger position than your cash alone would allow, but the borrowed amount must be repaid regardless of how the investment performs.

What is the difference between initial margin and maintenance margin?

Initial margin is the minimum percentage of a purchase you must fund with your own equity when you first buy (commonly 50% under U.S. Regulation T). Maintenance margin is a separate, usually lower threshold (often around 25%) that your equity percentage must stay above afterward — falling below it triggers a margin call.

What happens during a margin call?

A margin call requires you to bring your account equity back above the maintenance margin threshold, either by depositing more cash or securities, or by having the broker sell existing holdings — sometimes without further notice. This can force a sale at an inopportune time and lock in losses.

Does margin increase risk even if I use it carefully?

Yes, structurally. Margin scales gains and losses to the size of the total position, not just your own equity contribution, so a given percentage move in the underlying security produces a larger percentage move in your equity. This amplification applies symmetrically to both directions.

Is margin buying power the same as how much I can afford to lose?

No. Buying power measures how large a position you're permitted to open under margin rules, not how much risk is appropriate for your financial situation. Those are separate questions — a calculator showing available buying power says nothing about whether using all of it is a sound decision.

Source note: This guide uses standard Regulation T-style initial and maintenance margin formulas as a general illustration. All calculations happen in your browser — nothing you enter is sent to a server or stored. This is educational information, not investment advice — margin trading carries substantial risk, including the potential loss of more than your initial investment.