The concept
What margin buying power actually measures
Buying on margin means borrowing part of a purchase's cost from a broker, using your account's cash and securities as collateral. The initial margin requirement (commonly 50% under U.S. Regulation T for many securities) sets how much of a purchase must be covered by your own equity, which determines your gross buying power.
Once you hold a margin position, a separate maintenance margin requirement applies — typically lower, often around 25% — which sets the minimum equity percentage you must maintain. If the account value falls enough that your equity drops below that threshold, the broker can issue a margin call requiring you to deposit more funds or sell holdings.
The method
How the calculation works
Gross buying power is calculated by dividing account equity by the initial margin requirement. Any existing margin debt is subtracted to estimate remaining available buying power. Separately, the margin call threshold is calculated by dividing existing debt by one minus the maintenance margin requirement, showing the account value below which a call would be triggered.
Gross buying power scales inversely with the initial margin requirement, and the margin call threshold rises as existing debt grows or the maintenance requirement increases.
Worked example
A neutral example
For $10,000 in equity, a 50% initial margin requirement, a 25% maintenance requirement and $5,000 in existing margin debt, the calculator estimates remaining buying power and the account value that would trigger a margin call under those exact assumptions.
This example explains the method. It does not recommend a financial action or predict an outcome.
Frequently asked
Margin Buying Power Calculator questions
What is Regulation T margin?+
Regulation T is a U.S. Federal Reserve rule setting the initial margin requirement for many securities at 50%, meaning an investor must fund at least half of a new margin purchase with their own equity. Requirements can be higher for certain securities or accounts.
What happens if I get a margin call and can't meet it?+
A broker can sell securities in your account without further notice to bring equity back above the maintenance requirement. This can lock in losses and may trigger sales you didn't choose, which is a key risk of margin trading.
Does buying power increase my potential losses, not just gains?+
Yes. Margin amplifies both gains and losses relative to your own equity, since losses are calculated on the full position size, not just the equity portion you contributed. This calculator estimates capacity and thresholds only — it does not assess whether using margin is appropriate for your situation.