What Is Regulation T?
Behind every margin account is a decades-old federal rule that most investors never read directly, even though it quietly sets the outer boundary on how much a broker is allowed to lend.
The short answer
Regulation T is a federal rule that governs how much credit a brokerage firm can extend to a customer for purchasing securities, most notably by setting a minimum initial margin requirement. It establishes a floor — a percentage of a security’s purchase price that must come from the investor’s own funds rather than borrowed money — that applies broadly across brokers, though individual firms are permitted to set their own requirements higher.
What the rule actually sets
At its core, Regulation T addresses the initial margin requirement for opening a new leveraged position: a minimum percentage of the purchase price the investor must cover with their own money on the day the position is opened. It also covers related mechanics, like the general timeframe within which an investor must pay for securities purchased in a cash account, and rules around how credit can be extended by brokers more broadly. It doesn’t set the ongoing maintenance requirement that applies after a position is open — that’s a separate, continuously monitored threshold.
Why it exists as a floor, not a ceiling
Regulation T establishes the minimum a broker is permitted to require, not a rate every broker must charge identically. That distinction matters because it means the rule functions as a baseline rather than a fixed standard:
- Brokers can require more collateral. A firm’s own house requirement can sit above the Regulation T minimum, particularly for volatile or concentrated positions.
- Brokers cannot require less. The rule sets a hard floor that applies regardless of how comfortable an individual firm might otherwise be with more lending.
- Different security types can be treated differently. Certain securities may be restricted from margin purchases entirely or treated more conservatively under the rule.
Why the distinction matters day to day
For most investors using a standard margin account, Regulation T operates quietly in the background — it’s baked into how much buying power a brokerage platform shows for a given amount of cash, without the rule itself needing to be consulted directly. It becomes more visible in situations like a large or unusually leveraged purchase, where the difference between the federal minimum and a broker’s stricter internal requirement can actually change what’s allowed.
How it connects to margin calls
Because Regulation T governs the entry point into a leveraged position rather than what happens afterward, it isn’t the rule directly responsible for an existing position getting flagged as market values shift. That’s governed by maintenance requirements, which sit on top of the Regulation T framework rather than replacing it. The two work together: one sets the terms for getting in, the other for staying in good standing once there.
Where this leaves you
Regulation T is best understood as a baseline set by regulation, not a complete picture of what any individual account will actually require — the specific numbers that matter day to day come from the broker’s own margin agreement, built on top of that federal floor.