Regulation T Margin: The 50% Initial Equity Rule

Key Takeaways

• Regulation T sets the initial margin at 50 percent, meaning you must put up at least half the purchase price in cash or equity before borrowing the rest.
• Four round-trip day trades in five business days triggers the pattern day trader rule, requiring a $25,000 minimum account equity.
• Many investors assume every security is marginable at 50 percent; low-priced stocks, recent IPOs, and some leveraged ETFs are excluded entirely.
• Using margin amplifies both gains and losses, and the interest on the margin loan runs continuously even if the portfolio returns nothing.

What It Is

Regulation T (often written Reg T) is contained in 12 CFR Part 220. It governs the extension of credit by broker-dealers when the loan is collateralized by margin securities. The headline number most investors remember is the initial margin requirement: for a standard equity purchase, you must put up at least 50 percent of the purchase price in cash or marginable equity. The broker can lend you the other half.

A margin account is any brokerage account where that kind of borrowing is allowed. The opposite is a cash account, where every purchase must be fully paid by settlement date and no loans are involved.

The Intuition

Before 1934, buying stocks on 10 percent margin was routine. When prices fell in 1929, forced liquidations cascaded because every leveraged account blew up at once. The Securities Exchange Act of 1934 handed the Federal Reserve authority over stock margin specifically to prevent that dynamic from repeating.

Reg T is not a trading rule. It does not tell you when to buy or sell. It caps how much leverage the system as a whole can build up at once. FINRA Rule 4210 then adds a second layer: the maintenance margin that governs how much equity you must keep in the account after the trade is on. Reg T is about getting in. Rule 4210 is about staying in.

How It Works

The initial margin calculation for a standard long equity position is straightforward.

initial margin required = purchase price * 50%
maximum loan value     = purchase price * 50%

Not every security is marginable under Reg T. Common exclusions include most stocks trading below $5, many OTC bulletin board names, shares held less than 30 days after an IPO, and some leveraged or inverse ETFs that brokers treat as non-marginable by house rule. Options, futures, and single-stock futures use different regimes (options under Reg T Section 220.12, futures under SPAN at the exchange level).

Once the trade settles, the account sits under FINRA Rule 4210. The broker assigns each position a maintenance margin percentage, most commonly 25 percent for listed equities, often 30 to 40 percent house requirement, and more for concentrated or volatile names. If equity falls below that line, the broker issues a margin call.

Two other rules ride on top of Reg T for active traders. The first is Regulation U, which covers margin loans extended by banks rather than broker-dealers. The second, more practically relevant, is the FINRA pattern day trader rule. A customer who executes four or more day trades within five business days, where those trades exceed 6 percent of total trading activity, is flagged as a pattern day trader and required to keep at least $25,000 in equity. (FINRA filed a proposal in January 2026 to replace this with a more flexible intraday margin standard; watch the rule text for when and if it takes effect.)

Worked Example

You want to buy 200 shares of a stock at $100, a $20,000 purchase.

Initial margin required (Reg T 50%) = 20,000 * 0.50 = $10,000
Maximum loan from broker            = 20,000 * 0.50 = $10,000

You deposit $10,000 cash. The broker extends a $10,000 margin loan. You now own $20,000 of stock against $10,000 of your own equity. Your account equity percentage is 50 percent, right at the Reg T minimum.

If the stock drops to $80, the position is worth $16,000. The $10,000 loan is still outstanding. Your equity is now $6,000, or 37.5 percent of market value. Still above the 25 percent FINRA minimum, but likely below a 30 percent house requirement, so expect a margin call from the broker.

The margin loan itself accrues interest daily at the broker's published rate, which in a tightening cycle can exceed 10 percent annualized on retail tiers. That interest is an ongoing cost of the leverage, not a one-time fee.

Common Mistakes

1. Confusing initial margin with maintenance margin. Reg T governs the trade entry at 50 percent. FINRA Rule 4210 governs the trade after, typically at 25 percent. They are two different numbers from two different regulators doing two different jobs.

2. Assuming every security is marginable. Many low-priced stocks, recent IPOs, OTC names, and some leveraged or inverse ETFs are non-marginable by rule or by broker policy. A trader who expects to use 50 percent leverage on any ticker will be surprised when the buying power does not appear.

3. Underestimating the pattern day trader rule. Four round-trip day trades in five business days is easy to hit by accident. Once flagged, the $25,000 equity floor applies, and account restrictions follow if the balance dips below it.

4. Thinking Reg T applies to futures. Futures margin is not Reg T. It is set by the exchange through SPAN or similar portfolio-based systems, and the percentages are much lower, which also means far higher effective leverage.

5. Ignoring margin interest. Carrying a margin loan through a flat market still costs money. If the loan rate is 10 percent and the portfolio returns 10 percent, the leverage washed out to zero before taxes.

Frequently Asked Questions

Q: What is Regulation T margin in simple terms? Regulation T is a Federal Reserve rule requiring you to put up at least 50 percent of a stock purchase in cash. The broker can lend you the other half, and that loan accrues interest daily until you repay it.

Q: How does regulation T margin affect investment decisions? It sets the ceiling on how much leverage you can apply at entry. Using 50 percent margin doubles your position size but also doubles your loss on any given price move and adds daily interest as an ongoing drag.

Q: What is a real-world example of regulation T margin? You want to buy $20,000 of stock. Regulation T requires $10,000 of your own cash. If the stock drops to $16,000, you have $6,000 of equity against a $10,000 loan, 37.5 percent equity, below most brokers' 30 percent house minimum, triggering a margin call.

Q: How can investors use margin accounts effectively? Calculate your margin call price before opening any leveraged position. Keep enough buffer so that a 15 to 20 percent drawdown does not trigger a forced liquidation. Treat the loan interest rate as a hurdle your return must clear before leverage adds value.

Q: How is Regulation T different from FINRA Rule 4210 maintenance margin? Regulation T governs the initial equity required when entering a trade. Rule 4210 governs the ongoing minimum equity required while the trade is open, typically 25 percent on listed equities with most brokers requiring 30 to 40 percent in practice.

Sources

1. Board of Governors of the Federal Reserve System. "Regulation T (12 CFR Part 220): Credit by Brokers and Dealers." https://www.federalreserve.gov/supervisionreg/legalinterpretations/margin_requirements20080305.pdf
2. FINRA. "Rule 4210. Margin Requirements." https://www.finra.org/rules-guidance/rulebooks/finra-rules/4210
3. FINRA. "Margin Regulation Key Topics." https://www.finra.org/rules-guidance/key-topics/margin-accounts
4. Federal Register. "FINRA Notice of Filing of a Proposed Rule Change To Replace the Day Trading Margin Provisions With Intraday Margin Standards (January 2026)." https://www.federalregister.gov/documents/2026/01/14/2026-00519/self-regulatory-organizations-financial-industry-regulatory-authority-inc-notice-of-filing-of-a

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.