Reg T and FINRA Rule 4210: the regulatory floor.

[01]Federal Reserve Regulation T

Regulation T, promulgated by the Board of Governors of the Federal Reserve System under the Securities Exchange Act of 1934, governs the extension of credit by brokers and dealers in connection with securities transactions. The rule’s key provisions:

• 50 % initial margin: a customer purchasing securities on margin must deposit at least 50 % of the purchase price. The remaining 50 % may be borrowed from the broker.
• Reg T-eligible securities: most US-listed equity, ETFs, and many corporate bonds qualify. Penny stocks, OTC pink-sheet stocks, and recent IPOs may be excluded by broker policy or by SEC rule.
• 5-day settlement window: customer has T+5 to deposit the initial margin requirement; positions purchased without the deposit may be cancelled.

The 50 % initial-margin requirement has been in place since 1974 and has not been adjusted. The Federal Reserve has the authority to change the requirement — raising it would tighten leverage in the equity market, lowering it would loosen leverage — but has chosen not to in fifty years.

[02]FINRA Rule 4210 maintenance margin

FINRA Rule 4210 (succeeding NYSE Rule 431 and NASD Rule 2520 after the FINRA consolidation) governs maintenance margin for securities accounts. Key provisions:

• Long position maintenance: 25 % of current market value. If equity falls below 25 % of current value, a maintenance call is issued.
• Short position maintenance: 30 % of current market value (for stocks priced at $5+). Higher for low-priced stocks.
• Concentrated-position rules: when a single position represents more than a defined percentage of account equity, elevated maintenance requirements apply.
• Pattern day trader designation: customers executing 4+ day trades within 5 business days, where the day trades represent more than 6 % of total trading activity, are designated “pattern day traders” and subject to special rules.

The 25 % maintenance floor has been in place since 1974. Some brokers raise this to 30–40 % as their “house” requirement for retail accounts; the broker’s house requirement is the operational floor for any specific customer.

[03]House requirements: what your broker actually applies

Brokers may impose maintenance margin requirements above the FINRA floor. Common house-requirement frameworks:

• Standard equity: 30–35 % maintenance for stocks priced $5+ with adequate liquidity.
• Low-priced stocks (under $5): typically 100 % maintenance (effectively unmargined). Some brokers extend this to stocks under $3 or under $10 depending on policy.
• Recent IPOs: typically 100 % maintenance for the first 30–60 days post-IPO, then phasing to standard requirements.
• Volatile names: some brokers maintain a list of high-volatility “hard-to-borrow” or “watchlist” securities with elevated requirements (50–100 %).
• Concentration adjustments: when a single position exceeds a defined threshold (often 25–50 % of account equity), elevated requirements apply on the concentrated portion.
• Earnings-period adjustments: some brokers raise maintenance requirements on positions in the days surrounding scheduled earnings announcements.

The implication for the trader: the calculator’s default 25 % maintenance assumption is the regulatory floor, not your operational reality. Check your broker’s margin disclosure document for the actual house requirements; they are typically higher and the margin-call price is correspondingly tighter.

[04]The pattern day trader rule

FINRA defines a Pattern Day Trader (PDT) as a customer executing four or more day trades within five business days, where day trading represents more than 6 % of total trading activity in the same period. Designated PDTs are subject to special rules under FINRA Rule 4210(f)(8)(B)(iv):

• $25,000 minimum equity in the account at all times. Falling below $25,000 triggers a 5-business-day suspension of day trading until the equity is restored.
• 4× day-trading buying power on Reg T-eligible securities, computed as 4× the maintenance excess at the close of the previous trading day.
• Day-trading-call exposure: a customer who exceeds their day-trading buying power triggers a day-trading call, restricting buying power to 2× until cured (typically by deposit).

The PDT rule applies only to securities accounts (Reg T-margin); futures and forex accounts are governed by their own regulators and have no PDT equivalent. For active traders below the $25,000 threshold, futures accounts (with their own SPAN-based margining) or cash accounts (with their T+1 settlement constraints) are the operational alternatives.

[05]The special-memorandum-account (SMA) ledger

Reg T accounts maintain a Special Memorandum Account (SMA), a notional ledger that tracks the customer’s available borrowing capacity. SMA increases when the account’s long market value rises (fewer dollars are needed as initial margin against the now-larger position), when dividends are paid, and when cash is deposited. SMA does not decrease when long market value falls; it is “sticky” on the upside.

The practical implication: customers can use accumulated SMA as additional buying power on subsequent trades without depositing fresh cash. A long-time investor with a substantially appreciated long-only portfolio may have accumulated SMA equal to 50–100 % of current cash equity. The accumulated SMA can be deployed as leverage on new trades without violating Reg T.

The risk: SMA-based buying power is leverage. Using accumulated SMA to take a new position does not reduce the maintenance-margin requirement on the new position; it just bypasses the initial-margin requirement. The combined account is now more leveraged than the customer may realise. The calculator output for the new position should be examined on its own terms, ignoring the SMA convenience.

[06]Reg T extensions and the broker’s discretion

Reg T includes provisions allowing brokers to extend the time within which a customer must meet a margin call. The standard rule is one extension per 12-month period for accounts where the broker exercises judgment that the call will be met. Repeated extensions, however, attract regulatory attention and can result in account-level restrictions.

The broker’s discretion to extend is not the customer’s right to demand. If a margin call is issued, the customer is operationally required to meet it on the broker’s timeline (typically 1–3 business days for retail accounts, sometimes same-day for elevated-risk situations). Failure to meet the call is grounds for forced liquidation; repeated failure is grounds for account closure or restriction.