Buying power serves as a measurement of the dollar value of securities that one may purchase in a securities account without depositing additional funds. In the case of a cash account where, by definition, securities may not be purchased using funds borrowed from the broker and must be paid for in full, buying power is equal to the amount of settled cash on hand. Here, for example, an account holding $10,000 in cash may purchase up to $10,000 in stock.

In a margin account, buying power is increased through the use of leverage provided by the broker using cash as well as the value of stocks already held in the account as collateral. The amount of leverage depends upon whether the account is approved for Reg. T margin or Portfolio Margin. Here, a Reg. T account holding $10,000 in cash may purchase and hold overnight $20,000 in securities as Reg. T imposes an initial margin requirement of 50%, which translates to buying power of 2:1 (i.e., 1/.50). Similarly, a Reg. T account holding $10,000 in cash may purchase and hold on an intra-day basis $40,000 in securities given IB’s default intra-day maintenance margin requirement of 25%, which translates to buying power of 4:1 (i.e., 1/.25).

In the case of a Portfolio Margin account, greater leverage is available although, as the name suggests, the amount is highly dependent upon the make-up of the portfolio. Here, the requirement on individual stocks (initial = maintenance) generally ranges from 15% - 30%, translating to buying power of between 6.67 – 3.33:1. As the margin rate under this methodology can change daily as it considers risk factors such as the observed volatility of each stock and concentration, portfolios comprised of low-volatility stocks and which are diversified in nature tend to receive the most favorable margin treatment (e.g., higher buying power).

In addition to the cash examples above, buying power may be provided to securities held in the margin account, with the leverage dependent upon the loan value of the securities and the amount of funds, if any, borrowed to purchase them. Take, for example, an account which holds $10,000 in securities which are fully paid (i.e., no margin loan). Using the Reg. T initial margin requirement of 50%, these securities would have a loan value of $5,000 (= $10,000 * (1 - 0.50)) which, using that same initial requirement providing buying power of 2:1, could be applied to purchase and hold overnight an additional $10,000 of securities. Similarly, an account holding $10,000 in securities and a $1,000 margin loan (i.e., net liquidating equity of $9,000), has a remaining equity loan value of $4,000 which could be applied to purchase and hold overnight an additional $8,000 of securities. The same principles would hold true in a Portfolio Margin account, albeit with a potentially different level of buying power.

Finally, while the concept of buying power applies to the purchase of assets such as stocks, bonds, funds and forex, it does not translate in the same manner to derivatives. Most securities derivatives (e.g., short options and single stock futures) are not assets but rather contingent liabilities and long options, while an asset, are short-term in nature, considered a wasting asset and therefore generally have no loan value. The margin requirement on short options, therefore, is not based upon a percentage of the option premium value, but rather determined on the underlying stock as if the option were assigned (under Reg. T) or by estimating the cost to repurchase the option given adverse market changes (under Portfolio Margining).

Unlike the case of a stock, in which a dividend is taxable in the year in which it is paid, the SPDR S&P 500 ETF Trust (Symbol: SPY) represents itself as a Regulated Investment Company and its dividend is deemed taxable in the year in which the record date is determined.  As such, SPY dividends declared in either October, November or December and payable to shareholders of record on a specified date in one of those months will be considered taxable income income in that year despite the fact that such dividend will generally be paid in January of the following year.

Circular 230 Notice: These statements are provided for information purposes only, are not intended to constitute tax advice which may be relied upon to avoid penalties under any federal, state, local or other tax statutes or regulations, and do not resolve any tax issues in your favor.

The term "excess margin securities" refers to margin securities carried for the account of a customer having a market value in excess of 140 percent of the total debit balance in the customer's account. These securities are in excess of the securities held in a customer's margin account that are pledged by the customer as collateral for the margin loan and can be used to support the purchase of additional securities on margin

Example:

A customer whose account equity consists solely of a cash balance of USD 10,000 on Day 1 purchases 400 shares of stock ABC at USD 50 per share on Day 2.

On Day 2, the customer's excess margin securities total USD 6,000. This is calculated by subtracting 140% of the margin debit or loan balance from the market value of the stock position ($6,000 = $20,000 - {1.4 * $10,000}).

The term is relevant from a regulatory perspective as the SEC requires that U.S. broker dealers segregate and maintain in a good control location (e.g., DTC or bank) all customer securities which are deemed excess margin securities. Such securities cannot be pledged or loaned to finance the activities of the firm or other customers without specific written permission from the customer. The portion of the securities classified as margin securities ($20,000 - $6,000 or $14,000 in this example) are subject to a lien and may be pledged or loaned by the broker to others to assist in financing the loan made to the customer.

Note that securities which were excess margin at the date of acquisition may later be reclassified as margin securities based upon the customer's subsequent trade and/or margin borrowing activity. For example, if the loan value of excess margin securities is subsequently used to acquire additional securities on margin, a portion of securities will then be reclassified as margin securities and subject to a lien. If the customer subsequently deposits cash or sells securities to reduce or eliminate the margin loan, the securities will be reclassified as excess margin or fully paid and are required to be segregated.
See also "fully paid securities".