What Is a Variable Interest Rate?

A variable interest rate (sometimes called an 'adjustable' or 'floating' rate) is an interest rate on a loan or security that fluctuates over time because it is based on an underlying benchmark interest rate or index that changes periodically. The obvious advantage of a variable interest rate is that if the underlying interest rate or index declines, the borrower's interest payments also fall. Conversely, if the underlying index rises, interest payments increase.

Variable interest rates can be contrasted with a fixed interest rate, which does not fluctuate over its term.

How Variable Interest Rates Work

A variable interest rate is an interest rate that moves up and down with the rest of the market or along with an index. The underlying benchmark interest rate or index for a variable interest rate depends on the type of loan or security but is often associated with either LIBOR or the Fed funds rate. Variable interest rates for mortgages, automobiles and credit cards may be based on a benchmark rate such as the prime rate in a country. Banks and financial institutions charge consumers a spread over this benchmark rate, with the spread depending on a number of factors, such as the type of asset and the consumer's credit rating. So, a variable rate may bill itself as LIBOR + 200 basis points (+2%).

Residential mortgages, for instance, can be obtained with fixed interest rates, which are static and cannot change for the duration of the mortgage agreement, or with a floating or adjustable interest rate, which are variable and change periodically with the market. Variable interest rates can also be found in credit cards, corporate bond issues, swap contracts, and other securities.

Key Takeaways

  • A variable interest rate fluctuates over time because it is based on an underlying benchmark interest rate or index that changes periodically with the market.
  • The underlying benchmark interest rate or index for a variable interest rate depends on the type of loan or security but is often associated with either LIBOR or the Fed funds rate.
  • Variable interest rates can be found in mortgages, credit cards, corporate bonds, derivatives, and other securities or loans.

Variable Interest Rate Credit Cards

Variable interest rate credit cards have an annual percentage rate (APR) tied to a particular index, such as the prime rate. The prime rate most commonly changes when the Federal Reserve adjusts the federal funds rate, resulting in a change in the rate of the associated credit card. The rates on variable interest rate credit cards can change without advance notice to the cardholder.

Within the "terms and conditions" document associated with the credit card, the interest rate is most commonly expressed as the prime rate plus a particular percentage, with the listed percentage being tied to the credit-worthiness of the credit card holder. An example of the format is the prime rate +11.9%.

Variable Interest Rate Loans and Mortgages

Variable interest rate loans function similarly to credit cards except for the payment schedule. While a credit card is considered a revolving line of credit, most loans are installment loans, with a specified number of payments, leading to the loan being paid off by a particular date. As interest rates vary, the required payment will go up or down according to the change in rate and the number of payments remaining before completion.

When a mortgage has a variable interest rate, it is more commonly referred to as an adjustable-rate mortgage (ARM). Additionally, many ARMs start with a low, fixed interest rate for the first few years of the loan, only adjusting after that time period has expired. Common fixed interest rate periods on an ARM are three or five years, expressed as a 3/1 or 5/1 ARM respectively. See the calculator below to get an estimate of current interest rates on adjustable rate mortgages. In most cases, adjustable-rate mortgages (ARMs) have rates that adjust based on a preset margin and a major mortgage index such as Libor, the cost of funds index (COFI) or the monthly treasure average (MTA). For example, if someone takes out an ARM with a 2% margin based on Libor, and Libor is at 3% when the mortgage's rate adjusts, the rate resets at 5% (the margin plus the index).

Variable Interest Rate Bonds and Securities

For variable interest rate bonds, the benchmark rate may be the London Interbank Offered Rate  (LIBOR). Some variable-rate bonds also use the five-year, 10-year or 30-year U.S. Treasury bond yield as the benchmark interest rate, offering a coupon rate that is set at a certain spread above the yield on U.S. Treasuries.

Fixed income derivatives also can carry variable rates. An interest rate swap, for example, s a forward contract in which one stream of future interest payments is exchanged for another based on a specified principal amount. Interest rate swaps usually involve the exchange of a fixed interest rate for a floating rate, or vice versa, to reduce or increase exposure to fluctuations in interest rates or to obtain a marginally lower interest rate than would have been possible without the swap. A swap can also involve the exchange of one type of floating rate for another, which is called a basis swap.