### What is a Variable Interest Rate

A variable interest 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.

The underlying benchmark interest rate or index for a variable interest rate depends on the type of loan or security. 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.

### 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.

### Variable Interest Rate Bonds

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.