The interest rates for Series I savings bonds are available on the Treasury Direct website of the U.S. Department of the Treasury. The interest on Series I savings bonds is a combination of a fixed rate set semiannually and an inflation rate that changes every six months.
The U.S. Treasury announces the fixed rate for Series I savings bonds on the first business day of May and the first business day of November each year. Once the fixed interest rate is announced, it applies to the lifetime of all bonds purchased for the next six months. The variable inflation rate, based on the Consumer Price Index for all Urban Consumers, or CPI-U, causes part of the interest percentage of the bonds to change every six months. Inflation rates of bonds are adjusted every six months after the purchase date of the bonds.
The actual rate of interest or composite rate is a calculation of the fixed rate, plus two times the inflation rate, plus the fixed rate times the inflation rate. For instance, for Series I bonds issued from Nov. 1, 2014 to April 30, 2015, the fixed rate is 0 percent and the inflation rate is .74 percent. Therefore, the equation is 0 plus 2 times .0074 plus 0 times .0074, making the composite rate of interest 1.48 percent.