Series I Bonds are inflation-pegged bonds offered by the Treasury. The interest rate you earn is a calculation that takes into account the fixed rate of your particular bond, a rate set every 6 months and follows your bond for the rest of its life, and an inflation rate that changes every six months to correspond with the CPI-U inflation rate. Here is a list of the Series I bond’s historic rates ^{[3]}.

**This post was updated to reflect the November 1st, 2009 interest rate changes.**

The equation itself isn’t particularly complicated and is:

Composite rate = [Fixed rate + (2 x Semiannual inflation rate) + (Fixed rate x Semiannual inflation rate)]

Using it to calculate the latest inflation rate data (announced November 1st, 2009), we calculate bonds bought in this period to be earning an APY of 3.36%:

**Fixed rate** = 0.30% (this fixed portion remains the same until you redeem the bond)

**Semiannual inflation rate** = 1.53% (in May 2010, this number will change)

Composite rate = [Fixed rate + (2 x Semiannual inflation rate) + (Fixed rate x Semiannual inflation rate)]

Composite rate = [0.0030 + (2 x 0.0153) + (0.0030 x 0.0153)]

Composite rate = [0.0030 + 0.0306 + 0.0000459]

Composite rate = [0.0336459]

Composite rate = 0.0336

Composite rate = 3.36%

## Series I Bond Rate Calculator

Here’s a handy calculator to help you calculate the yield:

Remember, your fixed interest rate does not change, it is based on when you purchased your bond. You will need to look up your bond’s fixed interest rate to get an accurate calculation of your composite interest rate.

*(photo by allyrose18 ^{[4]})*