Back in September 2010, the CPI-U stood at 218.439, and it grew to 223.467 in March 2011.

FORBES: How To Predict Savings Bond Rates

The CPI-U increased from 218.439 in September 2010 to 223.467 in March 2011, a six-month increase of 2.30%.

FORBES: Treasury Confirms May 2011 Series I Savings Bond Rate

For the May 2011 rate update, you needed the March 2011 CPI-U numbers (released in mid-April) as well as the September 2010 CPI-U numbers.

FORBES: How To Predict Savings Bond Rates

Every six months your bonds are credited with the fixed rate plus the rate of inflation as determined by the Consumer Price Index for All Urban Consumers (CPI-U).

FORBES: Fact and Comment

The earnings rate combines a 0.00% fixed rate of return with the 4.60% annualized rate of inflation as measured by the Consumer Price Index for all Urban Consumers (CPI-U).

FORBES: Treasury Confirms May 2011 Series I Savings Bond Rate

It was implemented for the CPI-U in January 1983.

FORBES: Why QE2 Inflation Is Like A Tax On Disposable Income

This actually happened back in May 2009, when the CPI-U contracted by 2.78%, resulting in a 0% overall rate during that six month period for any I Bonds purchased since the Fall of 2001.

FORBES: How To Predict Savings Bond Rates