Now that the CPI-U values for March through September have been released, we can determine what the range of possible rates will be for the November I Bond.
The CPI-U increase from 223.467 to 226.889 results in an inflation-linked rate of 1.53%. Buying an I Bond in October (before the rate change) would result in 6 months of the current 4.60% composite rate followed by 6 months of the new composite rate of 3.06%.
The new November composite rate cannot be determined exactly since the fixed rate is not known until November. We can determine the possible range of composite rates based on the inflation-linked rate of 1.53% and calculating for different values of the new fixed rate. If you are debating whether or not to buy now or in November, the current fixed rate of 0% does not lend itself to long-term investing while the 4.60% composite rate is more attractive for shorter-term investing. That said, it doesn’t seem likely the fixed rate will rise above 0% in November, either.
| Theoretical Fixed Rate | Composite Rate |
| 0.00% | 3.06% |
| 0.10% | 3.16% |
| 0.20% | 3.26% |
| 0.30% | 3.36% |
| 0.40% | 3.47% |
| 0.50% | 3.57% |
| 0.60% | 3.67% |
| 0.70% | 3.77% |
| 0.80% | 3.87% |
| 0.90% | 3.97% |
| 1.00% | 4.08% |
| 1.10% | 4.18% |
| 1.20% | 4.28% |
| 1.30% | 4.38% |
| 1.40% | 4.48% |
Since the current 0% fixed rate still results in a competitive 3.06% composite rate for a new I Bond, I believe the fixed rate will remain at 0% in November. The official release of the new rate will be on Tuesday, November 1, 2011.
