You can find the methodology for calculating interest on I bonds on Bogleheads® wiki at the following link:

https://www.bogleheads.org/wiki/I_savings_bonds

The interest rates in a row represent the rates used to compute the bond value in the column "Accrued Value" in the same row. This is the value on the date in the first column in the same row. Redemption Value takes into account the three-month interest penalty for redemption prior to five years.

Composite rate is calculated as follows.

Composite rate = [fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate)]

The composite rate will never be less than zero. The interest income is computed by applying the composite rate based on the interest computation methodology.

The calculator assumes that you purchased the I bond on the first of the month, and therefore, it uses the CPI-U index for the previous month of purchase as the base CPI-U index.The inflation-adjusted value for a month is calculated as follows.

Inflation-adjusted value = (Purchase value of bond/base CPI-U index) X CPI-U index for the month

Inflation numbers will be updated every month soon after those numbers are released by BLS. Interest rates will be updated soon after they are released every six months by TreasuryDirect. Last update: CPI-U updated on 1/12/2026, I Bond Fixed Rate and Inflation Rate updated on 10/31/2025.

Learn more about I bonds at TreasuryDirect.
Also see the link on Bogleheads® forum: The I Bond Manifesto.