How the values are worked out
Every result here comes from a single set of rules, the same rules Treasury applies to electronic Series I bonds.
Composite rate
Your bond earns a composite rate every six months: fixed rate + 2 × semiannual inflation rate + fixed rate × semiannual inflation rate. It's rounded to 0.01% and never drops below 0%. The rate in force during the month you bought the bond is your fixed rate. For each six-month period of your bond, the inflation rate is the most recent one Treasury set before it began.
A unit priced at $25
For every I bond, Treasury works out the value as a multiple of the value of a $25 bond. Each month we add interest to a $25 bond, compound it every six months, and round to the cent at every step. Next we scale that figure to the amount you bought and round to the cent once more. That's why a bond of $36.73 carries its own exact value.
Cash-in value
You can't cash a bond during the first 12 months. If you cash in between 12 months and 5 years, you lose 3 months of interest. The cash-in value is then the value 3 months earlier. Once 5 years have passed, the two numbers match. After 30 years, bonds stop earning interest.
Rate data
The rate table draws on two sources: the Fiscal Data "I Bonds Interest Rates" dataset from Treasury and the rates page at TreasuryDirect. Each day a job checks both and rebuilds the site whenever Treasury sets a new rate. The latest rate on this site was set on November 1, 2026.
What we project
The "new I bond growth" projection assumes today's rate of 5.51% remains unchanged during every six months that you hold the bond. That's not true: the inflation portion changes on May 1 and November 1 every year. Read it as guidance, not a promise.
The checks we run
A test runs worked examples before each build, then compares them with values published by TreasuryDirect and independent sources. Take a bond of $10,000 bought in January 2022: on October 1, 2026 it was worth $12,364.00. Any build with a mismatch never ships. You can find the rules for buying and cashing bonds on TreasuryDirect: I bonds.