How an I bond earns interest
A Series I savings bond is a U.S. Treasury savings bond whose interest rate is tied to inflation. Its overall (composite) rate has two parts: a fixed rate that is set when you buy and never changes, and an inflation rate that resets every six months. Treasury announces both on May 1 and November 1.
The formula is: fixed rate + (2 × semiannual inflation rate) + (fixed rate × semiannual inflation rate), rounded to the nearest 0.01%. The current rate, for I bonds issued November 1, 2026 to April 30, 2027, is 5.51%: a fixed rate of 1.40% plus a 2.04% inflation rate for six months. If prices fall, the overall rate can drop, but Treasury never lets it go below 0%.
Your own rate changes every six months, counted from the month you bought, not on May 1 and November 1. A bond bought in August 2026 keeps its first rate until the end of January 2027, then switches to the rate announced in November 2026.
How this calculator works
Pick the amount, when you bought (or "buying now") and the month you plan to cash in. The calculator looks up your fixed rate and every six-month inflation rate in the official Treasury rate history, adds interest month by month, and compounds it every six months, as Treasury does. At the 5.51% rate, for example, $10,000 grows to about $10,276 after the first six months.
Rates for periods Treasury has not announced yet are unknown. For those periods the calculator asks for an assumed inflation rate and marks the rows as estimates. It starts with the latest official figure, which is only a placeholder and not a forecast. The next rate is due on November 1, 2026.
Rules that change your result
- 12-month wait – you cannot cash an I bond during its first year.
- 3-month interest penalty – if you cash in before 5 years, you lose the last 3 months of interest. Treasury's example: cash in after 18 months and you get the first 15 months of interest.
- 30 years – an I bond earns interest for up to 30 years, then stops.
- Buying limits – at least $25, and up to $10,000 per person per calendar year in electronic I bonds in a TreasuryDirect account. Since January 1, 2025, I bonds are sold only electronically, so buying paper I bonds with a tax refund is no longer possible.
- Taxes – I bond interest is subject to federal income tax but not state or local income tax. You can report the interest each year or all at once when you cash the bond.
This page is general information, not investment or tax advice. Your exact value appears in your TreasuryDirect account and can differ from this estimate by a few cents. To compare an I bond with other savings, try the CD calculator, or see what inflation does to money with the inflation calculator.
Frequently asked questions
When is the next I bond rate announced?
Treasury announces new I bond rates every May 1 and November 1. The next announcement is November 1, 2026. The new fixed rate applies for the life of bonds issued in the following six months, and the new inflation rate applies to all I bonds when their next six-month period begins.
Can I lose money on an I bond?
The overall rate cannot fall below 0%, so the value of an I bond does not go down. The 3-month penalty for cashing in before 5 years only removes recent interest, so after the 12-month wait you get back at least what you paid. The real risk is that the rate falls if inflation slows, and that your money is locked for the first year.
How much can I put into I bonds each year?
Up to $10,000 in electronic I bonds per person per calendar year, with a minimum of $25. The limit applies to the Social Security number (or Employer Identification Number) of the first-named owner in a TreasuryDirect account. Paper I bonds bought with a federal tax refund ended on January 1, 2025.