How U.S. Series I Savings Bonds actually earn interest (and a free calculator built on the official Treasury API)

If you own Series I savings bonds, you've probably opened TreasuryDirect, stared at a login wall, and given up on knowing what they're worth today. The rate math isn't secret — it's published in federal regulation — but it's not something most people want to do by hand every time they're curious. Here's how the math works, straight from the regulation, with a free calculator at the end built on the U.S. Treasury's own open data API.

The two rates that make up an I Bond's return

An I Bond's return each six-month period is a composite rate, combining two pieces (31 CFR Part 359):

• Fixed rate — set once, when you buy the bond, and never changes for its 30-year life.
• Semiannual inflation rate — resets every May 1 and November 1, tracking CPI-U, and applies to every I Bond ever issued, not just new ones.

The composite rate formula (31 CFR 359.4) is:

composite rate = fixed rate + (2 × semiannual inflation rate) + (fixed rate × semiannual inflation rate)

That cross term is the part people usually miss when they try to eyeball it — it's not just "fixed + 2×inflation," there's a small multiplicative adjustment too.

Two rules that change what you actually get back

• 12-month minimum (31 CFR 359.6(b)) — you cannot redeem an I Bond before it's 12 months old, period.
• 3-month interest penalty (31 CFR 359.7) — redeem anytime before 5 years and you forfeit the last 3 months of interest. This is why a bond's redemption value and its accrued value diverge until year 5.

The official data source

Treasury publishes every historical I Bond rate as structured data through Fiscal Data, no key required:

https://api.fiscaldata.treasury.gov/services/api/fiscal_service/v1/accounting/od/i_bonds_interest_rates?page[size]=10000&sort=issue_year_month,earning_period_start

That one call returns every fixed rate, inflation rate, and combined rate back to the program's 1998 launch — enough to reconstruct the exact value of any bond, any issue month, as of any redemption month.

A worked example

A $1,000 bond (issue price) bought in October 2022 (fixed rate 0%, riding the 9.62% inflation spike that made I Bonds famous that year), checked as of October 2026:

• 9 semiannual periods have applied, each compounding on the $25-unit base per 31 CFR Part 359 Appendix A
• The 3-month penalty applies (bond is under 5 years old) — the displayed value is already backed off by 3 months
• The 12-month minimum is long since met
• Result: $1,175.20 — $175.20 of interest, with the current semiannual rate (period starting 2026-05) at 3.34%.

Redo that same bond at the 5-year mark and the 3-month lookback disappears — the "Redeemable" column flips because the penalty window has closed.

Try it

Built this as a small static calculator against the Fiscal Data endpoint above: I Bond Value Calculator. Pick an issue month, a redemption month, paper or electronic, and an amount — it walks every semiannual period in between and shows the penalty and 12-month-minimum status alongside the number. There's also a standalone "compute composite rate from fixed + inflation" tool if you just want to check a rate announcement by hand.

It's free, no login, and it's exactly the regulation's formula — not an estimate from a rate-of-thumb. Figures are from the Bureau of the Fiscal Service's own "I Bonds Interest Rates" dataset; it's not tax or financial advice, and actual redemption amounts should be confirmed on TreasuryDirect.