Methodology & data sources
Transparency is the core of our E-E-A-T: this page documents where our data comes from, how often it is refreshed, and the formulas behind every calculation.
Data sources
| Source | Refresh cadence | License |
|---|---|---|
| TreasuryDirect — Series I Savings Bond Earnings Rates chart | none | Public domain (US government work) |
| TreasuryDirect — Series EE bond rates (May 2005 and later) | none | Public domain (US government work) |
| U.S. Treasury Fiscal Data — I Bonds Interest Rates dataset | none | Public domain (US government work) |
Where the rate data comes from
All rates are taken from the U.S. Treasury’s official, public-domain sources:
- I bond fixed rates and semiannual inflation rates come from the official
TreasuryDirect Series I Savings Bond Earnings Rates chart (the
i-bond-rate-chart.pdf) effective May 1, 2026, covering all 57 six-month issue periods from September 1998 to May 2026. - EE bond fixed rates come from TreasuryDirect’s “EE bonds issued May 2005 and later” page (the fixed-rate structure began in May 2005).
- Current figures were cross-checked against the Treasury press release of May 1, 2026 (Series I to earn 4.26%, Series EE to earn 2.40%).
How the composite rate is computed
The Series I composite rate uses the exact formula the Treasury publishes:
composite = fixed + (2 × semiannual inflation) + (fixed × semiannual inflation)
floored at 0%. We independently recomputed the composite rate for every one of the 57 periods from the published fixed and inflation rates and verified the result to the cent against the issue-period composite shown in the official chart — zero mismatches. Nothing on this site is fabricated; every rate is a dated snapshot from the source above.
How the calculators work
All calculators run entirely in your browser (no inputs are stored). The I bond value estimator compounds each of a bond’s own six-month interest windows at the composite rate in effect, models the 12-month lock-up and the 3-months-interest penalty for redemptions under five years, and floors the rate at 0%. It is a transparent approximation: the Treasury computes value per $25 of face value with exact monthly accrual factors and rounds to the cent, so figures can differ by a few cents to a couple of dollars. For the exact penny value of a real bond, always use TreasuryDirect’s official Savings Bond Calculator.
Limitations
Rates reset every 6 months (May 1 and November 1), so figures here are accurate as of May 2026 and will drift afterward. Calculator outputs are estimates for general information, not the official value or financial advice. Always verify against TreasuryDirect before relying on a figure. See our disclaimer.