I bond composite-rate calculator
The Series I bond composite rate is computed from two inputs using the official formula: composite = fixed + (2 × semiannual inflation) + (fixed × semiannual inflation), floored at 0%. Enter a fixed rate and a semiannual inflation rate below to see the composite. The current (May 2026 - October 2026) inputs are a 0.90% fixed rate and a 1.67% inflation rate, giving 4.26%.
Source: TreasuryDirect. Data as of May 2026.
The formula, step by step
composite = fixed + (2 × semiannual_inflation) + (fixed × semiannual_inflation)
- Convert both rates to decimals (e.g. 0.90% → 0.0090).
- Multiply the semiannual inflation rate by 2 to annualize it.
- Add the fixed rate and the cross term (fixed × inflation).
- If the result is below 0, use 0 (the floor).
Worked example for May 2026 - October 2026: 0.90% + (2 × 1.67%) + (0.90% × 1.67%) = 4.26%.
Frequently asked questions
What is the I bond composite rate formula?
Composite rate = fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate). The result is floored at 0%. The factor of 2 annualizes the semiannual inflation rate, and the cross term accounts for the fixed rate earning on the inflation-adjusted principal.
Why is the inflation rate multiplied by 2?
Treasury announces a six-month (semiannual) inflation rate, but the composite rate is expressed as an annual rate. Multiplying by 2 annualizes the semiannual figure. The small cross term (fixed x inflation) prevents double-counting.
Can the composite rate be negative?
No. Even when the semiannual inflation rate is negative (deflation), the composite rate is floored at 0%. That happened in 2009 and 2015 — bonds earned 0% for those six months but never lost value.
Related
Not investment or tax advice. BondValue is an independent reference, not affiliated with the U.S. Treasury or TreasuryDirect. Savings bond rates reset every 6 months (on May 1 and November 1), and any value shown here is an estimate. Verify current rates and the exact penny value of your bonds at TreasuryDirect’s official Savings Bond Calculator. Consult a qualified professional before making financial decisions.
Last updated: 2026-06-21