I-Bond / Series I Savings Bond Return Calculator
Calculate the return on a Series I savings bond based on the fixed rate and inflation rate components.
Estimate growth over your holding period.
What Is an I-Bond? A Series I savings bond (I-Bond) is a U.S. Treasury security that earns interest based on two components: a fixed rate that stays constant for the life of the bond, and a variable inflation rate that adjusts every six months based on CPI-U.
The Composite Rate Formula composite = fixed_rate + (2 × inflation_rate) + (fixed_rate × inflation_rate)
Where all rates are expressed as decimals. The composite rate is announced every May and November by the U.S. Treasury.
Key Rules
- Minimum holding period: 12 months (cannot redeem before then)
- If redeemed before 5 years: forfeit the last 3 months of interest
- Annual purchase limit: $10,000 per person (plus $5,000 in paper bonds via tax refund)
- Interest compounds semiannually
Example Fixed rate = 1.30%, Inflation rate = 3.40% Composite = 0.013 + (2 × 0.034) + (0.013 × 0.034) = 0.013 + 0.068 + 0.000442 = 8.14% Semiannual rate = 8.14% / 2 = 4.07% $10,000 held 5 years grows to: $10,000 × (1.0407)^10 ≈ $14,880