I Bond Calculator — Series I Savings Bond Value Estimator
Series I savings bonds are inflation-linked US Treasury instruments. Unlike Series EE bonds, which pay a fixed rate, I bonds combine a fixed rate (locked in at purchase) with a variable inflation component that adjusts every six months based on the Consumer Price Index (CPI-U). This makes I bonds a popular hedge against inflation.
Use the main savings bond calculator to estimate your I bond's current value, or continue reading for a full explanation of the composite rate formula.
Current I Bond Rate (May–October 2026)
Series I bonds issued from May 2026 through October 2026 earn a composite rate of 4.26%.
- Fixed rate component: 0.90% (set at purchase; never changes)
- Semiannual inflation rate: 1.67% (annualized: 3.34%)
- Composite rate: 4.26%
Source: TreasuryDirect.gov May 1, 2026 rate announcement.
The Composite Rate Formula — Step by Step
The Treasury uses the following formula to combine the fixed and inflation components:
- fixed = The fixed rate at purchase (e.g. 0.0090 for 0.90%)
- inflation = The semiannual (6-month) inflation rate as a decimal (e.g. 0.0167 for 1.67%)
Worked Example for May 2026
- fixed = 0.0090
- inflation = 0.0167
- Composite = 0.0090 + 2 × 0.0167 + (0.0090 × 0.0167)
- = 0.0090 + 0.0334 + 0.00015
- = 0.04255 ≈ 4.26%
The extra term (fixed × inflation) accounts for the fact that the fixed
rate earns interest on the inflation-adjusted principal, not just the original balance.
How I Bond Interest Compounds
I bonds accrue interest monthly and compound semiannually. At the end of each 6-month period, all accrued interest is added to the bond's principal balance. The next 6-month period's composite rate then applies to this new, higher balance.
Each new period may have a different composite rate (because the inflation component changes), but the fixed component within that rate is always the one locked in at purchase.
Estimation Approach in This Tool
Because every I bond has earned a different composite rate for each 6-month period of its life (reflecting actual CPI-U changes), computing an exact historical value requires TreasuryDirect's complete rate table for your specific issue date. This tool applies the current 4.26% composite rate as a single-rate approximation to give a reasonable estimate. The actual value of your bond may be higher or lower depending on historical inflation rates.
For the exact value, use the TreasuryDirect official calculator.
I Bond Purchase and Redemption Rules
- Purchase price: Purchased at face value ($100 = $100).
- Denominations: $25 minimum; $10,000 maximum per person per calendar year (electronic).
- Minimum hold: 12 months before any redemption.
- Early penalty: Redeem before 5 years, forfeit last 3 months of interest.
- Final maturity: 30 years — bond stops earning interest at that point.
- Tax: Federal income tax applies; exempt from state and local tax. Interest can be deferred until redemption.
I Bond Rate History (Recent Periods)
| Period | Fixed Rate | Inflation (Semi) | Composite Rate |
|---|---|---|---|
| May 2026 – Oct 2026 | 0.90% | 1.67% | 4.26% |
| Nov 2025 – Apr 2026 | 0.90% | 1.56% | 4.03% |
| May 2025 – Oct 2025 | 1.20% | 1.19% | 3.61% |
| Nov 2024 – Apr 2025 | 1.20% | 0.95% | 3.11% |
| May 2024 – Oct 2024 | 1.30% | 1.48% | 4.28% |
| Nov 2023 – Apr 2024 | 1.30% | 1.97% | 5.27% |
| May 2023 – Oct 2023 | 0.90% | 1.69% | 4.30% |
| Nov 2022 – Apr 2023 | 0.40% | 3.24% | 6.89% |
| May 2022 – Oct 2022 | 0.00% | 4.81% | 9.62% |
| Nov 2021 – Apr 2022 | 0.00% | 3.56% | 7.12% |
Source: TreasuryDirect.gov I Bonds Interest Rates (as of June 2026). Full rate chart for all issue dates available at TreasuryDirect.
Related Tools
- Savings Bond Calculator (EE + I)
- Series EE Bond Calculator
- EE vs I Bond Comparison
- How to Cash In Savings Bonds
Frequently Asked Questions
Series I bonds issued from May 2026 through October 2026 earn a composite rate of 4.26%. This is made up of a 0.90% fixed rate plus a 3.34% annualized inflation component (based on CPI-U). Source: TreasuryDirect.gov, May 1, 2026.
The composite rate formula is: Composite = fixed + 2 x inflation + (fixed x inflation). For the May 2026 period: 0.0090 + 2 x 0.0167 + (0.0090 x 0.0167) = 0.0426, or 4.26%. The formula accounts for the interaction between the fixed and inflation components.
Yes. The fixed rate portion is set when you purchase the bond and never changes. The inflation component adjusts every May 1 and November 1 based on changes in the CPI-U. Your composite rate changes every six months, but the fixed component within it stays constant.
The composite rate cannot go below 0%. If deflation is severe enough that the inflation component would produce a negative composite rate, the rate is floored at 0%. However, the fixed component is preserved and will still apply when inflation turns positive.
Each I bond earns a different composite rate every six months based on the exact inflation rate for that period. Replicating the full rate history for every past issue date would require TreasuryDirect's complete rate tables. This tool uses the current composite rate as an approximation to give a useful ballpark estimate. For the precise value of a specific I bond, use the TreasuryDirect calculator at treasurydirect.gov.