II-Bonds blend guaranteed principal, inflation protection, and Treasury backing, but their layered rate structure makes historical performance tough to evaluate. This calculator solves that by letting you pick any issuance window since 1998, plug in an investment amount and holding period, and instantly see the original fixed rate, the inflation component, the composite yield, and how your balance would have grown (with charts) through every six‑month reset up to 30 years or your chosen end date, using live data pulled straight from TreasuryDirect.
I-Bond Calculator
Enter your I-Bond details to estimate growth, cumulative gains, and the annualized return using the latest TreasuryDirect rate files.
Fixed Rate (%): —
Inflation Rate (%): —
Composite Rate (%): —
Total Value ($): —
Total Gain ($): —
Annualized Return (%): —
How to Use the I-Bond Calculator
- Select an Issuance Date. Pick one of the May/November offerings dating back to 1998. The fixed rate, inflation component, and composite rate shown in the summary will always correspond to this issue.
- Enter the Investment Amount. Use whole-dollar values of $25 or more. The calculator compounds this starting balance through every six-month reset until your chosen end date (or the 30-year maturity ceiling).
- Set the End Date. Choose any date after the issue (up to 30 years out). The tool automatically rounds to the next full month when necessary, mirroring Treasury’s accrual schedule.
- Click “Calculate.” The summary panel updates instantly with formatted results, and the two charts redraw without reloading the page.
Understanding the Results
- Fixed Rate (%): The permanent real yield the bond carries from its issue date—this never changes once set.
- Inflation Rate (%): The semiannual CPI‑based add-on announced for that issuance; multiply by two for the annualized figure. This is the same value shown in the JSON dataset.
- Composite Rate (%): The effective semiannual yield that combines the fixed and inflation portions. This governs the first six months of growth and serves as the baseline for chart comparisons.
- Total Value & Total Gain: The projected redemption amount (before any early-redeemer penalty) and the dollar increase versus your principal through the selected end date.
- Annualized Return (%): The compounded annual growth rate from issue through the end date, so you can compare I-Bonds with other investments over identical horizons.
- Growth Chart: Plots your balance month by month, making it easy to see how compounding accelerates after inflation spikes or resets.
- Rate Components Chart: Displays the fixed, inflation, and composite percentages for every six-month period actually encountered in your holding window, so you can trace how Treasury updates affected returns.
How to use the I-Bond Calculator
The I-Bond Calculator is designed to help you pressure-test portfolio growth, compounding, drawdowns, income, and asset-allocation decisions across funds, stocks, and portfolios before you make a real-world change. Instead of relying on one rough estimate, run a few scenarios with conservative, base-case, and optimistic assumptions so you can see how sensitive the result is to returns, contribution levels, inflation, taxes, or timing.
A calculator result is most useful when you connect it to the account or plan decisions you actually control. After reviewing the output, compare it with your current savings rate, employer match rules, investment menu, expense levels, and withdrawal or rollover options. That is where MyPlanIQ's plan pages and retirement research become useful companions to the raw number.
If the result looks weak, treat that as a planning signal rather than a dead end. Small changes such as contributing earlier in the year, capturing the full company match, lowering fees, adjusting withdrawal assumptions, or choosing a more suitable allocation can materially change long-term outcomes. Re-run the calculator after each change and use the related links below to keep moving from estimate to action.
Related resources
- See plan investment menus and ratings
- Read more about investment portfolio decisions
- Explore all calculators
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- Investment Comparison Calculator
Calculator FAQs
What is the best way to compare investment scenarios?
Keep the time horizon the same, change only one major assumption at a time, and compare total return, drawdown, income, and ending value together. That keeps the comparison focused and easier to trust.
Why do fees and allocation matter in portfolio calculators?
Even modest fee differences or allocation changes compound over long periods. A portfolio calculator helps you see how those seemingly small choices can change long-term wealth and income.
How should you use a portfolio result in your retirement planning?
Use the result to review whether your workplace plan menu, fund costs, and asset mix support the growth or income path you want. Then test another related calculator to pressure-test the decision.