About the I Bond Calculator
This I bond calculator estimates how much a US Series I savings bond will be worth after the number of years you plan to hold it. It combines the bond’s fixed rate, which never changes for the life of the bond, with an inflation rate you choose, using the Treasury’s composite-rate formula, and compounds the value every six months the way TreasuryDirect does.
Use it to decide whether I bonds beat a high-yield savings account or CD for money you will not need for at least a year, or to estimate what bonds you already own are worth. Enter the fixed rate of your bond and the current (or an assumed) annual inflation rate. The defaults use the rates TreasuryDirect announced for bonds issued from 1 May 2026 to 31 October 2026: a 0.90% fixed rate and a 1.67% semiannual inflation rate (3.34% annualized), giving a 4.26% composite rate. The Treasury announces new rates every 1 May and 1 November, so check TreasuryDirect for bonds bought from November 2026 onward.
I bonds cannot be cashed in during the first 12 months, and if you redeem before five years you lose the last three months of interest — the calculator applies that penalty automatically. Interest is exempt from state and local income tax but subject to federal tax, which you can defer until you cash the bond or it matures after 30 years. The model assumes one inflation rate for the whole holding period, so treat long-range figures as scenarios.
With the default inputs, the redemption value is $12,346.20. Change any value above to recalculate instantly.
How to use the i bond calculator
- 1Enter the amount you paid or plan to pay for the I bond.
- 2Enter the bond’s fixed rate from TreasuryDirect.
- 3Enter the annualized inflation rate (the six-month rate × 2) or your own assumption.
- 4Choose how many years you will hold the bond (at least one).
- 5Compare the redemption value and after-tax value with a CD or savings account.
Formula and method
The Treasury combines the fixed rate F with the semiannual inflation rate S (half the annualized inflation rate) to get the composite annual rate, rounded to two decimal places and never allowed below zero. The bond earns half the composite rate every six months and interest compounds semiannually; value accrues monthly within each six-month period.
If you redeem before five years, the value is calculated as of three months earlier, which is the three-month interest penalty. The after-tax value applies your federal rate to the interest; I bond interest is not taxed by states or localities. In reality the inflation component resets every six months, so this calculator uses one assumed rate for the whole period.
- F
- Fixed rate (decimal)
- S
- Semiannual inflation rate (decimal)
- P
- Purchase amount
- months
- Months held (minus 3 if under 60)
Worked examples
$10,000 held for 5 years
A 0.90% fixed rate plus 1.67% semiannual inflation gives 0.009 + 2 × 0.0167 + 0.009 × 0.0167 = 0.04255, which rounds to 4.26%. Compounded at 2.13% every six months for 10 periods, $10,000 grows to about $12,346. After 22% federal tax on the $2,346 of interest you keep about $11,830.
Cashed in after 3 years
Redeeming at 36 months means the value is calculated at 33 months: $10,000 × 1.0213^5.5 ≈ $11,229. The three lost months of interest cost about $119.
Higher fixed rate, 10 years
1.20% fixed plus 1% semiannual inflation gives a composite of 3.21%. Over 20 six-month periods $10,000 grows to about $13,750.
Deflation cannot push the rate below zero
With −1% semiannual inflation the formula gives −0.81%, but the composite rate is floored at 0%, so the bond keeps its value.
Frequently asked questions
How is the I bond rate calculated?+
The composite rate is fixed rate + 2 × semiannual inflation rate + fixed rate × semiannual inflation rate. For example, 0.90% fixed and 1.67% semiannual inflation give 4.26% (the rate for I bonds issued May–October 2026). The inflation part changes every six months; the fixed part never changes.
When can I cash in an I bond?+
After 12 months. If you redeem before the bond is five years old you lose the last three months of interest. After five years there is no penalty, and the bond keeps earning interest for up to 30 years.
How much can I buy in I bonds each year?+
You can buy up to $10,000 in electronic I bonds per person per calendar year through TreasuryDirect. Trusts and businesses can buy their own $10,000 allocation, and gifts count toward the recipient’s limit in the year they are delivered.
Are I bonds taxable?+
Interest is subject to federal income tax but exempt from state and local income tax. You can report it each year or defer it until you redeem or the bond matures, and it may be tax-free if used for qualified higher-education expenses.
Can I bonds lose value?+
No. Even if inflation is negative, the composite rate cannot go below zero, so your redemption value never falls — apart from the three-month interest penalty if you cash out before five years.
Results are estimates for educational purposes and are not financial advice. Rates, fees and terms vary — confirm with your lender or a licensed advisor before making decisions.