Series I Savings Bond Calculator

Introduction: How Series I Savings Bonds Build Value

Series I savings bonds are U.S. government-backed savings securities intended to combine a fixed return component with protection against inflation. I bonds have a fixed rate that stays with a bond for its life and an inflation component that changes every six months in response to the Consumer Price Index for All Urban Consumers (CPI-U). Interest accrues to the bond rather than being paid out, so its value can compound over time. This Series I Savings Bond Calculator estimates a theoretical future value from the purchase amount, fixed rate, semiannual inflation rate, and holding period you enter.

An I bond’s distinguishing feature is its semiannual inflation adjustment. A conventional fixed-income investment can lose purchasing power when prices rise, whereas an I bond’s composite rate incorporates the current inflation component. That component can rise or fall at each rate reset. The composite rate is not merely the fixed rate added to the inflation rate; its calculation includes an interaction between the two rates. As a result, the calculator is useful for seeing how a chosen fixed rate, an assumed inflation rate, and semiannual compounding work together in a bond-value projection.

Series I Savings Bond Composite Rate Formula

For Series I savings bonds, the U.S. Treasury announces composite-rate terms each May and November. The formula joins the bond’s fixed rate with its semiannual inflation rate. The current defaults in this calculator use the TreasuryDirect rate announcement for I bonds issued May 1, 2026 through October 31, 2026: a 0.90% fixed rate and a 1.67% semiannual inflation rate. Replace those inputs when TreasuryDirect announces rates applicable to the bond or scenario you want to examine. The calculator computes the composite rate as:

Formula: r = f + 2 i + f i

Here, is the fixed rate and is the semiannual inflation rate; the calculation uses both as decimals. With a 0.90% fixed rate (0.009) and a 1.67% semiannual inflation rate (0.0167), the resulting composite rate is:

Formula: r = 0.009 + 0.0334 + 0.0001503 = 0.0425503

That is a composite rate of approximately 4.26% per year. For each six-month earning period in this calculator’s model, one-half of that annualized composite rate is applied before the next semiannual period.

If a deflationary inflation component would drive the formula below zero, this calculator uses a 0% composite-rate floor, reflecting that an I bond does not receive a negative composite rate.

Series I Savings Bond Compounding and Future Value

Series I bond interest is modeled here with semiannual compounding. The calculator divides the annualized composite rate by two, applies that periodic rate twice for every entered year, and estimates future value with:

Formula: V = P (1+r/2)^2n

In this I bond projection, is the purchase amount, is the composite rate in decimal form, and is years held. The exponent contains two periods per year. After calculation, the page also creates a year-by-year estimated-value table so you can inspect the modeled accumulation rather than only the final amount.

Example Series I Savings Bond Growth Table

This Series I bond example shows a $5,000 purchase with a 4.26% composite rate held for five years, assuming that composite rate remains unchanged throughout the projection:

The displayed example values are rounded and depend on a constant-rate assumption. Actual I bond inflation components are reset in May and November, so a real bond’s later composite rates may be higher or lower than the rate used in this illustration. The table instead isolates the effect of repeated semiannual compounding at one assumed rate.

Series I Savings Bond Tax Treatment and Redemption Rules

Interest on I bonds is subject to federal income tax but is exempt from state and local income taxes. Federal tax can generally be deferred until redemption or final maturity, allowing accrued interest to remain in the bond while it compounds. I bond interest may also be excludable in some qualified higher-education situations when applicable program requirements and income limits are met. These tax characteristics can matter when comparing an I bond with other conservative savings choices.

Series I bonds also have redemption restrictions that this growth calculation does not apply. They cannot be redeemed during the first 12 months after purchase, and redemption within the first five years forfeits the most recent three months of interest. After five years, that early-redemption penalty no longer applies, while a bond can continue earning interest until final maturity 30 years after issuance. The calculator reports theoretical value before taxes and any early-redemption penalty. TreasuryDirect currently limits electronic I bond purchases to $10,000 per Social Security number per calendar year, and the Treasury stopped issuing paper savings bonds through IRS tax refunds after January 1, 2025.

How to use: Set Up a Series I Savings Bond Projection

Use this Series I Savings Bond Calculator as a simplified rate scenario, beginning with the purchase amount and the fixed and semiannual inflation rates relevant to the bond you are considering. The model holds the entered rates constant over the full holding period even though the inflation component will ordinarily be reset over time. Changing the semiannual inflation-rate input can show how the assumed inflation component changes projected growth. Entering 0% for that component, for example, leaves the fixed-rate portion as the source of the modeled composite rate.

The holding period is the other major driver of the Series I bond estimate. Since each modeled six-month period adds interest to the balance, a longer period produces more compounding periods and a larger projected final value when the rate assumption is unchanged. Submit the form to view both the final estimate and the generated table for each whole year in the selected period.

The output is not a redeemable-value quote. It does not distinguish accrued from redeemable value, account for the first-year redemption restriction, subtract the three-month penalty that can apply before five years, or calculate taxes. Actual redemption proceeds can also depend on timing within an interest cycle. Treat the displayed amount as a consistent-rate growth illustration, then consult current Treasury information for a transaction decision.

Series I Savings Bonds in Broader Financial Planning

Within a savings plan, Series I bonds can serve as an inflation-aware, low-risk portion of assets. They do not trade at changing market prices in the way stocks, bond funds, or Treasury Inflation-Protected Securities can. The fixed rate remains tied to an individual I bond, while its inflation component is revised over time; consequently, a projection based on today’s inputs should be revisited after relevant Treasury rate announcements. This calculator helps show the sensitivity of the estimate to those rate assumptions without claiming to forecast future inflation.

I bonds are one of several government-linked options that savers may compare. TIPS adjust principal for inflation and trade in a secondary market, while I bonds are savings bonds with their own purchase, holding, and redemption rules. Liquidity needs, tax treatment, purchase limits, expected holding period, and tolerance for changing market values can all affect the comparison. The calculator’s purchase amount and rate inputs are useful for exploring an I bond scenario, but they do not make that broader allocation choice.

Whether the goal is a future expense, an emergency reserve after the holding restriction, or a general inflation hedge, the Series I Savings Bond Calculator offers a focused way to examine projected accumulation. Adjust the rate assumptions and holding period, then read the result as an estimate based on those assumptions rather than as a promise of a future Treasury rate or redemption amount.

Series I Savings Bond Limitations and Assumptions

This Series I bond tool is a constant-rate projection rather than a full record of every Treasury accrual, tax, purchase, and redemption detail. Its estimate depends on entering the correct purchase amount and rate terms and on recognizing that future Treasury rate announcements and bond rules can change. It excludes taxes, redemption timing, and early-redemption interest forfeiture. Check current TreasuryDirect materials and seek qualified advice when an I bond purchase, redemption, or tax treatment is important to your decision.