📈 I Bond Calculator

Estimate the composite rate, accrued value, and early-redemption penalty for a Series I U.S. savings bond, using the current fixed rate and semiannual inflation rate as a starting point.

Net Redemption Value

—

Composite Rate — Gross Value — Early-Redemption Penalty — Net Value — Total Interest —

Semiannual Accrual Schedule

Period	Rate Applied	Interest	Running Value

T-Bill Calculator CD Ladder Calculator Inflation Calculator

ガイド

詳しく見る

What Is a Series I Savings Bond?

A Series I savings bond is a U.S. Treasury savings bond designed to protect your purchasing power from inflation. Its interest rate, called the composite rate, combines a fixed rate that stays the same for the life of the bond (30 years) with a semiannual inflation rate that adjusts every May 1 and November 1 based on changes in the non-seasonally-adjusted Consumer Price Index for All Urban Consumers (CPI-U). Individuals can buy up to $10,000 per calendar year electronically through TreasuryDirect.gov, plus up to $5,000 in paper bonds via IRS tax refund, making I bonds a popular tool for savers seeking a low-risk, inflation-protected place to park cash beyond a standard savings account.

How the Composite Rate Formula Works

The composite rate is calculated using a formula set by the Treasury: Composite Rate = Fixed Rate + (2 x Semiannual Inflation Rate) + (Fixed Rate x Semiannual Inflation Rate), with all rates expressed as decimals in the calculation and the result displayed as a percentage. For example, a fixed rate of 1.2% (0.012) and a semiannual inflation rate of 1.5% (0.015) produce a composite rate of 0.012 + (2 x 0.015) + (0.012 x 0.015) = 0.012 + 0.03 + 0.00018 = 0.04218, or 4.218%. This formula ensures that even if inflation were zero, you still earn at least the fixed rate, and the cross-term (fixed x semiannual) is a small compounding adjustment specified in Treasury regulations. If the calculated composite rate would be negative, it is floored at 0% you can never lose principal value on an I bond.

Current I Bond Rates for May 2026 - October 2026

For I bonds issued between May 2026 and October 2026, the U.S. Treasury has set the fixed rate at 0.90% and the semiannual (6-month) inflation rate at 1.67%, which annualizes to approximately 3.34%. Applying the composite rate formula gives approximately 4.26% (0.009 + 2x0.0167 + 0.009x0.0167 = 0.04255, which the Treasury rounds to 4.26% in its official publication). This rate applies to the first six months after purchase for any bond bought in this window; after that, it resets to whatever the new fixed and inflation rates are for the next six-month period, using the bond's own fixed rate (which never changes) combined with the newly announced semiannual inflation rate. Source: TreasuryDirect.gov, verified for the May-October 2026 rate-setting period. These figures are prefilled as defaults above but can be edited to model any historical or projected rate combination.

How Interest Compounds: Semiannual Compounding Explained

I bond interest compounds semiannually, meaning every six months the accrued interest for that period is added to the bond's principal, and the next six-month period earns interest on this new, larger balance. This calculator applies the composite rate divided by two for each completed six-month period, compounding the running value at each step, and uses a rate-appropriate fractional exponent (rather than simple linear interpolation) for any partial period shorter than six months. Although the Treasury updates the composite rate every six months from your bond's issue date (not the calendar), the underlying accrual mechanism, semiannual compounding of a fixed six-month rate, is the same throughout the bond's life; this calculator models one rate held constant for the full holding period you enter, which is accurate for holding periods of six months or less and a reasonable approximation beyond that if rates do not change.

The 12-Month Lockup and 5-Year Early-Redemption Penalty

I bonds cannot be redeemed at all during the first 12 months after purchase this is a hard lockup with no exceptions, even in cases of financial hardship (the one exception is bonds owned by residents of a federally declared disaster area, under specific Treasury rules). After the 12-month mark, bonds can be redeemed, but if you cash one out before it is 5 years old, you forfeit the most recent 3 months of interest as a penalty. For example, redeeming a bond after exactly 12 months means you actually only keep 9 months worth of accrued interest the last 3 months are clawed back. This calculator automatically computes this penalty for any holding period under 60 months, and shows a warning if you enter a holding period under 12 months, since redemption would not be permitted at all in that window.

I Bonds vs T-Bills vs CDs: Which Should You Choose?

I bonds, Treasury bills (see our T-Bill Calculator), and CDs (see our CD Ladder Calculator) all serve different roles in a conservative cash strategy. I bonds are unique in offering inflation protection built directly into the rate, making them attractive when inflation is elevated or uncertain, but they come with the $10,000/year purchase limit, the 12-month lockup, and the 3-month early-withdrawal penalty inside 5 years. T-bills offer more flexibility (any amount, tradable before maturity, terms as short as 4 weeks) and state-tax-exempt interest, but no inflation adjustment their rate is fixed at purchase. CDs may offer competitive fixed rates and are FDIC-insured, but typically carry stricter early-withdrawal penalties and no inflation protection. Many savers use all three together: I bonds for long-term inflation-protected savings, T-bills or CD ladders for shorter, more flexible needs.

Taxes on I Bond Interest

I bond interest is subject to federal income tax but exempt from state and local income taxes, similar to T-bills. Interest can be reported annually as it accrues or, more commonly, deferred and reported all at once in the year the bond is cashed or matures (after 30 years), whichever comes first. Some taxpayers can exclude I bond interest from federal tax entirely if proceeds are used to pay for qualified higher education expenses, subject to income phase-out limits and other IRS rules under the Education Savings Bond Program. As always, this calculator provides estimates for planning purposes only consult a tax professional for guidance specific to your situation, and refer to TreasuryDirect.gov for the official, currently published rates before making a purchase decision.

よくある質問

現在のI bondの複合金利はいくらですか？

2026年5月から2026年10月に発行されたI bondでは、TreasuryDirect.govによると、複合金利は約4.26%で、0.90%の固定金利と1.67%の半年ごとのインフレ率から算出されます。この計算機では、これらの数値を編集可能な初期値として使用しています。

I bondの複合金利はどのように計算されますか？

複合金利 = 固定金利 + (2 × 半年インフレ率) + (固定金利 × 半年インフレ率) です。式では小数の金利を使います。たとえば、固定金利1.2%と半年インフレ率1.5%なら、複合金利は4.218%になります。

I bondはいつでも換金できますか？

いいえ。I bondは購入後最初の12か月間は、どのような場合でも償還できません。12か月から5年の間は償還できますが、直近3か月分の利息がペナルティとして差し引かれます。

I bondの利息はどのくらいの頻度で複利計算されますか？

I bondの利息は半年ごと（6か月ごと）に複利計算され、発生した利息が元本に加算されるため、次の期間は増えた残高に対して利息がつきます。

I bondの利息は課税対象ですか？

I bondの利息は連邦所得税の対象ですが、州税と地方所得税は非課税です。税金は通常、換金時または満期時まで繰り延べることができ、教育費の用途によっては連邦税の非課税措置の対象になる場合があります。

このツールは投資助言ですか？

いいえ。この計算機は教育および概算のためのものであり、金融・投資・税務の助言ではありません。実際の複合金利は6か月ごとに変わるため、判断の前にTreasuryDirect.govまたは資格のある金融アドバイザーにご確認ください。