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Series I Savings Bond

Also called: I bond, Series I bond, inflation bond

Definition: A Series I savings bond (I bond) is a U.S. Treasury savings bond whose rate combines a fixed rate kept for 30 years with an inflation rate based on CPI-U that resets every six months.

Updated November 11, 2026 · How we check facts

I bonds are sold electronically through TreasuryDirect, from $25 up to $10,000 per Social Security Number per calendar year. They earn interest for 30 years, can be cashed after 12 months, and lose the last 3 months of interest if cashed before 5 years. Interest is subject to federal income tax but not state or local income tax.

Formula

combined rate = F + 2 × I + F × I
  • F: fixed rate, set every May 1 and November 1 and kept for the life of the bond.
  • I: semiannual inflation rate, based on the change in the non-seasonally adjusted CPI-U and reset every six months.

The combined rate can never fall below zero. Interest accrues monthly and compounds every six months at the combined rate ÷ 2.

Example

For bonds issued November 1, 2026 to April 30, 2027: 0.0140 + 2 × 0.0204 + 0.0140 × 0.0204 = 0.0550856, rounded to 5.51%. A $10,000 bond grows by 2.755% in its first six months, to $10,275.50. If 5.51% held for five years, it would be worth $13,122.89.

The savings bond calculator values a bond for any rate and holding period, and the guide how do I bonds work covers rate history, tax and the early-redemption penalty.

Calculate it

Free toolSavings Bond CalculatorValue of an I bond or EE bond, with penalty and tax.Free toolCompound Interest CalculatorSee what your savings grow to, year by year.

Guides that explain it

Related terms

Series EE Savings BondA Series EE savings bond is a U.S. Treasury savings bond with a fixed interest rate, guaranteed to be worth at least double its purchase price after 20 years and earning interest for 30 years.Consumer Price Index (CPI)The Consumer Price Index (CPI) is a Bureau of Labor Statistics measure of the average change over time in prices paid by consumers for a representative basket of goods and services.Compound InterestCompound interest is interest calculated on both the original principal and the interest already added to it, so the balance grows faster than it would with simple interest.Deposit InsuranceDeposit insurance protects money in checking, savings, money market deposit accounts and CDs at insured banks, up to $250,000 per depositor, per bank, for each account ownership category.401(k)A 401(k) is an employer-sponsored retirement savings plan in which employees contribute part of their pay, often with employer matching, up to annual limits set by the IRS.Annual Percentage Rate (APR)Annual percentage rate (APR) is the cost of credit expressed as a yearly rate; for many loans it combines the interest rate with certain fees charged when the loan is made.Annual Percentage Yield (APY)Annual percentage yield (APY) is the total interest a deposit account pays over a 365-day period, expressed as a percentage that includes the effect of compounding.Catch-Up ContributionA catch-up contribution is an extra amount that people age 50 or older may add to a 401(k) or similar retirement plan above the normal annual limit, if the plan permits it.

Sources

  1. TreasuryDirect — Series I savings bonds
  2. TreasuryDirect — I bonds interest rates