Series I Savings Bonds — commonly called I Bonds — are US government-backed savings instruments with a unique feature: their interest rate adjusts every six months to track inflation, protecting the purchasing power of your savings in ways that standard savings accounts and CDs cannot. After years of minimal attention, I Bonds surged in popularity during the 2021-2023 inflation surge when their rates reached 9 percent — levels unavailable in any comparable safe investment. Understanding I Bonds thoroughly — the rate formula, the purchase mechanics, the holding requirements, and the genuine decision about whether they belong in your savings plan — prevents both underpurchasing and overpurchasing these instruments.
The Interest Rate Formula
I Bond interest has two components. The fixed rate is set at purchase and remains for the life of the bond — it represents the real return above inflation and can be zero in some periods. The inflation rate adjusts every six months based on the Consumer Price Index for Urban Consumers (CPI-U), reset on May 1 and November 1 each year. The composite rate combines these components: composite rate = fixed rate + (2 × semiannual inflation rate) + (fixed rate × semiannual inflation rate). When inflation is high, the composite rate rises; when inflation falls, the rate falls accordingly. The rate can never go below zero — I Bonds cannot lose nominal value — but they can pay zero interest in deflationary periods.
The May and November rate resets create a strategic consideration for timing purchases. Buying before a rate reset allows locking in the current rate for the first six months; buying just after a reset gives six months at the new rate. Monitoring the rate announcements at TreasuryDirect.gov around reset dates allows timing purchases to maximize the higher of the two consecutive rates for your holding period.
Purchase Limits and Methods
The $10,000 annual purchase limit per person through TreasuryDirect is the primary constraint on using I Bonds at scale. A married couple can purchase $20,000 annually. An additional $5,000 can be purchased using your federal tax refund by directing it to purchase I Bonds through Form 8888 on your tax return — making this the only way to exceed the electronic purchase limit. Trusts and business entities can purchase additional amounts with separate $10,000 limits, which some families use to expand their effective annual I Bond capacity. These limits make I Bonds a supplemental savings vehicle rather than a primary investment for most households.
Holding Requirements and Early Redemption
I Bonds must be held a minimum of 12 months — they cannot be redeemed within the first year under any circumstances. Redemption between 12 and 60 months incurs a penalty of the most recent 3 months of interest. After 60 months, I Bonds can be redeemed without penalty and continue earning the composite rate for 30 years. These holding requirements make I Bonds appropriate for savings you are confident you will not need for at least 12 months — emergency fund supplementation above an accessible liquid tier, medium-term savings goals with timelines of 1-5 years, or a portion of a fixed income allocation in a retirement portfolio. They are not appropriate for money you might need in a financial emergency before 12 months have elapsed.