How the Series I Bond Rate Is Calculated

The Series I bond rate is calculated as the sum of two components: a fixed rate set at issuance and an inflation adjustment based on recent changes in the Consumer Price Index. This hybrid formula means your I bond yield adjusts every six months as inflation data updates, while the fixed component remains locked for the life of the bond.

The Two-Part Rate Formula

The U.S. Treasury Department uses a simple but elegant formula to set the I bond composite rate:

Composite Rate = Fixed Rate + (2 × Semi-annual Inflation Rate)

The fixed rate portion is announced every May and November and applies to all bonds purchased during that six-month period. This component never changes—it stays the same for the entire 30-year life of the bond. Recent fixed rates have ranged from 0.00% to around 1.00%, depending on the economic environment.

The inflation portion is recalculated every six months using the most recent data from the Consumer Price Index for All Urban Consumers (CPI-U). The Treasury takes the semi-annual change in the CPI-U (the percentage increase from, say, May to October), and doubles it to annualize the rate. This doubled figure is added to the fixed rate to produce the composite rate.

For example, if the fixed rate is 0.80% and the CPI-U rose 2.50% over the most recent six-month period, the inflation component would be 2.50% × 2 = 5.00%. The composite rate would be 0.80% + 5.00% = 5.80%.

Why the Formula Works That Way

The doubling of the six-month inflation rate converts a half-year figure into an annualized one. A 2.50% rise over six months, if sustained for a full year, would be roughly 5.00% (not exactly, but Treasury uses the simple double for clarity). This gives bond owners inflation protection while keeping the calculation transparent and easy to verify.

The fixed rate component ensures that even in a deflationary environment—where the CPI-U falls—your bond still earns something. If prices drop 1% over six months and the fixed rate is 1.00%, the composite rate would be 1.00% + (−1.00% × 2) = 1.00% − 2.00% = −1.00%. But Treasury has a floor: your composite rate can never drop below 0%. So your bond would earn 0% for that period, not negative interest.

The CPI-U Data Source

The inflation adjustment relies entirely on the Consumer Price Index for All Urban Consumers, published monthly by the U.S. Bureau of Labor Statistics. This index tracks the average price change for a basket of goods and services—food, energy, housing, medical care, transportation, and more—across urban areas.

Treasury uses specific CPI-U reference months to calculate the semi-annual inflation rate:

• May issuance rate: Uses CPI data from November and May
• November issuance rate: Uses CPI data from May and October

For example, the November 2024 composite rate was based on the CPI-U from May 2024 and October 2024. The next rate change occurs in May 2025, using May 2025 and October 2025 data.

This lag means the most recent month of CPI data is already several weeks old when the Treasury announces a new rate. The lag introduces a short data delay but ensures enough time for official data publication and auditing.

When the Rate Changes

I bonds earn interest in two ways: the principal earns interest monthly (credited as the composite rate each month), and the coupon rate itself resets every May and November.

If you own an I bond, your rate on a specific purchase date remains unchanged until the next rate-announcement date. If you buy a bond in June, you’ll earn the June rate until November, then the November rate takes over for the next six months, and so on.

This means your earnings can change significantly at each reset. During high-inflation periods (like 2021–2023), I bond rates rose to 5%+ as the CPI-U spiked. In low-inflation periods, rates can fall to 1–2% or even the floor of 0% if deflation were to occur.

How It Compares to Treasury Bills and Bonds

I bonds vs. Treasury bills: Treasury bills have fixed yields that never change (they’re short-term, typically four to twenty-six weeks). I bonds adjust every six months. A T-bill offers certainty; an I bond offers inflation protection.

I bonds vs. Treasury notes and bonds: Standard Treasury notes and bonds are fixed-rate instruments—the coupon rate never changes. But they expose you to interest-rate risk: if rates rise after you purchase, your bond’s market value falls if you need to sell early. I bonds avoid this because you can redeem them at par value (plus accrued interest) after five years, with no secondary-market sale needed.

Calculating Your Returns

To estimate your I bond earnings, you need the composite rate and the holding period.

Example 1: You buy a $10,000 I bond in June when the composite rate is 4.50%. You hold it for exactly one year (when the November rate takes effect). Your June six-month earning is $10,000 × 4.50% ÷ 2 = $225. Say the November rate is 3.80%. Your December–May earning is $10,225 × 3.80% ÷ 2 = $194. Total first-year earnings: about $419, or 4.19% annualized (accounting for the rate change mid-year).

Example 2: You buy a $25,000 I bond in May, hold it five years, then redeem. The rates vary each six-month period—say they average 4.25% over the five years. Your total interest earned is roughly $25,000 × 4.25% × 5 = $5,312.50 (this is approximate; actual earnings depend on month-by-month accrual and rate changes).

The exact amount requires tracking all six-month periods and the composite rate applicable to each. Treasury’s website and bond-tracking tools calculate this precisely.

Redemption and Tax Implications

I bonds must be held for at least one year before redemption. If you redeem between one and five years, you forfeit the most recent three months of interest. After five years, you can redeem at par without penalty.

Interest on I bonds is subject to federal income tax but exempt from state and local taxes. You can defer federal taxes until the bond matures (30 years) or until you redeem it. Some people use I bonds in education savings accounts to defer taxes until a beneficiary’s low-income years.

See also