The simple version

Two different dates get treated as one. There is the date the Treasury announces a new rate, and there is the date your own holding starts earning a new rate.

The announcements land on two fixed dates a year. Your own switch happens every six months, counted from the month yours was issued.

Those two schedules coincide for only two of the twelve issue months. For the other ten they do not, and the Treasury publishes the table that says so.

The numbers

• Composite rate of 4.26 percent for Series I savings bonds issued May 1, 2026 to October 31, 2026, which TreasuryDirect states includes a fixed rate of 0.90 percent. That window closes on October 31, 2026 and a new rate is announced for the window that follows it.
• The fixed rate is announced every May 1 and November 1, and TreasuryDirect states it then applies for the life of the bond to all bonds issued during the next 6 months. The fixed rate never changes.
• The inflation component is set every May 1 and November 1, based on changes in the non-seasonally adjusted Consumer Price Index for all Urban Consumers (CPI-U) for all items, including food and energy.
• Composite rate formula, as TreasuryDirect prints it: fixed rate, plus two times the semiannual inflation rate, plus the fixed rate times the semiannual inflation rate.
• TreasuryDirect's own worked example uses a fixed rate of 0.90 percent and a semiannual inflation rate of 1.67 percent: 0.0090 plus 0.0334 plus 0.0001503 gives 0.0425503, which rounds to 0.0426, or 4.26 percent.
• TreasuryDirect lists the fixed rate set on November 1, 2025 as 0.90 percent and the inflation rate set that day as 1.56 percent, which give a composite of 4.03 percent for bonds issued in that window. It lists the inflation rate set on May 1, 2026 as 1.67 percent.
• Its table of composite rates for the six-month earning period beginning May 2026 shows 4.26 percent for bonds bought May 2026 to October 2026 and also 4.26 percent for bonds bought November 2025 to April 2026, two cohorts issued at different composite rates earning the same one in that window, because they share a 0.90 percent fixed rate and are both on the May 2026 inflation figure.
• When your rate changes, by issue month: January, July 1 and January 1. February, August 1 and February 1. March, September 1 and March 1. April, October 1 and April 1. May, November 1 and May 1. June, December 1 and June 1.
• Continuing: July, January 1 and July 1. August, February 1 and August 1. September, March 1 and September 1. October, April 1 and October 1. November, May 1 and November 1. December, June 1 and December 1.
• On deflation, TreasuryDirect states that a negative inflation rate can pull the combined rate below the fixed rate, but that if it would pull the combined rate below zero, they stop at zero.
• Interest is earned from the first day of the month of purchase, and interest earned over each 6 months is added to the bond's value, so the next 6 months apply the new rate to the larger value.
• Figures and the table are the Treasury's own, read at TreasuryDirect on October 24, 2026.

Why the announcement date is not your date

The announcement sets a rate for a six-month issue window. Your holding then runs on its own six-month clock, started by the month it was issued.

Read down the Treasury's table and the pattern is mechanical. A holding issued in October switches on April 1 and October 1. One issued in March switches on September 1 and March 1.

Only two of the twelve issue months land on the announcement dates themselves. For the other ten, the announced number waits until your own clock comes round.

The actual math on a rate that is not the inflation rate

The combined number is built from two parts and a small third term. For the six months beginning May 2026 the Treasury lists a fixed rate of 0.90 percent and a semiannual inflation figure of 1.67 percent.

Doubling the semiannual figure gives 3.34 percent. Adding the 0.90 percent fixed part gives 4.24 percent, and the cross term of 0.0090 times 0.0167 adds the remaining 0.015 of a point, which brings it to 4.2550 percent before rounding to 4.26.

So the headline is neither the fixed part nor the doubled inflation part. It sits above both of them, and the gap between 3.34 and 4.26 is the fixed part plus that cross term doing the work.

What this means

If you hold one of these and you read that a new rate has been set, the next question is which month yours was issued. That month, not the announcement, tells you when the new number reaches you.

It also explains why two people can hold the same product and earn different rates in the same week. They are on different six-month clocks and may carry different fixed parts.

The transferable habit is to separate an announcement date from an application date. Plenty of rates work this way, and the two are reported as if they were the same day.

What this is NOT

This article explains when a Series I savings bond's interest rate changes and how the Treasury calculates the combined rate, using TreasuryDirect's own published figures, formula and issue-month table read on October 24, 2026 as the worked example. It is not a recommendation to buy, hold, or cash any savings bond or any other product, not a view on whether these bonds suit anyone, and not a comparison against any alternative. We do not give personalized advice, and nothing here addresses the tax treatment of these bonds, which is a separate subject this article does not touch. It does not predict any rate that has not yet been announced, or the inflation component behind it, because an unannounced figure is not a published one and a scheduled announcement is not a published number. Every rate figure here is tied to the six-month window stated beside it and is superseded for later windows. The count of ten issue months out of twelve is ours, read off the Treasury's table; the table itself is theirs. The arithmetic in the math section is the Treasury's own worked example, restated step by step, and the 4.26 percent figure and the 0.90 percent fixed rate are the ones TreasuryDirect states for bonds issued May 1 through October 31, 2026 and lists in its rate history for that period. The Treasury's example block states 4.03 percent as the composite rate for bonds issued November 2025 through April 2026, which its own rate tables support, since a 0.90 percent fixed rate and the 1.56 percent inflation rate it lists for November 1, 2025 give 4.03 percent. The arithmetic printed beneath that sentence instead uses the 1.67 percent inflation rate it lists for May 1, 2026, which is why that working produces 4.26 percent rather than the 4.03 percent stated above it. We have taken our inputs from the Treasury's fixed-rate and inflation-rate tables rather than from that example block. This is not a political endorsement or criticism of anyone.

Sources

• U.S. Department of the Treasury, TreasuryDirect, I bonds interest rates. Current rate and fixed rate, the announcement schedule, the composite rate formula and worked example, the issue-month table, the deflation floor, and the compounding description. Read October 24, 2026.