Every headline about Series I savings bonds quotes one number, and it’s the wrong one. Right now that number is 4.26%, the composite rate the Treasury set on May 1, 2026. It will be gone by Halloween. The number that actually determines whether an I bond is a good deal is the I bond fixed rate, currently 0.90%, and unlike the headline rate it never changes for as long as you hold the bond. Understanding the difference between those two numbers is the whole game, and it explains why the Treasury is about to make I bonds meaningfully more attractive for reasons that have nothing to do with inflation.
An I bond is two rates wearing one coat
The composite rate you see advertised is a blend. The Treasury sets a fixed rate twice a year, on May 1 and November 1, and it applies for the full 30-year life of any bond issued during that window. Separately, it sets a semiannual inflation rate derived from the Consumer Price Index, and that one applies to every I bond in existence regardless of when it was bought.
The two get combined with a formula that looks fussier than it is. TreasuryDirect publishes it as the fixed rate, plus twice the semiannual inflation rate, plus the product of the two. For bonds issued between May and October 2026, that’s 0.90% plus 3.34% plus a sliver of 0.015%, which rounds to the 4.26% in the press release. The middle term is doing almost all the work, and the middle term is temporary.
This is where most people misread the product. They compare 4.26% against a savings account paying around 4% and conclude the I bond is roughly a wash. It isn’t a wash in either direction, because those two rates are not the same kind of thing. A savings account rate is a nominal promise that can be cut tomorrow. The I bond rate is a promise about inflation, and the only permanent part of it is that 0.90%.
The fixed rate is a permanent promise to beat inflation
Strip out the inflation component and here’s what an I bond actually offers: your money will grow 0.90% per year faster than official U.S. inflation, forever, until you cash out or the bond matures in 30 years. If inflation runs at 8%, you earn roughly 8.9%. If inflation runs at 1%, you earn roughly 1.9%. If inflation goes negative, the composite rate floors at zero and you never lose principal.
That framing changes what the product competes against. An I bond is not competing with your savings account on yield this quarter. It’s competing on the question of whether your purchasing power survives a decade. A savings account paying 4.15% in a year when inflation runs 3.4% is delivering about 0.75% of real return, and that spread evaporates the moment the bank trims its rate. The I bond’s 0.90% real spread is contractual.
The people who bought I bonds in 2021 and 2022 learned this the hard way in reverse. Those bonds carried headline rates of 7.12% and 9.62%, the highest in the product’s history, and buyers piled in. But the fixed rate on most of those issues was 0.00%. Once inflation cooled, those bonds settled into paying exactly inflation and nothing more, which is a fine place to park cash and a poor place to compound wealth. Somebody who waited until the November 2023 reset got a 1.30% fixed rate on the same product, a gap you can see clearly in the Treasury’s own historical rate data.
Your six-month clock runs from your issue date, not from May and November
The Treasury announces new rates on May 1 and November 1, but your bond does not switch rates on those dates. It switches on the six-month anniversary of its own issue month. This is the piece that trips up nearly everyone who tries to time a purchase.
Buy an I bond in September 2026 and you earn the 4.26% composite for six full months, through February 2027, even though a new rate gets announced on November 1. Your bond picks up the November rate in March 2027, then the following May’s rate in September 2027, and so on in a rolling pattern set entirely by when you bought. TreasuryDirect’s rate chart is organized this way, by issue month rather than by calendar date, which is why it looks confusing until you realize each row is a separate six-month clock.
What a 0.40% difference in the I bond fixed rate is actually worth
The reason any of this matters right now is that the November 1, 2026 reset is coming and the fixed rate is almost certainly going up. David Enna of Tipswatch, who has tracked the Treasury’s behavior for over a decade, notes that the fixed rate has closely followed a simple rule: roughly 0.65 times the six-month average real yield on 5-year Treasury Inflation-Protected Securities. Real yields surged in 2026, with the 5-year TIPS yield climbing about 102 basis points since late February to 2.13% as of early August. Running that ratio through the May-to-August average produces a projected fixed rate of 1.20%, with 1.30% in play if yields hold near current levels.
Say you have $10,000 and a 30-year horizon. At a 0.90% fixed rate, that $10,000 grows to about $13,084 in inflation-adjusted purchasing power. At 1.30%, it grows to about $14,733. The gap is roughly $1,649 of real money, produced by four tenths of a percentage point that costs you nothing to wait for.
Then price the waiting itself. Parking that $10,000 in an online savings account at about 4.15% for September and October earns roughly $69. Buying the I bond today at 4.26% earns roughly $71 over the same two months. So you forgo about two dollars of interest for a shot at $1,649 of permanent advantage. Anyone who has already decided to buy I bonds this year and is sitting on the cash in August has a fairly easy call to make.
Treat the projection as a strong lean, not a certainty. The Treasury has never published a formula and the rate is legally the Treasury Secretary’s call. The 0.65 ratio is a pattern somebody noticed in the data, not a rule anybody agreed to, and a decade of it holding is good evidence rather than a promise. What is not in doubt is the direction. Real yields are far higher than they were when the current 0.90% was set, and the fixed rate has never moved in the opposite direction from them.
The lockups are the price of the deal
A guaranteed real return has to be paid for with something, and here that something is access to your own money. You cannot redeem an I bond for the first 12 months, full stop, and if you cash out before five years you forfeit the last three months of interest. TreasuryDirect’s displayed value for bonds under five years already nets that penalty out, so the number you see is the number you’d actually get. Purchases are capped at $10,000 per person per calendar year in electronic bonds, which is why the annual limit shapes so much of the strategy discussion around this product.
The penalty is best understood the way you’d understand a CD’s early withdrawal penalty: it is the other half of a trade, not a punishment. The Treasury is buying certainty about how long it gets your money, and it pays for that certainty with an inflation guarantee it doesn’t offer anywhere else in the savings-bond lineup.
It all reduces to one comparison. Your savings account gives you liquidity and a nominal rate that floats with whatever the Fed and your bank decide. An I bond gives you a locked real return and takes your liquidity for a year. The difference between real and nominal returns is not academic here; it’s the entire product. And once you see that the I bond fixed rate is the only durable part of the quoted yield, the November 1 reset stops looking like financial trivia and starts looking like a date worth writing down.