Buying I Bonds Before October 31? Waiting Until November Is Now Worth About $1,650.

Update, October 14, 2026: September inflation landed this morning, and it settles half of the November rate. The CPI-U index went from 330.213 in March to 336.940 in September, a 2.04% jump in six months. Run through Treasury’s formula, that’s an inflation rate of 4.08% a year, up from 3.34% now. Add a 1.30% fixed rate and a November bond starts at about 5.41%. A bond you buy in October starts at 4.26%. Waiting now wins in the first six months too, not just over 30 years.

Update, October 6, 2026: The fixed-rate projection rose to at least 1.30%, worth about $1,650 on a $10,000 bond. Numbers below are updated.

If you were about to buy I bonds before October 31 to lock in the 0.90% fixed rate, stop. The one number on an I bond that stays with you for 30 years is about to move, and it is moving up.

October is the worst month of the year to buy one.

The number you are actually shopping for

An I bond pays two rates stacked together. Treasury sets the fixed rate the month you buy, and it never changes for the life of the bond. The inflation rate resets every six months, for every I bond, no matter when it was bought. Treasury sets both on May 1 and November 1. Bonds bought through October 31 carry a 0.90% fixed rate and a 4.26% composite rate for their first six months.

Only one of those is a reason to hurry. It is not the 4.26%.

David Enna of TIPSwatch ran the numbers on August 9. The 5-year TIPS real yield, which is what the market pays for a guaranteed return above inflation, sat at 1.11% on February 27, the day before the Iran war began. It was 2.13% when he wrote, and Treasury’s data put it at 2.70% on October 5. Treasury does not publish a formula for setting the fixed rate, but Enna’s long-used rule of thumb is roughly 0.65 times the average of that real yield over the preceding six months. In August that pointed to 1.20%. By September 20 he had raised it, writing that there is “nearly zero chance that the fixed rate will fall below 1.30%,” and that a real yield holding at 2.50% or higher would push it to 1.40%.

We checked his math against Treasury’s daily real yield data. From May 1 through October 13, the 5-year real yield averaged 2.05%. Times 0.65, that’s 1.33%. It hasn’t closed below 2.50% since he wrote, and it was 2.67% on October 13. If it sits near there for the rest of October, the same math lands around 1.38%, which is why 1.40% is a live possibility.

What that gap is worth

Run it on a single $10,000 bond held the full 30 years, in today’s dollars, meaning on top of whatever inflation does.

At 0.90% fixed, you end up with about $13,080. At 1.30%, about $14,730. At 1.40%, about $15,170.

Waiting from October to November is worth roughly $1,650, and possibly $2,100, on one $10,000 purchase. Nothing else about the bond changes. Same issuer, same tax treatment.

There used to be a catch here. November’s composite rate depended on the September inflation report, and a soft number could have made the first six months pay less. The report came in hot. Gasoline jumped 9.1% in September alone, and the six-month inflation rate came to 2.04%, which Treasury’s formula doubles to 4.08% a year.

That’s locked. Treasury’s composite formula is the fixed rate, plus twice the six-month inflation rate, plus a sliver for the two multiplied together. At a 1.30% fixed, a November bond pays about 5.41% for its first six months. At 1.40%, about 5.51%. An October bond pays 4.26%. On $10,000, that’s roughly $270 of interest in the first six months instead of $213. The fixed rate would have to come in below about 0.2% for October to win even that stretch, and the real-yield data point the other way.

Do this

Park the money in a high-yield savings account through October. It stays liquid and it keeps earning while you wait. Our best savings rankings and the savings calculator will tell you what you are giving up in the meantime, which is not much.

Check TreasuryDirect.gov on November 1, read the new fixed rate, then buy. One thing to do first: TreasuryDirect’s old password login dies October 28, so switch your login to ID.me before then, or you’ll be locked out on the day you want to buy.

If you hold I bonds from 2022 with a 0.00% fixed rate, this is the swap worth running in November. You are years past the one-year lock, and trading a 0% fixed rate for something above 1% is real money for the next three decades. We ran the case for the current 0.90% bonds in June; the case gets stronger with a bigger fixed rate behind it.

Do not buy in October to beat a deadline that costs you money.

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