To recap:

The Series I savings bond is a US government bond offered to US citizens, with purchases limited to $10k per person per year. It pays interest set by a formula based on the official inflation rate, with a built-in lag. (...)

In particular, the interest rate formula for bonds bought before May 1, 2022 will be:

• 3.56% until October 1
• X% for the next 6 months (where X% is the inflation of CPI-U from Sept 2021 to March 2022)
• Y% for the following 6 months (where Y% is the inflation of CPI-U from March 2022 to Sept 2022)

...and so on. You'll forfeit the last 3 months of interest if you redeem anytime before 5 years (which I'm assuming will be correct to do).

When we checked in December, we'd seen 2 months of the Sept'21->Mar'22 period and I had conservatively estimated that the 6-month inflation rate would come in at 2.17%, implying a

Summary: The Series I savings bond is a US government bond offered to US citizens, with purchases limited to $10k per person per year. It pays interest set by a formula based on the official inflation rate, with a built-in lag. If inflation from November '21 to March '22 follows historical patterns, bonds purchased in December '21 and redeemed after 15 months will pay ~4.62% interest annualized. If inflation is higher (as it has been recently), the bonds will pay more; if it's lower, they will pay at least 3.56% interest when redeemed after 12 months.

All of those potential rates are a percentage points higher than any other bond that is even remotely as safe; this is because of the way the inflation adjustment rule works. Specifically, the inflation adjustment for the next six months is set based on what inflation was in the last six months. As a consequence, a Series I purchased between now and April 30 will pay its