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 4.57%/ann rate for redeeming after 12 months (and abandoning the last 3 months of interest). But now there are three more months of CPI-U out, and we can make a more-informed estimate.

Inflation has slightly accelerated since then -- Sept->Nov saw 2 months of 8.23%/ann inflation, and Nov->Feb saw 3 months of 8.56%/ann inflation. If we assume (very conservatively, I think) that Feb->Mar inflation will be halfway between 8.43%/ann and historical 2.62%/ann, we get 3.90% for the Sept->Mar interval, and a 6.03% annualized return for holding for the first 15

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 first 5-6 months of interest at 7.12% annualized (so 3.56% in 6 months), and then reset to some other rate that will depend on future inflation. If you don't want to stay invested after that, it's possible to redeem the bonds after 12 months.

If getting ~3-6% annualized interest on $20k of a US government bond is a thing that you want, then this might be the best way to do it.

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The Series I savings bond is a US government bond that is offered directly to US citizens, with purchases limited to $10k per person per year. (Apparently, there's a way to purchase an additional $5k