I hate to ask such a stupid question that I should be able to figure out. What is the lowest yield the I bond has ever paid. I get that everyone thinks it's a no brainer, but if it drops to the .40% what am I missing?
Answers

As for the .40% that you mention I suspect you think that is your lowest possible annualized return? If so, that’s not correct. Deflation would neutralize the fixed rate component as well, but with a 0.00% composite rate floor.



As for the purchase decision, it has been discussed in earlier threads so I don’t want to open that can of worms here again. But I think it would be a horrible mistake to buy I-bonds now. At a minimum wait until mid-April and you will know for certain what your full year (and minimum holding period) return will be. You give up no return by doing this and you cement your informational advantage. In addition you earn an inflated rate on your capital while you wait and that type of rate won’t likely be available on the tail end when you choose to redeem. Frequently argued ‘starting the clock earlier’ comments rarely make any sense, unless you know now of a future date when you will need the money back for an alternative purchase.





it is a brainer.
While in 2015 composite rate was 0%, it wasnt 0% paid!
Lowest ever paid, as far as I understand and Treasury reports as "Yield from Issue", was 0.4% for Bonds purchased in 2015 and redeemed in 2016.
There is nothing ANNUAL about Bonds, it is ANNUALIZED, where the ANN is 12 months in 6 months increments May-to-November and November-to-May.
Bonds do NOT pay INTEREST, Bonds generate Yield trough change of the Value between Bonds Purchase price and Redemption Value, where the Value is promised not to be less than the Price paid, the Yield will never go below Zero.
Since there is No Interest, there is nothing to lose. Bond current Value is calculated first of every month and available for Redemption until first of the next month when Yield and Value may be different or remain unchanged in 0% composite rate scenario.