You should bookmark this:
https://proconnect.intuit.com/articles/federal-irs-underpayment-interest-rates/
And this:
https://www.irs.gov/newsroom/interest-rates-remain-the-same-for-the-second-quarter-of-2020
And you can subscribe to federal e-newsletters and get all of that by email.
Is there an updated link to this page? I cannot find the same page for year 2021.
I think it is still 3%. You can find it on irs.gov
