How do the interest payments for delayed refunds work? What’s the interest rate and how specifically is the interest payment calculated?

https://www.cnbc.com/select/what-to-do-with-late-tax-return-interest-check/

I’ve seen this article and others similar to it, but how are they figuring $18 for an interest payment? That’s what I’m trying to figure out. 

If it’s an assumed 3-5% interest rate compounded daily, depending on how much the IRS owes you as a refund, that could be a sizable chunk of money—not a puddly $18. 

@kyounkman 

See https://www.irs.gov/payments/interest-on-underpayments-and-overpayments and

https://www.irs.gov/payments/quarterly-interest-rates-for-underpayment-and-overpayment-of-tax

Thanks @tagteam  I appreciate this.

So if I’m interpreting the information correctly, “whichever the later” is the filing deadline of May 17, then the time elapsed from then to which they release my refund should have 3% interest daily compounded and give as a payment, correct?

Not from the date that I had filed in a format that they could process, right?

It is the later of all four possibilities listed which could clearly be the date the return was received in a proper format.