What is the interest rate on the installment plan?

[AnthonyC · Accepted solution]

The interest rate is determined quarterly and is the federal short-term rate plus 3 percent.  Currently, it stands at 4% for the most recent quarter (see second link press release below). 

You can view information about the interest and penalties the IRS charges at this link (in addition to interest there is a late payment penalty of 0.25% per month when you have an installment agreement in effect - otherwise, its is 0.50%) -

https://www.irs.gov/taxtopics/tc653.html

https://www.irs.gov/pub/irs-news/ir-17-053.pdf