How to get interest rate when it has only penalty rate. It says Deficiency interest is applied as provided by La. R.S. 47:1601 and is calculated on the cumulative tax balance from one period due to the next. ***Interest continues to accrue until the entire tax liability is paid. *Refund interest is calculated provided by La. R.S. 47:1624(A)(1). How can I get interest rate from year 12/28/2017,12/31/2018, 2019?