Interest is calculated using the following formula: (Original liability x Rate x # of days delinquent divided by 365.25)
Example: Assume the liability on an entity’s monthly sales tax return for February is $1,000. This return was due by March 20, but was not filed until April 22 (33 days late). The interest rate is 7.5%. Interest is calculated as follows:
$1,000 x 7.5% x 33 divided by 365.25 = $6.78.