What is the interest and penalty if I don't pay my taxes on time?

Interest is 1% per month and is applied to your unpaid tax due amount the first day of each month.  If you miss your April 30 payment you can pay at any time before October 31 and stop the interest and penalty by paying the first half plus any interest and penalty due. 

If your payment is still delinquent on June 1 there is a one-time 3% penalty applied to the full tax amount due.

If your payment is delinquent on December 1 there is an additional one-time 8% applied to the full tax amount due.