10% Penalty for Failure to Timely File a Return:
If you do not file your return by the due date and at least 90% of the correct tax is not paid, you owe an additional 10% of the unpaid tax.
5% Penalty for Failure to Timely Pay the Tax Due:
If you file your return on time but do not pay at least 90% of the correct tax due, you owe an additional 5% of the unpaid tax.
You will never be subject to both the 5% and 10% penalties. The penalty will be the applicable 5% or 10%, but not a total of 15%.
Interest must be added to delinquent tax. Interest is added at a rate of 0.8% per month beginning on the due date of the return (May 1) and accrues each month until paid in full. Part of a month constitutes a whole month, so if you pay the tax on June 3, you are late for May and June for a total interest rate of 1.6% (2 x 0.8%).