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 penalties. The penalty will be the applicable 5% or 10%, but not 15%.

Interest
Interest must be added to delinquent tax. Interest is added at a rate of 0.5% per month beginning on the due date of the return (April 30) 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 1.0% (2 x 0.5%).

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