How much is the monthly interest if on the IRS payment plan and do you still pay a penalty if you are on the payment plan with the IRS

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Late Payment Penalties -  If you file on time but don't pay all amounts due on time, you'll generally have to pay a late payment penalty of one–half of one percent (0.5%) of the actual tax owed for each month, or part of a month, that the tax remains unpaid from the due date, until the tax is paid in full. The penalty is not waived if you are on a payment plan.

Interest Rate - For taxpayers other than corporations, the interest rate is calculated as  the federal short-term rate plus 3 percentage points. For the first quarter of 2017, it is 4%.