Interest Calculation


Effective March 30, 2020 all taxpayers who file a return less than $1,000,000 will have an additional 3 months to file their return, between now and July 31, 2020. To calculate interest for returns during this period, use a start date of August 1, 2020. For more information, see our COVID-19 page.

Taxpayers reporting $1,000,000 or more should not use the interest calculator to calculate interest for the months of March 2020 through June 2020. Monthly interest for these months can be calculated by multiplying the Tax or Fee Amount by the monthly interest factor (.00667). Example: $1,000,000*.00667 = $6,670 of interest due.


* All form fields are required. See instructions below.


Start Date:
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End Date:
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Enter the interest Start Date. The interest start date is the day after the due date of the return. For example, if the due date of the return was the end of the month (12-31-04), the interest start date will be 01-01-05. If the due date of the return was the 15th of the month (12-15-04), the interest start date should be 12-16-2004. If the due date of the return was the 25th of the month (12-25-04), the interest start date should be 12-26-2004. Use the appropriate drop down arrows to select the correct month, day and year.

Enter the interest End Date. This is the month the tax or fee amount will be paid. Use the appropriate drop down arrows to select the correct month and year.
Please note: For certain tax programs, a return may be due on a date other than the last day of the month. When selecting the month of payment, the calculator assumes the payment is made on or before the regular due date for that month. For example, a payment made in August for a return due on July 15 will accrue only one month's interest, because the calculator defaults to August 15. If the payment is actually made after August 15th, an additional month's worth of interest will be due so the End Date should be set to 09 (September) rather than 08 (August).

Enter the Due Date. This is the due date of the return. Use the appropriate drop down arrows to select either the "End of the Month," "15th," or "25th."

Enter the Tax or Fee Amount. This is the amount of tax or fee due. This amount should be entered as a whole number, do not enter dollar signs, periods or commas. For example, for $300.00 enter "300".

Click "Calculate" to calculate interest.
Click "Reset" to clear all entries on the form.

The results on this page are estimates based on the information provided by you. The actual interest due may be re-calculated and adjusted on your billing notice. Interest rates are subject to change in July and January. The calculator will not work for the International Fuel Tax Agreement (IFTA) program or for taxpayers reporting $1,000,000 or more in tax or fee amount due.


Taxpayers requiring assistance are encouraged to contact our Customer Service Center at 1-800-400-7115 (CRS:711). Customer service representatives are available Monday through Friday from 8:00 a.m. to 5:00 p.m. (Pacific time), except state holidays.

TDD service for the hearing impaired:

From TDD phones: 1-800-735-2929
From voice phones: 1-800-735-2922