Calculation rules

Interest will be calculated by applying to the amount of the undue deduction the default interest rate in force in each of the years between the due date of the declaration period of the year in which the undue deduction was made (or, where appropriate, from the day after the date on which the refund was obtained) and the date on which the declaration corresponding to the 2025 year is submitted.

The sum of the late payment interest corresponding to each of these years will determine the total amount of late payment interest corresponding to the undue deduction.

For these purposes, three periods can be distinguished when determining late payment interest:

• Initial period

It will include the number of days elapsed from the day after the end of the declaration period corresponding to the year in which the deduction that is now being restored was made (or, where applicable, from the day after the date on which the refund was obtained) and December 31 of said year.

The determination of the amount of late payment interest for this period can be carried out using the following calculation formula:

Interest on initial period delay = Deduction amount x (interest rate ÷ 100) x (period (number of days) ÷ 365 or 366)

The interest rate, expressed as a percentage of 100, will be the late payment interest rate in force in the fiscal year to which the initial period corresponds.

• Intermediate period

It will cover each of the full years following the initial period, up to December 31, 2025.

The determination of late payment interest corresponding to each of the calendar years included in this period can be carried out using the following formula:

Interest on arrears for each year = Deduction amount x (interest rate ÷ 100)

The interest rate, expressed as a percentage of 100, will be the late payment interest rate in effect in each of the years comprising this period.

However, since the same late payment interest rate (11%) was in force in 1994, 1995 and 1996, the interest corresponding to those years that form part of the interim period may be determined globally by multiplying the aforementioned late payment interest rate by the number of said years that make up the aforementioned period. This same rule may be applied in the years 1999 and 2000 for which the same rate of late payment interest was in force (5.5 percent); for the years 2002 and 2003 in which the same rate of late payment interest was in force (5.5 percent); for the years 2005 and 2006, in which the same late payment interest rate was in force (5%) and for the years 2008 and 2009 (until 31 March 2009), in which the late payment interest rate was 7%. From April 1, 2009 to December 31, 2014, the late payment interest rate is 5%. The late payment interest established for the year 2015 was 4.375 percent; for 2016, 2017, 2018, 2019, 2020, 2021 and 2022 it was 3.75 percent, and for the years 2023, 2024 and 2025 it was 4.0625 percent. 

• Final period 

It is the period between January 1, 2026 and the date of submission of the declaration for the 2025 fiscal year (June 30, 2026).

The determination of the interest corresponding to this period can be carried out using the following formula:

Late payment interest final period = Deduction amount x (4.0625 ÷ 100) x (T24 ÷ 365)

T24 represents the number of days of the delay period in the year 2026, that is, those that have elapsed between January 1 and the date of submission of the declaration for the year 2025.

Note: For 2026, the default interest rate of 4.0625 percent applies until the General State Budget Law for 2026 is approved and enters into force, which may establish a new interest rate.