If your company invoices less than six million euros per year, when calculating your installment payments of Corporate Income Tax (CIT), you must apply by default the "installment method" and pay 18% of the tax due from the last fiscal year whose filing deadline has expired. Thus, if your company's fiscal year coincides with the calendar year:

• In the April advance payment, you must use the tax due from the CIT of 2024 as a reference.
• However, in the advance payments of October and December, you must use the one from the CIT of 2025.

However, if you anticipate that your profits in 2026 are going to be lower, during the month of February you can choose to calculate the advance payments according to the "income-based system", in which case you can calculate the installment payments based on the accumulated profits of the current fiscal year. This will prevent the installment method from forcing you to make excessively high advance payments and then, in your tax return, having to request a refund.