The calculation you need, step by step

Late-payment interest calculator

Estimate simple interest from the principal, dates and annual rate, with separate periods before and after service of a complaint.

Reference results

Check the entries, then select Calculate.

The 5%, 6% and 12% annual rates have different conditions

Article 379 of the Korean Civil Act sets a statutory annual rate of 5% for interest-bearing claims unless another law or agreement provides otherwise. Article 54 of the Commercial Act sets an annual rate of 6% for debts arising from commercial transactions. Neither rate is a universal charge on all money transactions. First establish the nature of the claim and when default begins.

The rate prescribed by Presidential Decree under the Act on Special Cases Concerning Expedition, etc. of Legal Proceedings is 12% per year. In principle, it applies from the day after service of a complaint seeking payment, but exceptions include a period during which it is reasonable to dispute the existence or scope of the obligation. Service alone does not always make the 12% rate applicable.

How the periods are divided

Both the first day on which interest actually accrues and the last day are included. If a service date is entered, the first rate applies through that date and the second rate applies from the following day. If service predates the first day, the second rate applies throughout. If no service date is entered, only the first rate applies. Enter dates as calendar dates in Korea.

The formula is principal × annual rate ÷ 100 × days ÷ 365. Interest for each period is added without compounding, and the displayed result is rounded to the nearest won. This simplified method uses 365 even in leap years, so it may differ from an agreement or judgment using annual periods or a 366-day basis.

Example and limitations

For KRW 10 million at 5% per year over 365 days, simple interest is KRW 500,000. If principal was repaid partway through or the claim increased, check separate amounts and periods. This tool does not determine allocation of repayments, transitional rules for rate changes, whether an agreed rate is lawful, or the legally correct start date. It does not automatically apply old rates such as 15% or 20% to historical cases.

Calculation basis and official sources

This material is based on sources checked on November 7, 2026 (KST). Check subsequent amendments and transitional provisions for your particular case separately.