Overdraft (Revolving Credit Line) Account Interest Calculator

Enter the average balance you actually drew on a Korean overdraft/revolving credit line account (마이너스통장), the annual interest rate, and the number of days used to calculate the expected total interest and daily interest for the period. The annual rate is always prorated on a fixed 365-day-per-year basis, so the same inputs give the same result regardless of when you calculate (leap years included).

Average balance used (actual outstanding average, KRW)

Annual interest rate (%)

Days used

Overdraft credit limit (optional, KRW)

Enter the average balance used, the annual interest rate, and the number of days used to calculate the expected interest.

Expected interest

Total expected interest for the period

Daily interest (per day)

Monthly-equivalent interest (30-day basis, reference)

Reference table of expected interest by period (same balance and rate)

Period	Expected interest

This is a reference estimate only. Interest on a Korean overdraft/revolving credit line account (마이너스통장) accrues only on the amount actually drawn, and banks calculate it daily on the actual outstanding balance, which changes every day. This calculator assumes the average balance you enter stays constant for the entire period, and always prorates the annual rate using a fixed 365-day-per-year day-count regardless of the real calendar or leap years, so the same inputs always produce the same result no matter when you run the calculation. Actual loan balances fluctuate daily and banks may use different day-count conventions (some apply 366 days in leap years), so check your bank's app or product disclosure for the exact interest. This is not financial advice.

About overdraft (negative balance) accounts

An overdraft account (마이너스통장) is a Korean revolving credit line attached to a checking account that lets you spend below a zero balance up to an approved limit, charging interest only on the amount actually overdrawn and only for the days it stays overdrawn — unlike an installment loan where interest accrues on the full principal from day one. This tool estimates the interest charged on your overdraft usage.

How the interest is calculated

Interest is calculated daily on the outstanding overdrawn balance using the formula: overdrawn amount × annual interest rate × days used ÷ 365, then summed for the billing period if the balance changes over time. Because interest only applies to the portion actually drawn and only for the days it's outstanding, paying down the balance quickly — even temporarily — meaningfully reduces the total interest compared to carrying the full limit for the entire period.

Frequently asked questions

How is an overdraft account different from a regular loan?: A regular installment loan disburses a fixed principal upfront and charges interest on the full (or amortizing) balance regardless of whether you've spent it, while an overdraft account only charges interest on the specific amount you've actually drawn below zero and only for the days that amount stays outstanding — you can repay and redraw freely within the limit without reapplying.
Why does paying down the balance early save money?: Since interest accrues daily only on the currently outstanding overdrawn amount, reducing your balance even for part of the billing period lowers the base the daily interest formula is applied to for those days — so intermittent or partial repayment before the due date directly cuts your total interest cost compared to leaving the full amount drawn the whole time.
Is the overdraft interest rate fixed or variable?: Overdraft account rates are typically variable, tied to a reference rate plus a margin set by the bank based on your credit profile, and can change over the life of the account — so the rate you enter into this calculator should reflect your current applicable rate, which you should verify with your bank rather than assuming it stays constant.