Loan Interest Calculator

The Loan Interest Calculator is a comprehensive financial planning tool that computes monthly repayment amounts, total interest costs, and overall payment obligations across three widely used repayment structures: equal payment (annuity), equal principal, and bullet (interest-only) repayment. Understanding loan interest mechanics is essential for anyone considering a mortgage, car loan, personal loan, or business financing. In South Korea, the Interest Limitation Act caps maximum lending rates, while the Fair Transactions in Franchise Business Act and related banking regulations require lenders to clearly disclose repayment terms. This calculator allows borrowers to compare different repayment schedules side by side, visualize how principal and interest proportions shift over time through an interactive chart, and make data-driven decisions about which loan structure best fits their cash flow. The tool also integrates real-time Bank of Korea (BOK) base rate data, providing users with the current benchmark rate that influences all consumer and commercial lending rates in the Korean financial market. Whether you are a first-time homebuyer evaluating a 30-year mortgage or a small business owner considering a short-term operating loan, this calculator gives you complete transparency into the true cost of borrowing.

Step 1: Choose your repayment method by selecting one of the three tabs at the top — Equal Payment, Equal Principal, or Bullet Repayment. Each method distributes principal and interest differently over the loan term. Step 2: Enter the total loan amount in Korean Won. For example, enter 100000000 for a 100 million KRW mortgage. The field supports comma-separated formatting for easy reading. Step 3: Input the annual interest rate as a percentage. You can manually type a rate or click the displayed BOK base rate to auto-fill the current benchmark rate as a starting point. Most consumer loans carry a spread above the base rate. Step 4: Specify the loan term in months. Common terms include 120 months (10 years), 240 months (20 years), or 360 months (30 years) for mortgages, and 12-60 months for personal or auto loans. Step 5: Review the results panel showing your monthly payment amount, total interest over the life of the loan, and the combined total payment (principal + interest). The interactive chart below breaks down each monthly payment into its principal and interest components, clearly illustrating how these proportions change over time. For equal payment loans, early payments are interest-heavy, while later payments are principal-heavy.

Equal Payment (Annuity) Method: M = P × r(1+r)^n / ((1+r)^n - 1) Where M = monthly payment, P = loan principal, r = monthly interest rate (annual rate ÷ 12), n = total number of months. Example: 100M KRW at 4% for 30 years → r = 0.04/12 = 0.003333, n = 360 → M ≈ 477,415 KRW. Total payment = 171.87M KRW, total interest = 71.87M KRW. Equal Principal Method: Monthly principal = P / n (fixed each month) Monthly interest = Remaining balance × r Monthly payment = (P/n) + Remaining balance × r The payment decreases each month as the remaining balance shrinks. First month is highest, last month is lowest. Example: 100M KRW at 4% for 30 years → Monthly principal = 277,778 KRW. First month payment = 277,778 + 333,333 = 611,111 KRW. Last month ≈ 278,704 KRW. Total interest = 60.17M KRW. Bullet Repayment (Interest-Only): Monthly payment = P × r (interest only, every month) Final month = P + (P × r) (full principal + last month's interest) Example: 100M KRW at 4% for 30 years → Monthly interest = 333,333 KRW for 359 months. Final month = 100,333,333 KRW. Total interest = 120M KRW. Comparison Summary: For the same 100M KRW at 4% over 30 years, total interest is approximately 60M (equal principal) < 72M (equal payment) < 120M (bullet). The choice depends on your cash flow constraints and total cost tolerance.

Choosing the right repayment method is one of the most impactful financial decisions you can make. Here are detailed strategies and common pitfalls to consider: 1. Equal Payment is ideal for budgeting because the monthly amount never changes. This predictability makes it the most popular choice for long-term mortgages. However, in the early years, most of your payment goes toward interest rather than reducing principal, which means equity builds slowly. 2. Equal Principal results in the lowest total interest cost among the three methods. The trade-off is significantly higher initial payments — often 30-40% more than equal payment in the first month. This method is well-suited for borrowers who expect rising income or who want to minimize lifetime interest expense. 3. Bullet Repayment carries the highest total interest cost because the principal never decreases during the loan term. It is typically used for short-term bridge loans, business financing where cash flow is seasonal, or when you expect to refinance or sell an asset before maturity. Avoid using bullet repayment for long-term loans unless you have a specific exit strategy. 4. Early repayment (prepayment) can dramatically reduce total interest. Even small additional principal payments each month can shave years off your loan. However, check your loan contract for prepayment penalties — Korean regulations limit penalty rates, but they can still apply within the first 3 years. 5. The BOK base rate directly influences variable-rate loans. When the base rate rises, your monthly payment increases. Consider fixing your rate when rates are historically low, and opting for variable rates when rates are high and expected to decrease. 6. Always compare the Annual Percentage Rate (APR) rather than the nominal rate, as APR includes fees and charges that affect the true cost of borrowing.