How the loan payment formula works
This calculator uses the standard amortization formula: M = P × [r(1+r)^n] / [(1+r)^n − 1], where M is the monthly payment, P is the principal loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments. Each payment covers the month's interest on the remaining balance, with the rest reducing the principal. If the interest rate is zero, the monthly payment simply equals the principal divided by the number of months.
Understanding amortization
In an amortizing loan, the total monthly payment stays constant, but its composition shifts over time. Early payments are mostly interest; later payments are mostly principal repayment. This is why paying off a loan early can save substantial interest — you eliminate future periods where a large portion of each payment goes to interest. The total interest shown by this calculator represents the true cost of borrowing over the full loan term.
Mortgage vs. personal loan vs. auto loan
All three use standard amortization. The main differences are typical amounts, interest rates, and terms. Mortgages typically run 15–30 years with lower interest rates (secured by property). Auto loans are usually 3–7 years. Personal loans are often 2–7 years and carry higher rates since they're unsecured. Enter your specific numbers regardless of loan type — the formula is the same for all.
How to reduce total interest paid
The most effective ways to reduce total interest are: choose a shorter loan term (higher monthly payment, much less total interest), make extra principal payments when possible (reduces the balance faster), and shop for a lower interest rate. For example, a $250,000 mortgage at 6% over 30 years costs $289,595 in interest — but over 15 years at the same rate costs just $127,544, a savings of over $160,000.