How Loan Amortization Works
When you take out an installment loan — a mortgage, auto loan, student loan, or personal loan — your monthly payment is calculated to pay off both principal and interest in equal installments over the loan term. This is called amortization. Early in the loan, most of your payment goes toward interest. As the balance decreases, a growing share of each payment goes toward principal. On a 30-year mortgage at 7%, you don't reach 50/50 principal-to-interest split until about year 18.
Understanding amortization helps you see why extra principal payments early in a loan save so much more than the same payments made later. An extra $100 payment in month one reduces the principal on which all future interest is calculated — the savings compound forward through every remaining payment.
How Extra Payments Reduce Total Interest
Extra payments toward principal reduce the loan balance faster, which shrinks the interest charged each month, which means more of your regular payment goes to principal — a virtuous cycle. On a $200,000 auto or personal loan at 7% over 5 years, paying an extra $100/month can cut the payoff time by nearly a year and save thousands in interest. The impact is even larger on longer loans like mortgages. Use the calculator to see the exact impact of different extra payment amounts on your specific loan.
Comparing Loan Terms: Shorter vs. Longer
Shorter loan terms mean higher monthly payments but significantly less total interest paid. A $25,000 car loan at 6% over 48 months costs about $2,990 in total interest. The same loan over 72 months costs about $4,540 — 52% more in interest for the same car. The longer term lowers the monthly payment by about $170, but you pay nearly $1,600 more overall. When evaluating a loan, always calculate total cost, not just monthly payment. Monthly payment comparisons obscure the true cost of longer terms.
When to Refinance a Loan
Refinancing replaces your existing loan with a new one at a lower rate or different term. It makes sense when interest rates have fallen since you took out the loan, your credit score has improved enough to qualify for a better rate, or you want to adjust the term (shorter to save interest, longer to reduce cash flow pressure). The break-even point for refinancing is the closing costs divided by your monthly savings. If closing costs are $3,000 and you save $150/month, break-even is 20 months — worth it if you plan to keep the loan longer than that.
Types of Loans This Calculator Covers
This calculator works for any fixed-rate installment loan: personal loans, auto loans, student loans, home equity loans, and fixed-rate mortgages. For mortgages with property tax, insurance, and PMI, use the dedicated Mortgage Calculator, which breaks out all components of the total monthly payment. For student loans specifically, see the Student Loan Calculator, which models income-driven repayment options alongside standard amortization.