Simple Loan Calculator With Amortization

Early loan payments feel unfair because most of your money goes to interest, not reducing what you owe. This happens because interest is calculated on your remaining balance — when you owe $250,000, even a 6.5% rate costs $1,354 in interest the first month alone.

The amortization formula ensures your payment stays the same each month, but the split between principal and interest shifts over time. As your balance drops, less goes to interest and more chips away at what you actually borrowed. By year 20 of a 30-year loan, the split finally tips in favor of principal.

This tool assumes a fixed interest rate and equal monthly payments — the standard for most consumer loans. Variable-rate loans or interest-only payments require different calculations since the payment amount or interest rate changes during the loan term.

Use this calculator when you have a fixed-rate loan with equal monthly payments — the standard for mortgages, auto loans, and personal loans. It's perfect for comparing loan terms, determining affordability, or understanding how much interest you'll pay over time.

Don't use this for variable-rate loans, lines of credit, or credit cards where the rate or payment amount changes. Those require different models. It also doesn't apply to interest-only loans where you're not paying down principal, or balloon loans where you owe a large sum at the end.

The biggest mistake is focusing only on monthly payment without considering total interest cost. A longer loan term lowers the monthly payment but dramatically increases total interest — stretching a $250,000 loan from 15 to 30 years saves $845 monthly but costs an extra $203,000 in interest over the life of the loan.

Many borrowers also forget that early payments are mostly interest, creating unrealistic expectations about principal reduction. After one year of $1,580 payments on our example loan, you've paid $18,960 but reduced the balance by only $3,840 — the rest went to interest.

Using gross income instead of take-home pay when determining affordability overstates what you can handle. Lenders may approve you based on gross income, but your actual budget depends on what hits your bank account after taxes, which is typically 20-35% less than gross pay.

The standard amortization formula is M = P × [r(1+r)^n] / [(1+r)^n - 1], where M is monthly payment, P is principal, r is monthly interest rate (annual rate ÷ 12), and n is total number of payments. This formula ensures the loan balance reaches exactly zero after the final payment.

For a $250,000 loan at 6.5% for 30 years: monthly rate = 0.065 ÷ 12 = 0.00542, number of payments = 30 × 12 = 360. Plugging into the formula gives M = $1,580. Over 360 payments, you'll pay $568,800 total — $318,800 more than you borrowed.

The edge case is zero interest, where the formula breaks down because you're dividing by zero. For zero-interest loans, the calculation becomes simply: monthly payment = loan amount ÷ number of months. Most promotional 0% financing falls into this category.

Lenders quote APR (annual percentage rate) which includes fees, but this calculator uses the note rate — the actual interest rate on your loan agreement. For most loans the difference is small, but on mortgages with high closing costs, APR can be 0.25-0.5% higher than the note rate, making the real monthly payment slightly different from what this calculator shows.