The 15 vs 30 Year Mortgage Calculator is built around the same formulas your lender uses. Below is the math, in plain language, so you understand exactly what the numbers mean.

The core mortgage formula

Every amortizing mortgage payment follows the same equation: M = P × [r(1+r)n] / [(1+r)n − 1], where P is the principal loan amount, r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments. This formula produces a level payment that pays off the loan exactly over its term — front-loaded with interest, back-loaded with principal.

Why amortization matters

In the first year of a 30-year mortgage at 6.875%, roughly 86% of each payment goes to interest. By year 20, that flips — most of your payment is now retiring principal. Understanding this curve is the key to making smart decisions about extra payments, refinancing, and how long to stay in a home.

What this calculator includes

The 15-year saves on interest but locks in a higher payment. The 30-year frees cash flow — and if you invest the difference at a higher return than your rate, you might come out ahead. Run the numbers. Every input updates the result in real time — no submit button, no page reload, no data sent anywhere.