A mortgage calculator turns three inputs — loan amount, interest rate, and term — into a monthly payment figure. Understanding how it works helps you make better decisions about down payments, loan terms, and refinancing.

The Core Formula

Most mortgages use fixed equal monthly payments calculated with the annuity formula:

M = P × [r(1+r)^n] / [(1+r)^n – 1]

Where M is the monthly payment, P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (years × 12).

Breaking Down a Payment

Each monthly payment is split between interest and principal repayment. In early years, most of the payment goes to interest. Over time, the balance shifts — more goes toward principal. This is called amortisation.

How Down Payment Affects Costs

A larger down payment reduces the loan principal, which lowers both the monthly payment and total interest paid over the life of the loan. Putting down 20% or more also typically eliminates the need for private mortgage insurance (PMI).

15-Year vs 30-Year Mortgages

30-year: Lower monthly payments but significantly more total interest. Better for cash flow flexibility
15-year: Higher monthly payments but builds equity faster and costs far less in total interest

On a $300,000 loan at 6.5%, a 30-year mortgage costs about $1,896/month with ~$382,600 in total interest. A 15-year mortgage costs $2,613/month but only ~$170,400 in total interest — a savings of over $212,000.

Fixed vs Variable Rates

Fixed-rate mortgages lock your interest rate for the entire term — predictable payments, no surprises. Adjustable-rate mortgages (ARMs) offer a lower initial rate that resets periodically. ARMs can save money if you plan to sell or refinance within the initial fixed period.

What Calculators Don't Include

Most basic calculators show principal and interest only. Your actual monthly housing cost also includes:

Property taxes (varies widely by location)
Homeowner's insurance
Private mortgage insurance (PMI) if down payment is below 20%
HOA fees for condos or planned communities

Try It Yourself

Use our Mortgage Calculator to model different scenarios — adjust loan amount, rate, and term to see how each factor impacts your monthly payment and total cost.

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