Mortgage Calculator

Most mortgage calculators stop at the monthly payment. This one keeps going: enter a loan amount, an annual rate and a term, and you get the full picture — monthly payment, total amount paid over the life of the loan, total interest, and how the principal-to-interest ratio shifts across all 360 payments. The amortisation schedule is downloadable as a CSV so you can import it straight into a spreadsheet.

Add an extra monthly payment and the tool recalculates the whole schedule, showing how many months you shed and how much interest you avoid. On the lead sample a $200/month overpayment on a 30-year loan saves more than five years and cuts the interest bill by a meaningful fraction of the original principal.

Worked Example: $320,000 at 6.75% for 30 Years

Press Load Sample and the tool loads a $320,000 loan at 6.75% annual rate over 30 years. The monthly payment is $2,076.05. Over 360 payments the total outlay is $747,378, of which $427,378 is interest — roughly 1.34× the original loan amount. The bar at the bottom of the result card shows the principal/interest split at a glance: 42.8% principal, 57.2% interest.

Click Show to open the amortisation table. Month 1 has interest $1,800.00 and principal $276.05; by month 180 the split has flipped to roughly $1,075 interest and $1,001 principal. Click CSV to download the full 360-row schedule.

The Formula Behind the Payment

The standard fixed-rate amortisation formula is:

M = P × r(1 + r)^n / ((1 + r)^n − 1)

where P is the principal, r is the monthly rate (annual rate ÷ 12), and n is the total number of payments (years × 12). At a 0% rate the formula reduces to P / n — the tool handles this edge case so no division-by-zero occurs. Each row in the amortisation table is computed iteratively: interest = remaining balance × r; principal = payment − interest; new balance = old balance − principal paid.

Precision Limits and What Is Not Included

• The last payment is adjusted. Due to floating-point rounding across hundreds of payments, the final balance rarely lands on exactly zero. The amortisation loop caps the final payment at the remaining balance plus one month’s interest, so the schedule always totals to the exact loan amount.
• Extra payments are applied entirely to principal.The scheduled payment covers that month’s interest first; the extra amount then reduces the balance directly, which is why a small overpayment has a disproportionately large effect early in the term.
• Term is capped at 50 years (600 months). Rates above 50% produce the validation message “Annual rate must be between 0% and 50%.”
• This tool calculates principal and interest only.Property taxes, homeowner’s insurance and PMI are not included — your actual monthly outlay will be higher.