How to Calculate Loan and Mortgage Payments
A loan offer shows a monthly payment, but that number tells only part of the story. How much total interest will you pay over the life of the loan? How much can you save by making extra payments? How does a 15-year mortgage compare to a 30-year? A loan calculator answers all these questions.
How the calculator works
The ToolStand Loan and Mortgage Calculator uses the PMT formula to calculate monthly payments from principal, interest rate, and loan term. It generates a full amortization schedule showing each monthly payment broken into principal and interest. Over the life of a 30-year $300,000 mortgage at 6.5 percent, you will pay over $380,000 in interest alone — the schedule makes this painfully clear.
Early payoff simulation
The calculator lets you add extra monthly payments and shows how much interest you save and how many years you cut off the loan. Adding $200 per month to that $300,000 mortgage saves over $90,000 in interest and pays off the loan 7 years early. The amortization schedule recalculates instantly to show the new payoff date and remaining balance after each extra payment.
Paired with other financial tools
Before taking a loan, use the Compound Interest Calculator to understand opportunity cost — money spent on interest could have been invested. Use the Savings Calculator to model saving for a larger down payment instead of borrowing more. Use the Inflation Calculator to understand that fixed-rate mortgage payments become effectively cheaper over time as inflation erodes the value of each dollar.
Amortization front-loading — why most of your early payments go to interest
Amortization is designed so that interest is paid first. In the first year of a 30-year $300,000 mortgage at 6.5%, approximately 75% of each monthly payment goes to interest alone. Of the $1,896 monthly payment, roughly $1,625 is interest and only $271 reduces principal. Ten years in, the split is still about 60% interest, 40% principal. This front-loading means you pay the bulk of total loan interest in the first half of the term. The Loan Calculator visualizes this dynamic with a full amortization schedule. The practical takeaway: any extra principal payment early in the loan term has outsized impact because it skips years of future interest on that amount.
The monthly payment trap — why lower payments cost you more
Lenders often advertise the low monthly payment of a 30-year mortgage without showing the total cost. A $300,000 loan at 6.5% over 30 years costs $682,632 total — $382,632 in interest. The same loan over 15 years costs $469,296 total — only $169,296 in interest. You save $213,336 in interest by choosing the shorter term, but the monthly payment jumps from $1,896 to $2,607. The monthly payment trap is when borrowers optimize for the lowest possible monthly number, not the lowest total cost. Use the calculator to toggle between loan terms and watch the total interest column change. The right choice depends on your cash flow and whether you can invest the monthly difference at a higher return than your mortgage rate.
APR vs interest rate — what you actually pay
The interest rate on a loan is the cost of borrowing the principal. The Annual Percentage Rate (APR) includes the interest rate plus lender fees, points, and closing costs expressed as a yearly rate. APR is always higher than the interest rate when fees are involved. For a $300,000 loan at 6.0% with $5,000 in closing costs, the APR would be approximately 6.15%. Comparing APR between lenders gives you the true cost comparison, but APR assumes you keep the loan for its full term. For a mortgage you plan to refinance or sell within five years, upfront fees matter more than APR. The Loan Calculator works with interest rate (not APR), so factor fees in separately when comparing offers.
15-year vs 30-year mortgage — the opportunity cost question
Conventional wisdom says pick the 15-year mortgage to save on interest, but the math is more nuanced. A 30-year $300,000 mortgage at 6.5% costs $682,632 total. A 15-year at 5.8% (rates are typically lower for shorter terms) costs $469,296 total — a $213,336 saving. However, the 30-year payment is $711 lower per month. If you invest that $711 monthly in an S&P 500 index fund averaging 8% annually, the investment grows to approximately $481,000 after 30 years — more than the interest saved. The 15-year is the safer, guaranteed-return path. The 30-year-plus-investing path has higher expected returns but carries market risk and requires discipline. Run both scenarios in the Compound Interest Calculator to see the numbers for your specific rate difference.
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