Loan Calculator

How the Loan Calculator Works — and What Your Monthly Payment Really Means

When you take out a loan — for a car, a home, a student degree, or anything else — the lender tells you a monthly payment figure, and most people accept it without digging deeper. But that number is only part of the story. Behind it sits a formula that determines how much you borrow, how long you'll be paying, what the interest actually costs you, and how much of your hard-earned money goes to the bank versus how much chips away at the principal. Our loan calculator lays all of this out: enter your loan amount, interest rate, and term, and you'll see the monthly payment, the total interest paid over the life of the loan, and the total amount you'll hand over by the end.

The formula behind it

The monthly payment on a standard amortizing loan comes from a formula: M = P × [ r(1 + r)^n ] ÷ [ (1 + r)^n − 1 ]. Here, P is the principal (the amount borrowed), r is the monthly interest rate (the annual rate divided by 12), and n is the total number of monthly payments. For example, borrowing $20,000 at 6% annual interest over 5 years means P = 20,000, r = 0.06 ÷ 12 = 0.005, and n = 5 × 12 = 60. Plugging those in gives a monthly payment of about $386.66. It looks intimidating on paper, but the underlying concept is straightforward: the formula balances the loan to zero over the term by accounting for the fact that each payment reduces the principal slightly, which reduces the interest owed next month.

What “amortization” means

Amortization is the schedule that shows how each payment is split between interest and principal. In the early months of a loan, most of your payment goes to interest — because the outstanding balance is still large. As the loan progresses, the balance shrinks, so less interest accrues, and a bigger chunk of your payment chips away at the principal. By the final months, almost all of your payment is principal. This is why paying even a little extra early in the loan can save you a surprising amount over time: you reduce the principal faster, which reduces the interest charged on every subsequent month. On a 30-year mortgage, the first payment might be over 80% interest; the last payment is nearly all principal.

The three levers you control

Every loan has three variables you can adjust before signing — or refinance later. The principal is how much you borrow. Borrowing less obviously means paying less overall, but it might mean a bigger down payment or a cheaper purchase. The interest rate is what the lender charges you to borrow. Even a small difference matters: on a $250,000 mortgage over 30 years, the difference between 6% and 7% is roughly $60,000 in extra interest. The term is how long you'll take to repay. A longer term means a lower monthly payment, but you'll pay much more interest overall. A shorter term means higher monthly payments but dramatically less interest. There's no universally “right” answer — it depends on your cash flow, your goals, and how much you value paying off the debt sooner.

Limitations to keep in mind

This calculator assumes a fixed interest rate and standard amortization, which is the most common type of loan but not the only one. Variable-rate loans adjust their rate over time, making future payments uncertain. Interest-only loans let you pay just the interest for a period, after which payments jump sharply. Some loans include fees — origination fees, closing costs, insurance, or prepayment penalties — that this calculator doesn't account for. It also doesn't factor in tax deductions, which can significantly reduce the effective cost of a mortgage for many borrowers. Use this calculator to compare loan offers and understand the true cost of borrowing, but always read the full loan agreement and consider consulting a financial advisor before making a major decision.