Loan Calculator

How the loan calculator works

This calculator uses the standard amortizing-loan formula that banks and lenders use for mortgages, car loans, and personal loans. You tell it three things — how much you are borrowing, the annual interest rate, and how many years you have to pay it back — and it returns the fixed monthly payment that will clear the debt exactly on schedule, along with the total interest you will pay over the life of the loan.

The formula behind it

A fully amortizing loan is repaid in equal monthly installments. Each payment covers the interest that accrued that month, and whatever is left over reduces the outstanding balance. The payment amount is calculated with this equation:

M = P × r × (1 + r)ⁿ ÷ [(1 + r)ⁿ − 1]

Here M is the monthly payment, P is the principal (the amount borrowed), r is the monthly interest rate (the annual rate divided by twelve), and n is the total number of monthly payments (years multiplied by twelve). When the interest rate is zero, the formula simply becomes the principal divided by the number of months.

Why early payments are mostly interest

One thing many borrowers are surprised by is how little of an early payment actually reduces the balance. At the start of a loan the outstanding principal is large, so the interest portion of each payment is high and the principal portion is small. As the balance falls, the interest shrinks and more of every payment goes toward the principal. This is why the amortization table above shows the balance dropping slowly at first and then accelerating toward the end. It is also why making extra payments early in a loan saves so much interest — every extra dollar reduces the balance that future interest is charged on.

How to use the results

The monthly payment figure tells you what you need to budget each month. The total interest figure is arguably more important: it is the true cost of borrowing, separate from the amount you actually received. Comparing total interest across different terms is eye-opening. A longer term lowers the monthly payment but increases total interest, sometimes dramatically, because you are borrowing the money for longer. A shorter term does the opposite — higher monthly payments but far less interest overall.

Try adjusting the term to see this trade-off in action. Many people find a middle ground that keeps the monthly payment comfortable without paying tens of thousands in unnecessary interest. You can also test different interest rates to understand how much a better rate is worth, which is useful when shopping lenders or deciding whether to refinance.

Tips for borrowing smart

• Always compare the total interest, not just the monthly payment, when choosing between loan offers.
• Even a small rate difference compounds into large savings on big, long loans like mortgages.
• If your loan allows penalty-free extra payments, paying a little more each month can shorten the term significantly.
• Keep an emergency buffer — a slightly longer term with lower payments can be safer than an aggressive one you cannot sustain.