Equal installments, moving interest
An installment payment is P×r×(1+r)^n / ((1+r)^n−1), where r is the monthly rate and n is the number of monthly payments. A zero-rate loan is the special case: principal divided by months. The level payment covers interest accrued since the prior due date and then reduces the balance.
The default payment in detail
For $25,000 at 6.5% over 60 months, the payment is $489.15. Paying that amount 60 times totals $29,349.22, so the modeled interest is $4,349.22. In the first month, interest is about $135.42; the remainder reduces principal.
What the formula holds fixed
The model assumes a fixed rate, no skipped installments, and one payment each month. It treats the amount entered as the balance that actually earns interest. A lender can round each monthly interest charge differently, so a statement may differ by small cents even when the rate and term match.
Term is a cost decision
A smaller payment does not necessarily make borrowing cheaper. Extending this example to seven years lowers the monthly strain but adds months in which interest can accrue. Compare total repayment beside the payment amount before choosing a term.
Charges outside an installment
Origination charges, late fees, prepayment penalties, insurance products, and taxes are absent. This page is not a cash-to-close estimate and it does not model a variable-rate note. Add compulsory charges separately when deciding whether an offer fits a budget.
A useful offer comparison
Use this calculation for a plain fixed installment loan after the financed balance is known. Keep the quote date, stated rate, term, and payment frequency with the result; those are the facts needed to compare two offers that have similar monthly payments.
A term comparison worth making
At the same $25,000 and 6.5%, changing only the term changes both the required payment and the number of interest periods. Treat the term as a separate decision from the amount borrowed: the lowest installment can be the most expensive total commitment. A payment calendar should also match the contract; biweekly or irregular-payment arrangements need their own schedule.
Keep the quoted payment frequency visible
If a contract quotes a biweekly payment, converting it to a monthly-looking number can obscure that 26 half-payments are made in a year. Match the calculation period to the document before comparing totals.
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It is an educational estimate, not financial, tax, or legal advice.