Loan Calculator — Monthly Payment & Total Interest

A loan calculator computes your monthly payment, total interest paid, and total cost for any fixed-rate installment loan — including personal loans, auto loans, student loans, and small business loans. It uses the standard amortization formula recognized by the Consumer Financial Protection Bureau (CFPB) and required by U.S. law for APR disclosure on all consumer credit products.

In a fixed-rate installment loan, you borrow a lump sum (the principal), agree to a fixed interest rate and loan term, and repay through equal monthly payments that fully retire the debt by the end of the term. Each payment is split between interest (the lender's fee for providing the loan) and principal reduction (paying down the balance). Because interest is charged on the remaining balance, the split shifts over time — early payments are mostly interest, later payments are mostly principal.

Knowing your true loan cost before borrowing is essential for informed financial decisions. A seemingly affordable monthly payment can hide a very high total cost when stretched over a long term. This calculator makes the full picture visible — monthly payment, total interest, and total cost — so you can compare loan offers accurately and choose the option that minimizes your total borrowing cost.

Example 1 — Personal loan: $10,000 at 9% APR over 36 months. r = 0.09/12 = 0.0075, n = 36 M = 10,000 × [0.0075 × (1.0075)^36] / [(1.0075)^36 − 1] = $317.99/month Total paid: $11,448 | Total interest: $1,448

Example 2 — Auto loan comparison (36 vs 60 months): $22,000 at 6.5% APR. 36 months: $674/month | Total interest: $2,278 60 months: $430/month | Total interest: $3,796 Longer term saves $244/month but costs $1,518 extra in total interest.

Example 3 — Impact of interest rate (same loan, different credit score): $15,000 over 48 months. 6% APR: $352/month | Total interest: $1,883 15% APR: $417/month | Total interest: $5,010 Good vs poor credit costs an extra $3,127 on the same loan.

Consumer loans are the second most significant financial commitment most people make after mortgages. Americans collectively carry over $1.6 trillion in auto loan debt and nearly $250 billion in personal loan balances. Understanding the true cost of a loan — not just the monthly payment — is the most important step in responsible borrowing.

Interest rate differences have a compounding effect on total loan cost. The difference between 6% and 12% APR on a $20,000 auto loan over 60 months is $3,200 in extra interest paid — nearly 16% of the loan principal paid purely in additional borrowing costs for the same car. Your credit score is the primary determinant of your interest rate, making credit health one of the most financially impactful personal finance activities: improving your credit score from 'Fair' to 'Good' can save thousands of dollars on any significant loan.

Loan term is equally consequential. Dealers and lenders often advertise monthly payment rather than total cost precisely because it conceals how expensive long-term financing is. A $35,000 car financed at 7% over 72 months has a monthly payment of $533 — but you will pay $38,400 over the life of the loan: $3,400 more than the car's price. This calculator puts total cost front and center so you can make the decision with full information.

Debt-to-income ratio (DTI) is the metric lenders use to evaluate your loan application. Total monthly debt payments divided by gross monthly income should stay below 36% for most lenders — ideally below 28% excluding housing costs. If you are approaching these thresholds, taking on additional loan debt even at a favorable rate may jeopardize your ability to qualify for future credit, including a mortgage. Use this calculator alongside a budget review to ensure that a new loan payment keeps you comfortably within sustainable DTI limits.

This calculator computes principal and interest only. For auto loans, the actual monthly obligation may include gap insurance, extended warranty costs, or taxes and fees financed into the loan amount. For personal loans, origination fees (typically 1–8% of the loan amount, deducted from the disbursed amount) effectively raise the real APR above the stated rate — confirm whether quoted rates include or exclude origination fees.

This calculator assumes a fixed interest rate for the entire loan term. Variable-rate personal loans or HELOCs (home equity lines of credit) have rates that reset periodically — the amortization formula does not apply to those products in the same way. Similarly, balloon payment loans — which have small monthly payments followed by a large lump-sum payment at maturity — cannot be accurately modeled with standard amortization.

For student loans, this calculator works for fixed-rate private loans but not for federal income-driven repayment plans (IDR, PAYE, SAVE), which base monthly payments on income rather than loan balance and include forgiveness provisions after 10–25 years. Always consult your loan servicer or a certified financial counselor for student loan repayment strategy.