How to Calculate Loan Payments (Formula & Examples)
For informational purposes only, not financial advice. Full disclaimer
Every fixed-rate installment loan — personal loans, auto loans, mortgages, student loans — uses the same payment formula. Once you learn it, you can calculate any loan payment, compare offers from different lenders, and understand exactly how much interest you will pay. A $20,000 personal loan at 8% for 5 years costs $406/month with $4,332 in total interest. Change the rate to 10%: $425/month, $5,496 interest. Those numbers matter.
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Try the Loan CalculatorWhen I took out my first business loan, I focused on the monthly payment and almost missed the origination fee buried in the fine print. That fee raised the effective APR by nearly a full percentage point. I learned to always read the full loan disclosure and calculate the true cost myself before signing anything.
Alex B.
The Universal Loan Payment Formula
M = P × [r(1+r)^n] / [(1+r)^n - 1]M = monthly payment, P = principal (loan amount), r = monthly interest rate (APR ÷ 12), n = number of total payments (years × 12). This formula calculates the fixed monthly payment that amortizes the loan completely — meaning both principal and interest are fully paid by the final payment.
Example Calculation
You take a $15,000 personal loan at 9% APR for 3 years.
- P = $15,000
- r = 9% / 12 = 0.75% = 0.0075
- n = 3 × 12 = 36 months
- Calculate (1 + 0.0075)^36 = 1.3086
- Numerator: 0.0075 × 1.3086 = 0.009815
- Denominator: 1.3086 - 1 = 0.3086
- M = $15,000 × (0.009815 / 0.3086) = $15,000 × 0.03181 = $477
Monthly payment: $477. Total paid: $477 × 36 = $17,172. Total interest: $17,172 - $15,000 = $2,172.
How Loan Term Affects Total Cost
On a $25,000 loan at 8%: a 3-year term means $783/month, $3,195 interest. A 5-year term: $507/month, $5,416 interest. A 7-year term: $390/month, $7,694 interest. The monthly payment drops $393 between the 3 and 7-year option, but total interest more than doubles. Longer terms always cost more in total.
Calculating Total Interest
Total Interest = (Monthly Payment × Number of Payments) - PrincipalThis is the simplest way to see the true borrowing cost. Multiply your monthly payment by the total number of payments and subtract the original loan amount. Everything above the principal is interest — the cost of borrowing money over time.
APR vs. Interest Rate
APR (Annual Percentage Rate) includes the interest rate PLUS fees and charges, expressed as a yearly rate. A loan with a 7% interest rate and a 2% origination fee might have an APR of 7.8%. When comparing loan offers, always use APR — it is the true cost of borrowing. The Truth in Lending Act requires lenders to disclose APR, making it your best apples-to-apples comparison tool.
Extra Payments: Saving on Interest
Extra payments reduce the principal faster, which means less interest accrues on subsequent payments. On a $25,000, 5-year loan at 8% ($507/month), adding $100/month saves $1,118 in interest and pays off the loan 10 months early. Adding $200/month saves $1,874 and cuts the term by 17 months. The interest savings come from the reduced balance generating less interest every single month.
When making extra payments, explicitly instruct your lender to apply the extra amount to principal, not to advance future payments. Some lenders will credit extra payments to the next month's payment, which does not reduce interest — it just means you have a paid-ahead status with no financial benefit.
Lenders love to advertise low monthly payments achieved through longer terms. A $30,000 loan at $350/month sounds affordable until you realize it is an 8-year term costing $13,600 in interest. The same loan at $570/month for 5 years costs only $4,200 in interest — saving $9,400.
Frequently Asked Questions
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Disclaimer: This article is for informational purposes only and does not constitute financial advice. Consult a qualified financial advisor for decisions about your specific situation.