Accurate Monthly Payment Calculation
Computes exact principal and interest payments using standard banking amortization formulas with full mathematical transparency.
Loan Calculator
Free loan calculator to find fixed monthly repayments, total interest, and amortization schedules for personal, auto, or business loans.
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Fixed-Rate Loan Amortization Formula
Last reviewed: September 2026Calculation Formula
M = P × [r(1 + r)^n] / [(1 + r)^n - 1] | Total Interest = (M × n) - P
Variables & Definitions
- M:
- Fixed monthly amortized payment
- P:
- Initial principal loan balance
- r:
- Monthly periodic interest rate (annual APR ÷ 12 ÷ 100)
- n:
- Total repayment periods in months (years × 12)
- Total Interest:
- Total cash paid over the loan term minus the original principal borrowed
Core Assumptions
- Fixed interest rate throughout the entire borrowing term.
- Interest is compounded monthly based on standard 30/360 or actual/365 banking conventions.
- Payments are made monthly at the end of each billing period.
- Extra principal payments are applied directly to reducing the outstanding principal balance.
Disclaimer: Calculations provided are estimates for financial planning purposes only and exclude potential lender origination fees, closing costs, credit insurance, and late payment penalties. Consult a certified financial advisor or loan officer for official disclosures.
Worked Example: $25,000 Auto or Personal Loan at 6.50% APR (5-Year Term)
A borrower takes out a $25,000 fixed installment loan with a 60-month (5-year) repayment schedule at 6.50% annual interest:
| Monthly Payment (P & I) | $489.15 per month |
| Total Principal Borrowed | $25,000.00 |
| Total Lifetime Interest | $4,349.00 |
| Total Lifetime Outflow | $29,349.00 |
💡 Adding an extra $50 per month directly to principal reduces total lifetime interest by $492 and pays off the loan 6 months ahead of schedule.
How to use Loan Calculator
- 1
Enter the loan amount (principal borrowed), annual interest rate (APR), and loan term in years or months.
- 2
Optionally enter an extra monthly payment amount to calculate how much interest you save and how many months you eliminate.
- 3
View your exact monthly amortized payment, total lifetime interest, and interactive month-by-month payoff schedule.
Why use this tool?
Extra Principal Savings Estimator
See how making extra monthly payments reduces your payoff date and saves thousands in compounding interest charges.
Complete Month-by-Month Amortization Schedule
Inspect the shifting ratio between principal and interest over time, watching your remaining loan balance drop to zero.
Continue your workflow
Evaluating competing quotes from multiple banks or dealers? Use the Compare 3 competing loan offers side by side.
Financing real estate with property taxes and homeowner insurance? Try the Calculate home mortgage payments with escrow.
Planning short-term electronics, appliance, or retail purchases? Use the Calculate retail and consumer installment plans.
Frequently asked questions
Can this calculator be used for auto, student, and personal loans?
Yes. Any fixed-rate installment loan with regular monthly payments (such as car financing, private student loans, or bank personal loans) uses the standard amortizing payment formula modeled by this tool.
How is monthly loan interest calculated by banks?
Each month, the bank divides your annual interest rate by 12 to find the monthly periodic rate, then multiplies it by your remaining loan balance. The rest of your fixed monthly payment goes directly toward reducing the principal balance.
Does paying extra every month really lower my total loan cost?
Yes. Every dollar paid beyond your required monthly payment is credited directly toward reducing the principal balance. Because subsequent interest is calculated on a smaller remaining balance, extra payments compound into significant interest savings and shorten the loan payoff date.
What is the difference between APR and simple interest rate?
The simple interest rate is the base percentage cost of borrowing the principal. The APR (Annual Percentage Rate) incorporates both the base interest rate and mandatory lender fees, such as origination charges or document fees, giving a more accurate representation of the true annual cost.
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