Personal Loan Amortization Calculator with Extra Payments
This advanced amortization tool models how extra principal payments affect a fixed-rate personal loan. It supports standard amortization, recurring extra principal per payment, and a single lump-sum paydown applied at a chosen payment number.
Use the calculator to estimate new payoff timing, approximate months saved and total interest under each scenario. Results use closed-form amortization formulas; the app includes guidance for edge cases and accuracy limits.
Models a fixed recurring extra amount applied each payment period to principal. Calculates new payoff schedule using closed-form approximation for number of payments and total interest.
Inputs
Results
Payment with extra principal
$188.71
Estimated payments to payoff
60
Total interest (with extra)
$1,322.74
Approximate months saved
-0
| Output | Value | Unit |
|---|---|---|
| Payment with extra principal | $188.71 | currency |
| Estimated payments to payoff | 60 | payments |
| Total interest (with extra) | $1,322.74 | currency |
| Approximate months saved | -0 | months |
Visualization
Methodology
Periodic rate is derived from the nominal annual rate divided by the number of payments per year. Regular periodic payment uses the standard fixed-rate annuity formula.
Recurring extra payments are modeled by adding the extra amount to the regular payment and using the logarithmic closed-form solution to estimate how many periods are required to amortize the balance. A single lump-sum paydown uses the closed-form balance formula to compute the outstanding principal immediately before the lump and then recalculates remaining payoff using the annuity solution.
These closed-form formulas are widely used in financial calculations. Where the periodic rate is zero, the tool falls back to simple division (principal divided by number of payments).
Worked examples
Example: $10,000 loan, 5% APR, 5 years, monthly. Regular monthly payment is computed by the annuity formula. Adding $100 recurring reduces number of payments and total interest; the tool estimates months saved and revised total interest.
Example: Same loan with a $2,000 lump payment on payment 12. The tool computes the outstanding balance at month 12, subtracts the lump, and estimates the new remaining term and interest.
Key takeaways
This advanced calculator provides fast, closed-form estimates for how extra payments affect payoff timing and interest. Use recurring-extra mode for steady additional principal and lump mode for one-time paydowns.
For full legal, tax, or loan-servicer-specific calculations, obtain official payoff quotes from your lender. This tool is an estimator and includes accuracy caveats and guidance consistent with best practices for financial calculators.
Further resources
Expert Q&A
Does the calculator consider fees, prepayment penalties or changing rates?
No. This tool models only fixed-rate, fixed-term loans without additional fees or prepayment penalties. Results do not include third-party fees, escrow, insurance, taxes, or variable-rate adjustments.
Is the APR the same as the periodic interest rate?
No. APR is an annual percentage rate; the calculator converts APR to a periodic rate by dividing by the number of payments per year. For loans with financed fees or compounding definitions differing from simple periodic compounding, consult loan documents or a licensed professional.
What happens if I enter zero interest rate?
If the periodic interest rate is zero, the calculator uses straightforward division: remaining principal divided by payments remaining. Some closed-form logarithmic formulas are undefined at zero; the tool handles that as a special case.
How accurate are the months saved and remaining-payments estimates?
Estimates use closed-form amortization formulas and assume every payment is applied on schedule and exactly as specified. They do not simulate irregular timing, partial periods, or mid-period interest accrual; for precise schedules or lender-specific rules, request an amortization schedule from your servicer.
Sources & citations
- NIST - Framework and Best Practices for Software Measurement and Validation — https://www.nist.gov
- ISO - Standards on financial services and software quality — https://www.iso.org
- IEEE - Recommended practices for software and systems engineering — https://standards.ieee.org
- OSHA - Guidance on trustworthy documentation and safety (organizational standards) — https://www.osha.gov