Cernarus

Personal Loan APR Calculator with Extra Payments

This tool models personal loan amortization and the effect of extra payments. Use it to compare scheduled payments to scenarios where you add a recurring extra amount or make a one-time lump-sum principal reduction.

Results estimate payoff time, total interest paid, interest saved, and payments saved. Calculations use standard amortization formulas; lender fees or prepayment penalties are not included unless entered as adjustments to loan amount or APR.

Updated Nov 22, 2025

Standard amortization using periodic interest rate with a fixed additional payment applied each period until payoff.

Inputs

Results

Updates as you type

Scheduled periodic payment (no extra)

-$0.83

Periodic payment including recurring extra

-$0.83

Estimated payments until payoff (with extra)

—

Total interest paid (with extra)

—

Interest saved

—

Payments saved

—

OutputValueUnit
Scheduled periodic payment (no extra)-$0.83currency
Periodic payment including recurring extra-$0.83currency
Estimated payments until payoff (with extra)—payments
Total interest paid (with extra)—currency
Interest saved—currency
Payments saved—payments
Primary result-$0.83

Visualization

Methodology

Calculations use periodic compounding: periodic_rate = APR / payments_per_year and standard amortization algebra to compute payment amounts and remaining balances.

When estimating reduced term from higher payments or lump sums, the tool solves for the number of payments using logarithmic inversion of the amortization equation. Edge cases (zero or negative remaining balance) are handled conservatively; see accuracy notes below.

For traceability and testability we follow best practices for numerical stability and input validation inspired by standards for measurement and verification (NIST), quality management principles (ISO), and engineering numerics (IEEE). This calculator is informational only and not a regulatory disclosure.

Key takeaways

Use the recurring-extra method to see the impact of adding the same extra amount to every payment. Use the lump-sum method to model a one-time principal reduction at a specific payment number.

This calculator is a model based on standard amortization mathematics and references measurement and numerical best practices. It is not a substitute for official loan disclosures.

Further resources

Expert Q&A

Does this calculator compute the legally defined APR used in loan disclosures?

No. This tool computes interest accrual and amortization using APR as an annual nominal rate for periodic compounding. It does not include lender fees, points, insurance, or finance charge disclosures required by law. For legally binding APR disclosures consult your loan documents or your regulator.

Can I model biweekly payments or irregular extra payments?

Yes. Use Payments per year = 26 for biweekly schedules. For irregular or ad-hoc extras, model each event as a lump-sum at the appropriate payment number or use the recurring extra field for constant additional amounts.

What if my lender applies the lump sum before interest accrues for the period?

This calculator assumes extras are applied at the same schedule timing described in the scenario: recurring extras are applied each payment period and lump sums are applied immediately at the specified payment number. If your lender applies payments differently, results will vary.

How accurate are the 'payments saved' and 'interest saved' values?

Values are computed from closed-form amortization formulas and are accurate within rounding tolerance for the mathematical model. They do not account for rounding rules, day-count conventions, compounding differences, or lender-specific processing. See accuracy & standards below.

Sources & citations