Personal Loan Interest Calculator with Bi-Weekly Payments
This calculator computes bi-weekly (or any periodic) loan payments from a nominal annual percentage rate (APR), and shows total interest and total cost over the loan term. It also produces an equivalent monthly payment and the interest difference between the monthly schedule and the chosen periodic schedule.
Defaults assume standard amortizing loans with fixed nominal APR and equal periodic payments. Change the payment frequency to model weekly, semi-monthly, monthly, or custom schedules.
Inputs
Results
Payment per period
$138.53
Number of payments
130
Total amount paid (principal + interest)
$18,009.24
Total interest paid
$3,009.24
Equivalent monthly payment
$300.57
Total interest if monthly payments
$3,034.15
Interest difference (monthly minus bi-weekly)
$24.91
| Output | Value | Unit |
|---|---|---|
| Payment per period | $138.53 | — |
| Number of payments | 130 | — |
| Total amount paid (principal + interest) | $18,009.24 | — |
| Total interest paid | $3,009.24 | — |
| Equivalent monthly payment | $300.57 | — |
| Total interest if monthly payments | $3,034.15 | — |
| Interest difference (monthly minus bi-weekly) | $24.91 | — |
Visualization
Methodology
The tool uses the standard annuity formula for amortizing loans: periodic rate = APR / payments_per_year; payment = P * r / (1 - (1 + r)^(-n)) where n = term_years * payments_per_year. This assumes interest compounds at the payment frequency implied by the chosen payments_per_year.
Assumptions, rounding, and software quality follow best-practice guidance for numerical calculations and testing. Implementation and verification should observe guidance from ISO software quality standards (https://www.iso.org), IEEE floating-point and software engineering recommendations (https://www.ieee.org), and NIST guidance for algorithm validation (https://www.nist.gov).
Further resources
Expert Q&A
Why are bi-weekly payments often advertised as saving interest?
Bi-weekly schedules make more frequent payments, which reduces principal slightly faster between interest accrual periods compared to monthly payments when both use the same APR and compounding frequency. Savings depend on exact timing and whether the lender applies bi-weekly compounding or converts payments to an equivalent monthly schedule.
Is the APR converted or the effective interest rate used?
This calculator uses the nominal APR and assumes compounding at the payment frequency you select. It does not convert to APRs that incorporate fees or use effective annual rate unless you enter an adjusted APR that includes those costs.
How accurate are the results?
Results use standard amortization formulas with double-precision arithmetic assumptions. Minor rounding differences may occur compared to lender statements due to daily interest conventions, day-count rules, or lender-specific posting rules. Verify final payoff figures with your loan servicer.
Can I model extra payments or variable rates?
This version models fixed-rate, fixed-payment amortization only. For extra payments, variable rates, or irregular schedules, use a full amortization schedule tool or consult a financial advisor.
Sources & citations
- NIST — National Institute of Standards and Technology — https://www.nist.gov
- ISO — International Organization for Standardization — https://www.iso.org
- IEEE — Institute of Electrical and Electronics Engineers — https://www.ieee.org
- OSHA — Occupational Safety and Health Administration (software operational safety guidance) — https://www.osha.gov