SBN Loan Interest Rate Calculator

This tool estimates the implied interest rate on a loan based on the loan amount, payment amount, payment frequency, and the dates the loan starts and is fully repaid. It also generates a monthly amortization summary showing total payments, interest, principal, and ending balance by month.

What to enter

• Loan Amount — The amount advanced (principal at the start).
• Payment Amount — The regular payment made at the chosen frequency.
• Date of Loan Proceeds — The date funds were advanced / loan begins.
• Date Loan is Repaid in Full — The date the balance reaches (approximately) zero.
• Payment Frequency — Weekly, bi-weekly, semi-monthly, monthly, or annual.

How the calculation works (plain English)

• The calculator determines how many payment periods occur between the start and end dates.
• It then solves for the periodic interest rate that makes the loan amortize to (approximately) zero over that number of periods.
• It converts the periodic rate to an annual equivalent rate based on the payment frequency.
• A monthly summary is built by grouping the schedule into months.

How to read the results

• Periodic Rate — The interest rate per payment period (weekly/bi-weekly/monthly, etc.).
• Annual Equivalent Rate — The compounded annual rate implied by the periodic rate.
• Monthly Amortization Summary — Totals by month for payments, interest, principal, and ending balance.

Important notes

• This is an estimate. Results depend on how many periods fit between your dates and how the payment timing aligns.
• If the loan has irregular payments, fees, deferred payments, or changing rates, the implied rate may not match the legal contract rate.
• For accounting/tax work, keep your source documents (loan agreement, payment history) with the output.