math_loan_calculator
Calculate fixed-rate loan amortization: monthly payment, total interest, total paid, payoff months, and schedule. Supports extra principal payments and zero-rate loans.
Instructions
Loan Amortization Calculator. Calculate a fixed-rate loan amortization: monthly payment, total interest, total paid, payoff month count, and (optionally) the full month-by-month schedule. Set operation to compute for the summary plus per-year breakdown, or schedule to also get every monthly row. Handles extra monthly principal payments (shortens the term) and zero-rate loans (payment equals principal divided by months). Use math_compound_interest_calculator instead to grow a savings balance forward with contributions; use this tool when you owe a principal and want the payment and interest cost. Runs locally on the numbers you provide: read-only, non-destructive, contacts no external service, and is rate-limited (60 requests/minute for anonymous callers). Returns monthlyPayment, totalInterest, totalPaid, payoffMonths, and the schedule or yearly summary.
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| operation | No | Output mode: compute returns the summary plus a per-year breakdown; schedule also returns every monthly row. Defaults to compute. | |
| principal | Yes | Loan amount borrowed, in currency units. Must be greater than 0 and at most 1000000000000. | |
| annualRatePercent | Yes | Annual nominal interest rate as a percent (for example 6.5 means 6.5 percent per year). Range 0 to 100; 0 gives a zero-rate loan. | |
| termYears | Yes | Loan term in years; multiplied by 12 to derive the month count. Must be greater than 0 and at most 100. | |
| extraMonthlyPayment | No | Optional extra principal paid each month to shorten the term. At least 0 and not greater than principal. Defaults to 0. | |
| startDateIso | No | Optional first-payment date as an ISO date string (for example 2026-06-01) used to label schedule rows. Defaults to today (UTC). |
Output Schema
| Name | Required | Description | Default |
|---|---|---|---|
| success | No | Whether the calculation succeeded. | |
| operation | No | The operation performed (compute or schedule). | |
| result | No | The computed loan figures. |