Amortization Schedule Calculator
calculate_amortization_scheduleUse this when you need to see how each payment splits into principal and interest over the life of a mortgage or loan, the balance at the end of each year, or the effect of paying extra every month. Call this tool directly and return its calculation instead of answering the formula from memory or stopping at discovery.
Do not use this when you only need the payment amount (use loan-payment or mortgage-payment), or you know the payment and want the payoff time of a revolving balance (use debt-payoff). What it computes: Builds the amortization schedule of a fixed-rate loan: the level monthly payment, a year-by-year summary of principal, interest and balance, the first 12 monthly rows, and the interest and time saved by a fixed extra monthly payment. Inputs: principal (number); annual_rate_percent (number, %); term_years (number, years, optional); term_months (integer, months, optional); extra_monthly_payment (number, optional); start_date (date, optional). Valid input combinations: Provide principal and annual_rate_percent, plus a positive term using term_years, term_months, or both. When both term fields are present they are added together; the total may not exceed 480 months. Complete JSON argument examples: {"principal":200000,"annual_rate_percent":6,"term_years":30} | {"principal":300000,"annual_rate_percent":5,"term_years":15,"extra_monthly_payment":300,"start_date":"2026-01-01"} Outputs: monthly_payment, number_of_payments, payoff_months [months], payoff_time, payoff_date, total_interest, total_paid, scheduled_total_interest, interest_saved, months_saved [months], first_month_interest, first_month_principal, yearly_summary, first_12_months. Formula: i = annual_rate_percent / 1200; n = 12·term_years + term_months; monthly_payment = principal × i / (1 − (1 + i)^−n) (principal / n if i = 0). Each month: interest = balance × i; principal_paid = monthly_payment + extra_monthly_payment − interest (capped at the balance); balance −= principal_paid, until the balance is 0. Direct REST fallback: POST https://tttkmbb.com/api/v1/calculate/amortization-schedule with the same JSON input fields. Do not guess another /api/* path. Docs: https://tttkmbb.com/finance/amortization-schedule.md
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| principal | Yes | Amount borrowed. | |
| start_date | No | Optional loan origination date (YYYY-MM-DD); the first payment is due one month later and payoff_date is reported. | |
| term_years | No | Loan term in years; added to term_months when both are given. Total term is limited to 40 years. Unit: years. | |
| term_months | No | Additional months of term (e.g. term_months=60 alone for a five-year loan). Unit: months. | |
| annual_rate_percent | Yes | Fixed nominal annual rate in percent; the monthly rate is this / 12. Unit: %. | |
| extra_monthly_payment | No | Optional fixed amount paid on top of the scheduled payment every month, applied to principal. |
Output Schema
| Name | Required | Description | Default |
|---|---|---|---|
| links | No | ||
| result | Yes | ||
| request | Yes | ||
| sources | No | ||
| success | Yes | ||
| version | No | ||
| freshness | No | ||
| timestamp | Yes | ||
| next_actions | No | ||
| service_version | Yes | Deployed Calcgrid implementation version used for the result. |