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Mortgage Amortization Calculator
x402
Mortgage Amortization Calculator is a paid API for AI agents from mortgage-amortization.underscoredone.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-09-14).
Computes precise monthly mortgage payments and generates a full amortization schedule with support for extra monthly payments and one-time lump-sum payments.
Price / call$0.01
Success—
StatusUnknown
How this service works
Give this API a loan amount, an interest rate, and a repayment term, and it returns the exact monthly payment plus a full month-by-month schedule: interest paid, principal repaid, and balance remaining for every payment. Ask what an extra $300 a month, or a $10,000 lump sum in month 24, would do and it reports the months and interest saved. Every figure uses banker's rounding at pinned precision, so two correct systems agree to the cent. You supply the rate; the API does the math.
Output
Returns the fixed monthly payment amount, a complete row-by-row amortization table (one row per month) showing interest portion, principal portion, and remaining balance for each payment, plus summary totals. When extra payments or lump sums are specified, also returns total interest saved and the number of months by which the loan is shortened. All monetary values use banker's rounding to the specified decimal places.
Example use cases
Example prompt
“Calculate the full month-by-month amortization schedule for a $350,000 mortgage at 6.5% annual interest over 30 years (360 months), starting payments on 2026-01-01, and show me how much interest and principal I pay each month — also run a 'what if' with an extra $300 per month toward principal.”
Technical details
Endpoint
POSThttps://mortgage-amortization.underscoredone.com/calculateWhen to prefer
Choose this endpoint when you need deterministic, reproducible mortgage or loan math with full amortization schedules — especially when banker's rounding and a pinned arithmetic rule are required for auditability. Prefer this over general financial APIs when you supply your own rate and need exact cent-level accuracy, scenario modeling (extra payments, lump sums), or a structured schedule for downstream processing. Not suitable if you need live rate lookups or market data.
Side effects
- Principal <= 0 returns validation error
- Term months < 1 returns validation error
- Negative annual rate returns validation error
- Invalid start_date format (not YYYY-MM-DD) returns parse error
- One-time payment month number exceeds term_months returns validation error
- Rounding value outside 2–6 range returns validation error
- Missing required fields (principal, annual_rate_percent, term_months) returns 400 error
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| rounding | integer | Decimal places for money values, 2 to 6. Defaults to 2. |
| principal | number | Amount borrowed, in dollars. Must be greater than zero. |
| start_date | string | Date of the first payment, YYYY-MM-DD. |
| term_months | integer | Number of monthly payments. 360 for a 30-year mortgage. Minimum 1. |
| summary_only | boolean | Set true to return totals only, without the row-by-row schedule. |
| one_time_payments | array | Optional lump-sum principal payments; each item needs 'month' and 'amount'. |
| annual_rate_percent | number | Yearly interest rate as a percentage - send 6.5 for 6.5%. Use 0 for an interest-free loan. |
| extra_monthly_payment | number | Optional extra principal paid every month. Defaults to 0. |
Output
Schema · Response
{
"type": "json",
"example": {
"summary": {
"total_paid": 910179.81,
"payoff_date": "2056-06-01",
"final_payment": 2530.88,
"total_interest": 510179.81,
"scheduled_term_months": 360
},
"rounding": {
"mode": "half_even",
"decimals": 2,
"final_payment_adjusted": true
},
"schedule": [
{
"date": "2026-07-01",
"balance": 399338.4,
"payment": 2828.27,
"interest": 2166.67,
"principal": 661.6,
"extra_applied": 300,
"payment_number": 1,
"scheduled_payment": 2528.27
}
],
"api_version": "1.0.0",
"inputs_echo": {
"principal": 400000,
"start_date": "2026-07-01",
"term_months": 360,
"annual_rate_percent": 6.5
},
"scheduled_payment": 2528.27,
"with_extra_payments": {
"total_paid": 734237.48,
"payoff_date": "2047-11-01",
"months_saved": 103,
"final_payment": 200.36,
"interest_saved": 175942.33,
"total_interest": 334237.48,
"actual_term_months": 257
}
}
}Tags
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