Mortgage & Loan Amortization Calculator API

Exact monthly payment and full amortization schedule for any fixed-rate loan

Give this API a loan amount, an interest rate, and a repayment term, and it hands back the precise monthly payment plus a complete schedule showing — for every single month — how much goes to interest, how much reduces the balance, and what you still owe. You can also ask 'what if I pay an extra $300 a month?' or 'what if I make a $10,000 lump payment in month 24?' and it will tell you exactly how many months you save and how much interest you avoid. Every number is computed with banker's rounding and a pinned arithmetic rule so that two correct systems running the same inputs produce identical results, down to the cent. No rate lookups, no market data — you supply the rate, the API does the math.

$0.01 per request — pay via x402/USDC. No account needed.

POST to /calculate with principal (positive number), annual_rate_percent (0 or above), term_months (1 or above), and start_date (YYYY-MM-DD for the first payment). Optional: extra_monthly_payment (recurring extra principal each month), one_time_payments (list of objects with 'month' and 'amount' keys for lump-sum payments), summary_only (true to skip the row-by-row schedule array), and rounding (decimal places, default 2). The top-level scheduled_payment field is always the base principal-and-interest payment without extras. Use with_extra_payments block when extras are supplied. The schedule array reflects actual cash paid each month including extras. All dates are YYYY-MM-DD. Raises 400 for invalid inputs such as negative principal or zero term.

{
  "principal": 400000,
  "annual_rate_percent": 6.5,
  "term_months": 360,
  "start_date": "2026-07-01",
  "extra_monthly_payment": 300,
  "one_time_payments": [
    {
      "month": 24,
      "amount": 10000
    }
  ],
  "summary_only": false,
  "rounding": 2
}

{
  "principal": 10000,
  "annual_rate_percent": 0,
  "term_months": 12,
  "start_date": "2026-01-01",
  "extra_monthly_payment": 0,
  "one_time_payments": [],
  "summary_only": true,
  "rounding": 2
}

{
  "principal": -5000,
  "annual_rate_percent": 6.5,
  "term_months": 0,
  "start_date": "not-a-date"
}

{
  "principal": 2500000,
  "annual_rate_percent": 7.25,
  "term_months": 360,
  "start_date": "2025-01-01",
  "extra_monthly_payment": 1000,
  "one_time_payments": [
    {
      "month": 12,
      "amount": 50000
    },
    {
      "month": 60,
      "amount": 100000
    }
  ],
  "summary_only": false,
  "rounding": 2
}

{
  "api_version": "1.0.0",
  "inputs_echo": {
    "principal": 400000,
    "annual_rate_percent": 6.5,
    "term_months": 360,
    "start_date": "2026-07-01"
  },
  "rounding": {
    "mode": "half_even",
    "decimals": 2,
    "final_payment_adjusted": true
  },
  "scheduled_payment": 2528.27,
  "summary": {
    "total_paid": 910179.81,
    "total_interest": 510179.81,
    "final_payment": 2530.88,
    "payoff_date": "2056-06-01",
    "scheduled_term_months": 360
  },
  "with_extra_payments": null,
  "schedule": []
}