Interest-only loan calculator API

Computes the interest-only payment (P*r) during the IO period and the fully-amortizing payment afterward over the remaining months, plus the payment jump at reset. Answers 'What is my interest-only payment?','How much will my payment jump when the loan starts amortizing?'.

Price$0.01per request

MethodPOST

Route/v1/calc/loan-interest-only

StatusLive

MIME typeapplication/json

Rate limit120/minute

CacheNo cache

loaninterest-onlyamortizationmortgagepayment-shockinterestcalculator

API URLhttps://x402.hexl.dev/v1/calc/loan-interest-only

Integration docs

Example request

{
  "principal": 400000,
  "annualRatePct": 6,
  "interestOnlyMonths": 60,
  "totalTermMonths": 360
}

Example response

{
  "interestOnlyPayment": 2000,
  "amortizingPayment": 2577.21,
  "paymentJumpAtReset": 577.21,
  "totalPaid": 893161.68,
  "totalInterest": 493161.68,
  "formula": "IO payment = P*r; after IO, amortize P over remaining months",
  "breakdown": {
    "principal": 400000,
    "annualRatePct": 6,
    "interestOnlyMonths": 60,
    "amortizingMonths": 300
  }
}

Input schema

{
  "type": "object",
  "required": [
    "principal",
    "annualRatePct",
    "interestOnlyMonths",
    "totalTermMonths"
  ],
  "properties": {
    "principal": {
      "type": "number",
      "description": "Loan principal.",
      "examples": [
        400000
      ]
    },
    "annualRatePct": {
      "type": "number",
      "description": "Annual nominal rate in percent.",
      "examples": [
        6
      ]
    },
    "interestOnlyMonths": {
      "type": "number",
      "description": "Length of the interest-only period in months.",
      "examples": [
        60
      ]
    },
    "totalTermMonths": {
      "type": "number",
      "description": "Total loan term in months (> IO period).",
      "examples": [
        360
      ]
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}