Loan Calculator API

A lightweight REST API that calculates loan payment metrics given amount, annual rate, and term in years. It returns monthly payment, total payment, and total interest using the standard amortization formula.

Base URL

https://www.promptbox.cn/api/loan.php

Methods

GET and POST (JSON)

Query Parameters (GET)

Parameter	Type	Required	Example	Description
`amount`	float	Yes	200000	Principal loan amount
`rate`	float	Yes	5.5	Annual interest rate in percent
`years`	integer	Yes	30	Term length in years

Request Body (POST JSON)

{
  "amount": 200000,
  "rate": 5.5,
  "years": 30
}

Response

Success

{
  "monthlyPayment": 1135.58,
  "totalPayment": 408808.80,
  "totalInterest": 208808.80,
  "inputs": {
    "amount": 200000.00,
    "rate": 5.50,
    "years": 30.00
  }
}

Error

{
  "error": "Missing or invalid 'amount'. Must be > 0."
}

Examples

1. GET Example

GET /api/loan.php?amount=200000&rate=5.5&years=30

{
  "monthlyPayment": 1135.58,
  "totalPayment": 408808.80,
  "totalInterest": 208808.80,
  "inputs": {
    "amount": 200000.00,
    "rate": 5.50,
    "years": 30.00
  }
}

2. POST Example

POST /api/loan.php
Content-Type: application/json

{
  "amount": 200000,
  "rate": 5.5,
  "years": 30
}

{
  "monthlyPayment": 1135.58,
  "totalPayment": 408808.80,
  "totalInterest": 208808.80,
  "inputs": {
    "amount": 200000.00,
    "rate": 5.50,
    "years": 30.00
  }
}

3. Error Example

GET /api/loan.php?amount=0&rate=5&years=30

{
  "error": "Missing or invalid 'amount'. Must be > 0."
}

Notes

Uses standard amortization formula: monthly payment based on principal, annual rate, and term.
rate is annual percentage; internally converted to monthly decimal.
Values are rounded to two decimals for readability.
CORS enabled; can be called from browser, curl, mobile apps, or any HTTP client.
Both GET and POST (JSON) are supported.