# ARM Mortgage Calculator API

> For the complete documentation index, see [llms.txt](/llms.txt).

Worst-case payment path of an adjustable-rate mortgage under its initial, periodic, and lifetime caps.

Use this endpoint to add the tool to your app, site, workflow, or agent without rebuilding formulas, validation, examples, and docs.

- Slug: `arm-mortgage-calculator`
- Category: `finance`
- Version: `2026-04-22`
- Credits per successful call: 2
- Starts on: free
- Max payload: 65536 bytes
- Privacy mode: `hash_only`
- Available on: free, starter, pro, business, scale
- Browser tool: https://miniwebtool.com/arm-mortgage-calculator/

## Endpoint

`POST https://api.miniwebtool.com/v1/tools/arm-mortgage-calculator/run`

## Request body

| Field | Type | Required | Description |
| --- | --- | --- | --- |
| `loan_amount` | `float` | no | loan principal |
| `initial_rate` | `float` | no | introductory annual rate % |
| `loan_term` | `str` | no | 30 | 20 | 15 Choices: 30, 20, 15 |
| `arm_type` | `str` | no | 5 | 3 | 7 | 10 Choices: 5, 3, 7, 10 |
| `init_cap` | `float` | no | first adjustment cap in percentage points |
| `periodic_cap` | `float` | no | later adjustment cap in percentage points |
| `life_cap` | `float` | no | lifetime cap in percentage points |
| `compare_rate` | `float` | no | fixed-rate comparison % |

## Example request

```json
{
  "loan_amount": 300000,
  "initial_rate": 5.5,
  "loan_term": "30",
  "arm_type": "5",
  "init_cap": 2,
  "periodic_cap": 2,
  "life_cap": 5,
  "compare_rate": 6.5
}
```

## cURL

```bash
curl -X POST https://api.miniwebtool.com/v1/tools/arm-mortgage-calculator/run \
  -H 'Authorization: Bearer mwt_live_YOUR_KEY' \
  -H 'Content-Type: application/json' \
  -d '{"loan_amount":300000,"initial_rate":5.5,"loan_term":"30","arm_type":"5","init_cap":2,"periodic_cap":2,"life_cap":5,"compare_rate":6.5}'
```

## Response envelope

```json
{
  "request_id": "01K...",
  "tool": "arm-mortgage-calculator",
  "tool_version": "2026-04-22",
  "credits_used": 2,
  "result": {}
}
```

## Errors

Errors are RFC 7807 `application/problem+json`. Branch on `code`, not `title`.

## MCP

Agents can discover and call this tool through `https://api.miniwebtool.com/mcp` or `https://api.miniwebtool.com/v1/mcp` using MCP `tools/list` and `tools/call`.

## Frequently asked

### What does the ARM Mortgage Calculator API do?

Worst-case payment path of an adjustable-rate mortgage under its initial, periodic, and lifetime caps. Use it to add this utility without rebuilding formulas, validation, examples, and documentation.

### How much does one call cost?

2 credits per successful call. Failed calls do not bill.

### What parameters are required?

Required fields: (none). Full schema at `https://api.miniwebtool.com/v1/openapi.json`.

### Is this deterministic?

Yes. Same input, same output. Tool version: 2026-04-22.