# Compound interest

- Human page: https://happytails.ai/en/finance-tools/compound-interest/
- Markdown: https://happytails.ai/en/finance-tools/compound-interest.md
- Structured JSON: https://happytails.ai/agents/en/finance-tools/compound-interest/index.json
- Visibility: public

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `compound-interest` |
| Category | finance-tools |
| Implementation | Registered implementation. Registration alone is not a production-quality guarantee. |
| Browser execution | Automatic updates after valid input or file selection. |
| Browser processing | Browser; see tool notes for network behavior. |
| HTTP API | /api/v1/tools/compound-interest/run |
| Browser-agent interface | run_current_tool |

### Inputs and settings

No free-text input is required. Use the file inputs and/or settings below.

| Setting | Meaning | Control type | Default | Constraints |
| --- | --- | --- | --- | --- |
| `principal` | Starting amount | number | 1000 |  |
| `rate` | Nominal annual rate (%) | number | 5 |  |
| `years` | Years | number | 10 |  |
| `n` | Compounds per year | number | 12 |  |

### HTTP contract

API calls process supplied data on the server. The website origin is `https://happytails.ai`. Server API hosting is not connected on the public website yet. Use your local server origin to test these requests.

```http
POST https://happytails.ai/api/v1/tools/compound-interest/run
Content-Type: application/json
```

[Detailed operation reference](https://happytails.ai/en/api/tools/compound-interest.md) · [Shared API guide](https://happytails.ai/en/api.md)

#### Limits

| Limit | Value |
| --- | --- |
| request_bytes | 8388608 |
| input_characters | 1000000 |
| files_bytes | 5242880 |
| max_files | 0 |
| timeout_seconds | 12 |

#### Complete input schema

```json
{
  "type": "object",
  "additionalProperties": false,
  "properties": {
    "input": {
      "type": "string",
      "maxLength": 1000000,
      "default": "",
      "description": "Plain text input. File tools use files instead unless otherwise documented."
    },
    "options": {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "principal": {
          "type": "number",
          "description": "Starting amount",
          "default": 1000.0
        },
        "rate": {
          "type": "number",
          "description": "Nominal annual rate (%)",
          "default": 5.0
        },
        "years": {
          "type": "number",
          "description": "Years",
          "default": 10.0
        },
        "n": {
          "type": "number",
          "description": "Compounds per year",
          "default": 12.0
        }
      }
    }
  }
}
```

#### Complete output schema

```json
{
  "type": "object",
  "required": [
    "tool",
    "result"
  ],
  "properties": {
    "tool": {
      "type": "string"
    },
    "result": {
      "type": "object",
      "required": [
        "text",
        "files"
      ],
      "properties": {
        "text": {
          "type": [
            "string",
            "null"
          ]
        },
        "data": {
          "description": "Parsed JSON when the textual result is JSON."
        },
        "name": {
          "type": "string"
        },
        "mime": {
          "type": "string"
        },
        "files": {
          "type": "array",
          "items": {
            "type": "object",
            "required": [
              "name",
              "mime",
              "bytes",
              "base64"
            ],
            "properties": {
              "name": {
                "type": "string"
              },
              "mime": {
                "type": "string"
              },
              "bytes": {
                "type": "integer"
              },
              "base64": {
                "type": "string",
                "contentEncoding": "base64"
              }
            }
          }
        }
      }
    }
  }
}
```

#### Error and retry handling

Failures return a non-200 status and `error.code` plus `error.message`. Correct invalid input before retrying; retry server-busy responses with bounded backoff. Returned files contain Base64 data, not persistent download URLs. Decode and inspect the output before treating conversion as successful. See the shared API guide for the complete status and timeout rules.

### Browser-agent workflow

1. Open the human page in a browser that supports WebMCP.
2. Discover the interface exposed by that page; use its actual schema.
3. Select files on the page first when the tool requires files.
4. Call `run_current_tool` with valid input and settings.
5. Inspect the returned result or error and the displayed output. Retrieve files from the displayed download links.

Calls do not automatically copy or download results. Browser permissions still apply. Interface availability alone is not proof of successful execution.

### Planning specification

Contribution timing, compounding frequency and inflation assumptions; independently verify formulas.

This describes intended requirements. Use the implementation notes, interface schema and observed output to determine current support.

### Complete machine-readable capability record

```json
{
  "id": "compound-interest",
  "category": "finance-tools",
  "built": true,
  "requirements": "Contribution timing, compounding frequency and inflation assumptions; independently verify formulas.",
  "implementation": {
    "fields": [
      {
        "key": "principal",
        "label": "Starting amount",
        "value": 1000,
        "type": "number"
      },
      {
        "key": "rate",
        "label": "Nominal annual rate (%)",
        "value": 5,
        "type": "number"
      },
      {
        "key": "years",
        "label": "Years",
        "value": 10,
        "type": "number"
      },
      {
        "key": "n",
        "label": "Compounds per year",
        "value": 12,
        "type": "number"
      }
    ],
    "input": false,
    "note": "",
    "live": true,
    "presentation": "metrics",
    "experience": {
      "automatic": true,
      "single": false,
      "label": "Your text",
      "placeholder": "Type or paste your input…",
      "help": "Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.",
      "action": null
    },
    "mode": "automatic",
    "interactive": false
  },
  "execution": "POST /api/v1/tools/compound-interest/run",
  "api": {
    "id": "compound-interest",
    "name": "Compound interest",
    "category": "finance-tools",
    "description": "Calculate growth from regular compounding at a fixed rate.",
    "notes": "",
    "human_path": "/en/finance-tools/compound-interest/",
    "method": "POST",
    "endpoint": "/api/v1/tools/compound-interest/run",
    "schema_url": "/api/v1/tools/compound-interest",
    "input_schema": {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "input": {
          "type": "string",
          "maxLength": 1000000,
          "default": "",
          "description": "Plain text input. File tools use files instead unless otherwise documented."
        },
        "options": {
          "type": "object",
          "additionalProperties": false,
          "properties": {
            "principal": {
              "type": "number",
              "description": "Starting amount",
              "default": 1000.0
            },
            "rate": {
              "type": "number",
              "description": "Nominal annual rate (%)",
              "default": 5.0
            },
            "years": {
              "type": "number",
              "description": "Years",
              "default": 10.0
            },
            "n": {
              "type": "number",
              "description": "Compounds per year",
              "default": 12.0
            }
          }
        }
      }
    },
    "output_schema": {
      "type": "object",
      "required": [
        "tool",
        "result"
      ],
      "properties": {
        "tool": {
          "type": "string"
        },
        "result": {
          "type": "object",
          "required": [
            "text",
            "files"
          ],
          "properties": {
            "text": {
              "type": [
                "string",
                "null"
              ]
            },
            "data": {
              "description": "Parsed JSON when the textual result is JSON."
            },
            "name": {
              "type": "string"
            },
            "mime": {
              "type": "string"
            },
            "files": {
              "type": "array",
              "items": {
                "type": "object",
                "required": [
                  "name",
                  "mime",
                  "bytes",
                  "base64"
                ],
                "properties": {
                  "name": {
                    "type": "string"
                  },
                  "mime": {
                    "type": "string"
                  },
                  "bytes": {
                    "type": "integer"
                  },
                  "base64": {
                    "type": "string",
                    "contentEncoding": "base64"
                  }
                }
              }
            }
          }
        }
      }
    },
    "processing": "Server; same transformation code as browser tools",
    "limits": {
      "request_bytes": 8388608,
      "input_characters": 1000000,
      "files_bytes": 5242880,
      "max_files": 0,
      "timeout_seconds": 12
    }
  },
  "agent_processing": "Server; same transformation code as browser tools",
  "browser_agent": {
    "supported": true,
    "name": "run_current_tool",
    "discovery": "WebMCP on the human page in a supporting browser",
    "verification": "See audit/API-AUDIT.md; availability is not verification"
  }
}
```

## Page guide

Calculate growth from regular compounding at a fixed rate.

## How to use Compound interest

1. Enter the values you want to calculate and choose the relevant units or settings.
2. The result updates as you type. Adjust the available settings if needed.
3. Review the result, then use Copy result or a download link when available.

## What to expect

Calculate growth from regular compounding at a fixed rate. Enter the starting amount, nominal annual rate, years, and compounding periods per year. The calculation assumes no additional deposits or withdrawals.

## Useful next steps

- [Loan payment](https://happytails.ai/en/finance-tools/loan-payment/): Estimate monthly payments on a fixed-rate loan. ([Markdown](https://happytails.ai/en/finance-tools/loan-payment.md))

- [Mortgage payment](https://happytails.ai/en/finance-tools/mortgage-payment/): Estimate principal and interest payments for a fixed-rate mortgage. ([Markdown](https://happytails.ai/en/finance-tools/mortgage-payment.md))

- [Savings goal](https://happytails.ai/en/finance-tools/savings-goal/): Estimate the monthly contribution needed to reach a savings goal. ([Markdown](https://happytails.ai/en/finance-tools/savings-goal.md))