# Simple interest calculator

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

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `simple-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 | Not callable through HTTP. |
| Browser-agent interface | run_current_tool |

### Implementation notes

Simple interest = principal × annual rate × years. Interest does not compound. The estimate excludes fees, taxes and changes in rate.

### 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 | min=0 max=1000000000000000 step=any |
| `rate` | Annual interest (%) | number | 5 | min=0 max=10000 step=any |
| `years` | Years | number | 3 | min=0 max=1000 step=any |

### 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

Interest is calculated on the starting principal only. Fees, taxes, payment schedules and changing rates are excluded. For interest earned on previous interest, use the Compound interest tool.

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

### Complete machine-readable capability record

```json
{
  "id": "simple-interest",
  "category": "finance-tools",
  "built": true,
  "requirements": "Interest is calculated on the starting principal only. Fees, taxes, payment schedules and changing rates are excluded. For interest earned on previous interest, use the Compound interest tool.",
  "implementation": {
    "fields": [
      {
        "key": "principal",
        "label": "Starting amount",
        "value": 1000,
        "type": "number",
        "min": 0,
        "max": 1000000000000000,
        "step": "any"
      },
      {
        "key": "rate",
        "label": "Annual interest (%)",
        "value": 5,
        "type": "number",
        "min": 0,
        "max": 10000,
        "step": "any"
      },
      {
        "key": "years",
        "label": "Years",
        "value": 3,
        "type": "number",
        "min": 0,
        "max": 1000,
        "step": "any"
      }
    ],
    "input": false,
    "live": true,
    "presentation": "metrics",
    "note": "Simple interest = principal × annual rate × years. Interest does not compound. The estimate excludes fees, taxes and changes in rate.",
    "experience": {
      "automatic": true,
      "single": false,
      "inputMaxLength": 1000000,
      "inputMode": "text",
      "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": "Browser interface",
  "api": null,
  "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 interest without compounding.

How can we improve this tool?

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### How to use Simple interest calculator

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.

## Useful next steps

- [Compound interest](https://happytails.ai/en/finance-tools/compound-interest/) ([Markdown](https://happytails.ai/en/finance-tools/compound-interest.md))

- [Percentage](https://happytails.ai/en/calculators/percentage/) ([Markdown](https://happytails.ai/en/calculators/percentage.md))

- [Percentage change](https://happytails.ai/en/calculators/percentage-change/) ([Markdown](https://happytails.ai/en/calculators/percentage-change.md))

## How to use it

Enter the principal, annual percentage rate and number of years. Simple interest is principal × rate × time, with the rate expressed as a decimal.

### A worked example

A principal of 1,000 at 5% a year for 3 years earns 150 in simple interest. The final amount is 1,150.

### What to know

Interest is calculated on the starting principal only. Fees, taxes, payment schedules and changing rates are excluded. For interest earned on previous interest, use the Compound interest tool.