> ## Documentation Index
> Fetch the complete documentation index at: https://sdk.grid.is/llms.txt
> Use this file to discover all available pages before exploring further.

> A simple HTTP API to calculate the monthly payment for a loan.

# Loan calculator

<Tip>
  More details on our Python examples can be found in our [GitHub
  repository](https://github.com/GRID-is/api-sdk-py/tree/main/examples).
</Tip>

You will need to download the [simple loan calculator and amortization table](https://create.microsoft.com/en-us/template/simple-loan-calculator-and-amortization-table-923c86b5-63f8-42d1-99cb-c6ae4f4b679e) from Microsoft,
upload it to your GRID account and replace `REPLACE_WITH_YOUR_SPREADSHEET_ID` with its workbook id.
See our [quick start guide](https://docs.grid.is/api-reference/getting-started) for more details.

```python theme={null}
# /// script
# requires-python = ">=3.9"
# dependencies = [
#     "fastapi[standard]",
#     "grid-api",
#     "uvicorn",
# ]
# ///
import uvicorn
from fastapi import FastAPI

from grid_api import AsyncGrid, APIStatusError, RateLimitError, APIConnectionError

app = FastAPI()


@app.get("/")
async def get_loan_calculations(loan_amount: float = 100000.0, years: int = 25, interest_rate: float = 2.5):
    client = AsyncGrid()

    loan_calculator_spreadsheet_id = "REPLACE_WITH_YOUR_SPREADSHEET_ID"
    first_row_to_read = 14
    end_row_to_read = years * 12 + first_row_to_read - 1

    try:
        response = await client.workbooks.query(
            id=loan_calculator_spreadsheet_id,
            apply=[
                {"target": "'Loan calculator'!D3", "value": loan_amount},
                {"target": "'Loan calculator'!D4", "value": interest_rate / 100},
                {"target": "'Loan calculator'!D5", "value": years},
            ],
            read=[
                "'Loan calculator'!D8",
                f"'Loan calculator'!B{first_row_to_read}:'Loan calculator'!H{end_row_to_read}",
            ],
        )
    except APIConnectionError:
        return {"error": "The server could not be reached."}
    except RateLimitError:
        return {"error": "A 429 status code was received; we should back off a bit."}
    except APIStatusError as e:
        return {"error": e.message}

    monthly_payments = response.read[0].data[0][0].v
    rows = [[cell.v for cell in row] for row in response.read[1].data]

    return {"monthly_payments": monthly_payments, "payment_schedule": rows}


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)
```

Make sure your [API key](https://app.grid.is/account/api-key) is set in the environment variable `GRID_API_TOKEN`, or passed as an argument to the `Grid` class.

```Python theme={null}
client = Grid(api_key="REPLACE_WITH_YOUR_API_KEY")
```

To start the loan calculator server, run the following:

```term theme={null}
uv run loan_calculator.py
```

Open this URL in your browser:

[http://localhost:8000/?loan\_amount=1000\&years=1\&interest\_rate=2](http://localhost:8000/?loan_amount=1000\&years=1\&interest_rate=2)

You should get a response like:

```json theme={null}
{
  "monthly_payments": 84.2388672841109,
  "payment_schedule": [
    [
      1.0,
      45758.0,
      1000.0,
      84.2388672841109,
      82.5722006174442,
      1.66666666666667,
      917.427799382554
    ],
    [
      2.0,
      45788.0,
      917.427799382554,
      84.2388672841109,
      82.7098209518066,
      1.52904633230426,
      834.717978430745
    ],
    [
      3.0,
      45819.0,
      834.717978430745,
      84.2388672841109,
      82.847670653393,
      1.39119663071791,
      751.870307777351
    ],
    ...
  ]
}
```