To convert negative Celsius temperatures to Fahrenheit in Excel, you can use the following formula:

F = (C * 9/5) + 32

where C is the temperature in Celsius and F is the temperature in Fahrenheit.

Here is a step-by-step guide for converting negative Celsius temperatures to Fahrenheit in Excel:

- Set up your Excel sheet: Open a new Excel sheet and add a column heading for "Celsius" and another column heading for "Fahrenheit".
- Enter the Celsius temperatures: In the "Celsius" column, enter the negative Celsius temperatures that you want to convert to Fahrenheit.
- Enter the formula: In the "Fahrenheit" column, enter the formula "=(C * 9/5) + 32", where C is the temperature in Celsius. To reference the Celsius temperature, simply click on the appropriate cell in the "Celsius" column.
- Copy the formula: Once you have entered the formula in the first row, you can copy it down to the other rows by clicking and dragging the bottom right corner of the cell. This will automatically apply the formula to all of the other rows.
- Format the sheet: To make the sheet easier to read, you can format the cells to display the temperatures with a specific number of decimal places. You can also add conditional formatting to highlight cells with specific values.
- Test the calculator: Once you have finished entering the data and the formula, be sure to test the calculator by checking the results against a known conversion table.

Here are some tips to keep in mind when converting negative Celsius temperatures to Fahrenheit in Excel:

- Use proper units: Make sure to use proper units when entering the data into the calculator, such as degrees Celsius for the temperature.
- Consider rounding: Depending on the level of accuracy you require, you may need to round the results of the calculator.
- Add error checking: To ensure that the calculator is as accurate as possible, consider adding error checking to the formula. For example, you could add an error message if the temperature is less than -273.15, which is absolute zero in Celsius.
- Keep it simple: Try to keep the calculator as simple and straightforward as possible to minimize the chance of errors.

By following these steps and tips, you should be able to successfully convert negative Celsius temperatures to Fahrenheit in Excel.