To display 1,000,000 as 1M in Microsoft Excel, you can use a custom number format. Here's how:

1. Select the cell or range of cells that you want to format.
2. Right-click and select "Format Cells" from the context menu.
3. In the Format Cells dialog box, select "Custom" from the Category list.
4. In the Type field, enter the following format code:
   Code:

   #,##0,,"M"

5. Click "OK" to apply the format.

Now, any number you enter in the selected cells that is greater than or equal to 1,000,000 will be displayed as 1M.

To use the formatted number in a sum function as a million, you can use a formula to convert it back to its original value. Here's an example:

1. Enter the numbers you want to sum in a column or row.
2. In the cell where you want to display the sum, enter the following formula:
   Code:

   =SUM(A1:A10)/1000000

3. Replace "A1:A10" with the range of cells that contain the numbers you want to sum.
4. Press Enter to calculate the sum.

The formula divides the sum of the selected cells by 1,000,000 to convert it back to millions. The result will be displayed in the cell with the formula.