Microsoft Excel is a spreadsheet application made specifically to work with numbers. In larger spreadsheets, you can create multiple worksheets to separate out logical numerical topics. For example, a financial spreadsheet can encompass four worksheets named Quarter1, Quarter2, Quarter3 and Quarter4. When the numbers are totaled and divided among these worksheets, you can sum across the worksheets by entering a formula. Two ways exist to accomplish adding cells from different spreadsheets. One is to type in the cell reference directly, and the other is to use the mouse to highlight each cell while the formula is being built.
Sum by Typing

Open the worksheet in Microsoft Excel.

Click inside the cell in which you want the total to reside to select it.

Type the equals sign to begin the function and then type "sum(" to start the formula. Type the first cell reference to be added from the current spreadsheet. For example, type "=sum(D3" to use cell "D3" in the formula.

Type a comma, and then type in the worksheet name of the next cell to be added in the formula followed by an exclamation point. For example: type ",Quarter2!" to use the worksheet named "Quarter2."

Type the cell reference of the next cell to be added to the formula, and then close the formula with a closed parenthesis. The whole function will look similar to this: "=sum(D3,Quarter2!D8)" if creating a sum total of current worksheet's cell D3 to Quarter2's worksheet's D8.
Sum by Using the Mouse

Open the worksheet in Microsoft Excel.

Click inside the cell in which you want the total to reside to select it.

Type the equals sign to begin the function, and then type "sum(" to start the formula. With the mouse, navigate to the first cell to be added to the formula and click on it.

Type a comma, and then click on the next worksheet's tab and navigate to the second cell to be added to the formula. Click on the cell, and then type the close parenthesis to complete the function.
Tips & Warnings
 Add multiple cells to the function by typing a comma after the second added cell and then selecting a third cell to add. Close the parenthesis when finished adding all cells to be totaled in the function.
 Add cell ranges in the function. The range needs to be separated by a colon. For example, to add cell D3 in the current worksheet to the range A3 through D3 in the Quarter2 worksheet, the formula would be "=sum(D3,Quarter2!A3:D3)."
 When using the mouse method if a cell is accidentally clicked on, it will add itself to the formula. In this case, press the "Esc" key on the keyboard to abort the function and start again.
References
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