You can use formulas in a workbook in the browser exactly as you can use them in Excel. Just as in Excel, the author of the formula determines the way formulas are calculated or recalculated. This article describes different options for calculation that a workbook author can select..
Typically, a workbook recalculates formulas automatically when a value in a cell changes that the formula depends on. For example, suppose that you use a formula to add all the individual values in a column. If you insert rows in the workbook, and therefore, add cells to the range of cells in the column, the workbook adjusts the formula to include the new cells automatically, and recalculates the total.
However, a workbook author can use a different option so that the workbook does not calculate formulas automatically. When an author creates a formula in Excel, the author can select different calculation options. For example, the author might require that all formulas have to be recalculated manually when a user clicks a command. After the author selects a calculation option and saves the workbook on a site, the published workbook keeps the calculation option that the author specified in the Excel workbook.
The following table provides a brief description of the different calculation options.
Calculation option 
Description 
Automatic Calculation 
Formulas are recalculated automatically every time that a formula changes, and every time that a cell changes that a formula references. This is the default option. 
Manual Calculation 
Formulas are never recalculated automatically. A user must select the Calculate Workbook option to recalculate any formulas. 
Automatic except Tables 
Formulas and referenced cells that are contained in an Excel Data Table as part of WhatIf analysis are not recalculated. Other formulas are recalculated automatically every time that a formula changes, and every time that a cell changes that a formula references. 
Recalculate Before Save 
Formulas are recalculated automatically every time that the workbook is saved. Important: Excel Services does not support this option because you cannot save a workbook. 
If the workbook author configured a workbook for manual calculation, you can follow these steps to calculate the formulas in a workbook in the browser.

Open the workbook.

On the workbook toolbar, click the Data menu, and then click Calculate Workbook.
or, you can use the keyboard shortcut F9 to calculate the formulas in a workbook.
Notes:

To use formulas in a workbook in Excel Services, the workbook author must first create a workbook in Excel that contains formulas. If Excel Online is enabled on the site, you can enter formulas directly in a workbook exactly as you do in Excel.