Calculating area, length, and other geometric properties
The Calculate Geometry tool allows you to access the geometry of the features in a layer. The tool can calculate coordinate values, lengths, and areas, depending on the geometry of the input layer. You can only calculate the area, length, or perimeter of features if the coordinate system being used is projected. Keep in mind that different projections have different spatial properties and distortions. If the coordinate system of the data source and data frame are not the same, you may get a different result if you calculate geometry using the data frame's coordinate system than when you calculate using the data source's coordinate system. It is recommended that an equalarea projection be used when calculating areas.
You can use the Calculate Geometry dialog box to update the area, length, or perimeter of shapefile features, since these properties are not automatically updated when you edit features in shapefiles.
You can only calculate zcoordinate values or 3D measurements if the feature is z aware. Zcoordinate values and 3D measurements can be calculated regardless of the chosen coordinate system. The units listed for z and 3D calculations are planar (miles, meters, and so on) as long as a vertical coordinate system has been defined for the layer. If the data does not have a vertical coordinate system defined, the units are listed as unknown. For more information on zvalues and feature geometry types, see Feature class basics.

Start an edit session.
You can make calculations without being in an editing session; however, in that case, there is no way to undo the results.

Rightclick the layer and click Open Attribute Table.
You can only perform geometric calculations on attribute tables.

Rightclick the field heading for which you want to make a calculation and click Calculate Geometry.
Optionally, you can press CTRL+SHIFT+G to open the Calculate Geometry dialog box.

Click the geometric property you want to calculate.
Different properties are available depending on the type of layer you're using.
 Click to use either the coordinate system of the data source or the coordinate system of the data frame.

Click the units of the output calculations.
Tip:
If you are calculating into a text field, you can choose to add a units abbreviation to the calculation. For instance, 47.5673 sq m is an example of the output of area calculated into a text field with the units abbreviation.
 Optionally, if you have selected records in the table, choose whether to apply the calculations to all records or just the selected ones.
 Click OK.
You can't undo a field calculation when performed outside an edit session.
To avoid seeing a warning message when you attempt to calculate values outside an edit session, you can check the Don't warn me again box on the message. You can turn on the warning message again from the Tables tab on the ArcMap Options dialog box.
The Calculate Geometry dialog box respects the number of decimal places (three, by default) specified on the General tab of the Editing Options dialog box. To change this setting, click the Editor menu on the Editor toolbar and click Options. This setting is saved in the map document.