How to calculate the area of polygons using QGIS

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To calculate the area of polygons in a shapefile using QGIS, you can use the following steps:

1. Open QGIS and load the shapefile containing the polygons that you want to calculate the area for.
2. Select the layer containing the polygons in the Layers panel.
3. Click the Field Calculator button in the Attributes toolbar.
4. In the Field Calculator dialog, select the field that you want to use to store the calculated area values. If you want to create a new field, click the New Field button and enter a name and data type for the field.
5. In the Expression field, enter the following expression: $area. This will calculate the area of each polygon in the shapefile.
6. Click the OK button to calculate the area for each polygon and store the values in the selected field.

You can then view the calculated area values for each polygon by opening the attribute table for the layer.

As with most tasks in QGIS, there is more than one way to achieve this. Here are a few other options.

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Using the Measure Area tool to calculate the area of polygons

You can use the Measure Area tool, found under View → Measure → Measure Area, to manually measure the area of individual polygons. Select the tool, then click on the polygon whose area you want to measure. The area will be displayed in the Measure dialog.

1. Open QGIS and load the data containing the polygons that you want to measure.
2. Open the Measure Area tool via View → Measure → Measure Area.
3. Click on the polygon whose area you want to measure. The area will be displayed in the Measure dialog.

Using the Basic statistics for fields tool to calculate the area of polygons

You can use the Basic statistics for fields tool in the Processing Toolbox to compute summary statistics (such as sum, mean, minimum, and maximum) on an existing numeric field in the layer. To use it for area calculations, first add an area field using the Field Calculator (see above), then open the Processing Toolbox, search for Basic statistics for fields, and run it against that field to obtain aggregate statistics across all polygons.

1. Open QGIS and load the shapefile containing the polygons that you want to measure.
2. Open the Processing Toolbox by clicking the Processing menu and selecting Toolbox.
3. In the Toolbox, search for the Basic statistics for fields tool and select it.
4. In the Basic statistics for fields dialog, specify the input layer and the numeric field you want to analyse.
5. Click the Run button to compute aggregate statistics for the selected field.

Using the Field Calculator to calculate the area of polygons

You can also use the Field Calculator with the area($geometry) expression to calculate the area of polygons. The result is returned in the native units of the layer's coordinate reference system (CRS), so ensure the layer is projected in an appropriate CRS for accurate area measurements.

1. Open QGIS and load the shapefile containing the polygons that you want to measure.
2. Select the layer containing the polygons in the Layers panel.
3. Click the Field Calculator button in the Attributes toolbar.
4. In the Field Calculator dialog, select the field that you want to use to store the calculated area values. If you want to create a new field, click the New Field button and enter a name and data type for the field.
5. In the Expression field, enter the following expression: area($geometry). The result will be in the native units of the layer's CRS.
6. Click the OK button to calculate the area for each polygon and store the values in the selected field.

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