Calculating km² represented by same pixel value in a 1km² resolution raster?

Question

From a raster resolution of 1km² we want to calculate the area that represents each pixel of the same color. The calculation must be done in QGIS.

Update : what parameters should I insert here:

Answer 1

Your screenshot indicates that you're using a continuous raster that is simply being symbolized as discrete classes. Consider running Reclassify by table to cement those classes into the data, converting the different ranges into coded integers.

With a reclassified raster, you can easily run the Raster layer unique values report tool to obtain the information you're looking for:

+-------+-------------+-------------------+
| Value | Pixel count |    Area (ft²)     |
+-------+-------------+-------------------+
|     1 |        1396 |  3454412666071.69 |
|     2 |       14581 |  36080795905437.9 |
|     3 |       26366 | 65242868448170.61 |
|     4 |         505 | 1249626358428.512 |
+-------+-------------+-------------------+

Update: Here is what my reclassification table looked like. Also, I used the Range boundaries type min < value <= max and checked the box for Use no data when no range matches value.

+-----+-----+-------+
| Min | Max | Value |
+-----+-----+-------+
|   1 |   5 |     1 |
|   5 |   8 |     2 |
|   8 |  12 |     3 |
|  12 |  20 |     4 |
+-----+-----+-------+