To calculate the area of an ROI in ENVI 4.7, you can choose Options > Report Area of ROIs in the ROI Tool dialog. - Peg Exelis VIS

From that option i can olny calculate the area of a roi i created. But after supervized calssification i want to calculate the specific Roi for the entire image, to find the total amount of vegetation not only the Roi-regions for (vegetation) i created.

Ah, I see. It was a terminology issue. In ENVI, a Region of Interest is usually what we call a specific set of polygons, polylines and/or points that the user identifies using the ROI Tool. It sounds like you want to calculate the area from the image that is classified as belonging to vegetation classes by some supervised classification that you have applied. In ENVI Classic, you can do that using the tool found under Classification > Post Classification > Class Statistics. If the input image to the classification had the pixel size specified in its header, then the resulting class statistics will include the area covered by each selected class. - Peg Exelis VIS