I try to measure area in QGIS, but encounter some problems. The results seem to be quite inexact. When testing it with some administrative units (polygons from OpenStreetMap) and compare the results with the known real surface, results in some cases are quite close, while in other cases differ up to 20% or more from the "real" surface. So there seems to be a problem with my measurement. I saw some answers in this forum connected to projection choice. I couldn't understand which projection (the name I have to choose) is adequate for area measurements?
Your projection should be in the same unit as your area for example projection in meters and then calculate area in square meters.
I don't know where is your area, so I can't tell you what projection you should choose. As example if polygon is located in Poland you can use projection with epsg 2180