how can i find polygon's length and width in Qgis(regular polygon and irregular polygon)? i have area and perimeter but possible to have length and width ?.
You could use the "Oriented minimum bounding boxes" first then run the "Polygons to lines" tool.
The output of Polygons to lines will have area, perimeter, angle, width and height of the minimum bounding box already calculated.
These results will be exact for square and rectangular shape and get worse the more your input polygon is irregular....
ETGeoWizards has the functionality to do this: http://www.ian-ko.com/ET_GeoWizards/UserGuide/polygonCharacteristics.htm#ToolBox_implementation