A:Just by stretching your muscles you are burning calories. Not much as an aerobic exercise. Some times while practicing Yoga you are not raising your heart beat enough. It depends on the amount you are practicing it and which Yoga you have chosen to practice. I think the best plan to loose weight with yoga is to combine it with a healthy diet and some walking or running to give you the workout you need. I would choose the Vinyasa style yoga . Power yoga also works much on cardiovascular activity.
Good Luck.