It is a good day in the answer forum. Good answers all around.
Classes like yoga are so self determined, intensity wise, that it is almost impossible to quantify calorie burn. Participating in yoga has many benefits. Some of which can be helpful to body composition management. But calorie expendature is usually not comparable to other forms of exercise. Luckily, calorie output is not the most important part of body composition management. So, I would suggest focusing on the mind/body component of yoga to apply discipline and mindfulness to what ever else you are looking to accomplish in life and fitness.
And if you want to find out how many calories you are burning in yoga anyway, your only real alternative would be something like the fitbit activity tracker. If you don’t want to buy one, maybe you could borrow one for a class.