If your question relates to tracking your own calorie expenditure wearing a fit bit is probably a practical solution.
If your question relates to answering questions from your class I would suggest that such questions are an opportunity to speak more broadly about why we do our yoga. Cross training is a way of practicing Hatha yoga…. we focus on different aspects of our physical self to create all over health. The role of yoga in weight control tends to be one of creating mindfulness that can help with making better food choices, in battling stress that is associated with weight gain, and so on. The total number of calories burned is not an index to its usefulness toward this goal.
That said, you might have students who find it helpful to track total calories in/calories out and want at least an estimate. The general answers you were given above are spot on…. it varies a lot. However, there is a lot of research on yoga, and a lot of it is done with flow style. The ashtanga of Patthabi Jois, and the Bikram practice lend themselves to research as the movements are done in the same way each time. Here is something from the Journal of Alternative Therapies July/August 2014. It kind of gives you an idea of how complex the question is, even when you control for the natural variation in how each of us practices a flow, as well as all the variations mentioned above:
Table 1. Relative Oxygen Consumption, METS, and Intensity Level for Each Posturea
(mL ├ù kg-1 ├ù min-1) METS
Standing Deep Breathing 5.85 1.67 Light 95
Half-Moon 7.79 2.23 Light 141
Awkward 9.39 2.68 Light 153
Eagle 13.00 3.71 Moderate 147
Standing Head to Knee 13.15 3.76 Moderate 160
Standing Bow 13.93 3.98 Moderate 162
Balancing Stick 13.24 3.78 Moderate 165
St. Sep. Leg Stretching 11.52 3.29 Moderate 130
Triangle 12.23 3.49 Moderate 168
St. Sep. Leg Head to Knee 11.15 3.19 Moderate 143
Tree 9.43 2.69 Light 150
Toe Stand 7.18 2.05 Light 137
Savasana 6.55 1.87 Light 105
Dead Body 7.65 2.18 Light 118
Cobra 8.47 2.42 Light 142
Locust 9.43 2.69 Light 148
Full Locust 9.59 2.74 Light 153
Bow 10.10 2.89 Light 150
Fixed Firm 9.42 2.69 Light 124
Half Tortoise 9.18 2.62 Light 119
Camel 8.95 2.56 Light 152
Rabbit 9.70 2.77 Light 145
Head to Knee 8.97 2.56 Light 139
Spine Twisting 8.28 2.37 Light 145
Blowing in Firm 8.60 2.46 Light 141
Savasana 5.71 1.63 Light 108
Full Practice Average 9.56 2.73 Light 140
Abbreviations: VO2 = oxygen uptake; BPM = beats per minute; St = standing;
Sep = separate; METS = metabolic equivalent values.
Values are expressed as means.
And this is for a style that is intensely rigid in its practice….
I would like to recommend to you the book “The Science of Yoga” by William J Broad. It is an interesting read, and although he does the journalist thing of bringing in as much salacious stuff as possible (as that is what sells) it is well researched and very interesting.
I would also suggest Mel Robin’s book “A Physiological Handbook for Teachers of Yogasana”. This is not a book to read all at once, but a really good, detailed reference for fairly specific questions one sometimes gets from students.