calories burned doing p90x?

The most accurate way is to go by your heart rate.
If you just want an estimate (i just do it the old fashioned way) I get a timer for 6 seconds and everytime I go to get a drink or if there's a break (in Insanity there are) I check my heart rate (you times whatever that number was by 10).
They have a bunch of calculator/formulas to figuring it out from there.
Hope that helps!

Here are the formulas:
Step 3
For males, estimate the calories that you burned during your exercise session. This is given by the equation C = (0.6309 x H + 0.09036 x W + 0.2017 x A -- 55.0969) x T / 4.184. C is the number of calories that you burned, H is your average heart rate, W is your weight, A is your age and T is the length of your exercise session in minutes.
Step 4
For females, derive the calories that you burned. This is given by the equation C = (0.4472 x H -- 0.05741 x W + 0.074 x A -- 20.4022) x T / 4.184. Assume that you're a 28-year-old female weighing 146 pounds. Your average heart rate during an exercise session that lasted 36 minutes was 138 bpm. You burned C = (0.4472 x 138 -- 0.05741 x 146 + 0.074 x 28 -- 20.4022) x 36 / 4.184 = 301 calories.


Read more: http://www.livestrong.com/article/78365-estimate-calories-burned-heart-rate/#ixzz1L8GRJNbH