Until a few weeks ago I had gone through a lot of P90X and Insanity without a heart rate monitor. Both programs give you a generic guesstimate at calories burned. P90X says estimate 600 calories per workout, Insanity advertises 1,000 (which is a gross overestimate). But making estimates like this is like never looking at your paycheck from work-- you WANT to make sure every penny is there.
Calories burned during a workout vary depending on your sex, height, weight, and heart rate. Smaller women burn fewer calories than large men (which women sometimes gripe about).
Here's the basic formula, taken from someone's fitness website:
Men use the following formula:
Calories Burned = [(Age x 0.2017) + (Weight x 0.09036) + (Heart Rate x 0.6309) -- 55.0969] x Time / 4.184.
Women use the following formula:
Calories Burned = [(Age x 0.074) -- (Weight x 0.05741) + (Heart Rate x 0.4472) -- 20.4022] x Time / 4.184.
But how do you know what your heart rate is without a heart rate monitor (which would calculate calories for you anyway)?
What you can do with Insanity and P90X, both of which have periodic breaks as part of the sort of interval training, is take your heart rate at the breaks. Look at the clock on the screen and take your pulse for 10 seconds. Write that number down each time (I logged it in my phone as I jogged, stretched, whatever).
I put all those numbers down in a spreadsheet and averaged them. Since the intervals are irregular in some workouts, I looked what the time was for each part and calculated a weighted average. Multiply your average by 6 and you have your BPM to plug into the formula above.
Since I've bought a heart monitor, I've been able to check my calculated averages. I had varying results. For some Insanity exercises, the break is right after a lower-impact exercise in which my heart rate had already come down anyway, so I was never really measuring the time in between when my heart rate spiked. In other cases, vice-versa.
My advice? Avoid the hassle of focusing on counting your pulse correctly and later averaging it by spending the money on a heart rate monitor. I got a Polar FT7 for $80 from Amazon and love it, it does all the work for me and has other cool features.