I've read that if I do anaerobic exercise it can trigger an "afterburn" effect which lasts for at most 48 hours. Suppose I do this every other day which means that the afterburn effect is quasi-constant. The question is how can I calculate or approximate the calories burned by this?
Afterburn is the layman's term for EPOC, or Excess Post exercise Oxygen Consumption. EPOC is your body converting from higher yield energy systems back to lower yield energy systems. Think of your energy systems as a series of dimmer switches-you can't turn any of them off completely, so they're running all the time, but you can tweak the settings.
Now the level of epoc is completely dependent on intensity of exercise-the more vigorous the exercise, the higher and longer the level of epoc. For example, I did a high intensity conditioning test yesterday, and my heart rate stayed elevated for about an hour afterwards.
High level EPOC is uncomfortable, and higher elevated heart rate only lasts for about two hours-it's hard to say when the cut off is, as there are many factors in play.
EPOC is a downward slope, and will continue for many hours, but that afterburn only continues for so long. Because you're pursuing anaroebic training (EPOC can happen with either aerobic or anaroebic, it's all intensity), I believe it would be very difficult to calculate accurately without a heart rate monitor. You must estimate your average heart rate at rest, and plug that into an estimated VO2 calculator or formula. After exercise, record your heart rate, and get the VO2 for that value. Subtract the two, and plug the difference into a VO2 to kcal converter or formula.
Not particularly user friendly, and it wouldn't give you real time data. I use a Suunto Quest (NOT PLUGGING THIS PRODUCT), and it gives me EPOC and heart rate. I believe the HR is more valuable on telling me my post exercise state than the EPOC calculator.
So in summary, monitor your heart rate before exercise, and post exercise. Note the difference, and how long it takes you to get back to normal. Greater intensities mean greater EPOC, which means more calories burned, although not a massive amount.