When I look for the calorie count of ground beef, for example, I see 480 for 8oz 85% lean ground beef. Fair enough. But this site shows 34g of fat, precisely 15% of the 8oz I started with. Seeing how much liquid is left in the pan, I imagine some of the fat had to have melted out. Yet, I'm unable to find a site that will at least hint at a better estimate based on the cooked product. Another site shows cooked calorie/fat count but the fat calories are actually higher for 8oz than with the uncooked from the first site. Clearly, I'm missing something. I was expecting to find a pre-cooked count, then a post-cooked with a warning "based on medium well" or similar. Obviously the cooked products won't be identical.
You can easily calculate a maximum calories difference, and not so easily approximate an actual difference.
If you measure the weight of your meat pre- and post-cooking, you can regard the difference as the maximum amount of fat that has been lost from the meat. Multiply that difference by the number of calories per same unit, and you will have a good idea of the maximum difference in calories.
If you want a more exact number, catch all the drippings and try to determine the percentage comprised by fat. You could boil your drippings for some period, and what is left should be (mostly) fat. That seems like an awful lot of work, but you may be able to do it once and decide that for your purposes whatever you came up with on that one calculation is good enough to apply universally.
In some cases, cooking food makes the calories more accessible, and so it could be thought of as making them higher in calories.
- Eggs have more calories when cooked because raw eggs contain enzymes that interfere with absorption (this is thought to be a defense mechanism against predators). Cooking the eggs deactivates these enzymes.
- Plants, in many cases, take longer to extract all of the nutrients than our omnivorous digestive systems allow. Cows have four stomachs to fully digest grass! Because the cooking process breaks down the plant matter, there is more accessible after cooking.