How to calculate the calorie content of cooked food?

If you are concerned about the impact of whatever difference in calorie count cooked vs. raw makes, you are cutting it pretty fine. The margin for error is likely very small--probably smaller than your measuring errors, or the inaccuracy of your kitchen (or bathroom) scale.

Keeping an eye on calories is fine for weight loss. Reducing calorie intake and/or increasing calorie burning is the only proven way to do it. But looking out to avoid that 20 calorie margin of error suggests that either you're not cutting enough calories in the first place that 20 matters, or you're tormenting yourself over things you shouldn't be worried about. Cut yourself some slack, and don't sweat the small stuff. The simple act of realistically monitoring how much you eat is more likely to help you than obsessing over every single calorie.

Calories are a measure of energy, so technically warm food has more energy than cold food. It's possible that the way that it's cooked might add fat (saute, frying, etc), which will add to the chemical energy available.

But the real issue is a factor of absorption -- cooking makes more nutrients available that the body wouldn't otherwise be able to use. Do those nutrients have calories? It's possible, I guess, but to get the same nutrients, you'd have to eat more of the raw food.

I really don't know calories are calculated these days -- it used to be a measure of how much energy was given off when the dehydrated food was burned, but with the advent of things like Olestra that are considered '0 calorie' are only so because they can't be absorbed by the body.

I've heard that one of the suspected reasons for the advent of human civilization was because of cooking that might be the source of your question. From the Publisher's Weekly summary of Catching Fire: How Cooking Made Us Human:

By making food more digestible and easier to extract energy from, Wrangham reasons, cooking enabled hominids' jaws, teeth and guts to shrink, freeing up calories to fuel their expanding brains.

... but that doesn't specifically say that it added calories, as it "liberated mankind from the drudgery of chewing", which would've required energy. (sort of like the 'negative calories' of raw celery)

update (many years later):

I'm going to have to change my assumption that the articles I cited suggested that the changes made by cooking other than the addition of heat energy were purely mechanical.

A recent article made the news about the problem with the 'calorie' as a measure for dieters (which should technically have been 'Calorie', ie, 'kilocalorie'). But they mention both that the number printed on packages are not the same as bomb calorimeter, but modified by 'Atwater values' ... which assume that the digestability of all fats are the same, as are all carbohydrates, etc. But studies have shown that how you cook food can change its digestability, which significantly reduces the calories that you absorb. As mentioned in the recent calorie article, this has been known for decades, but isn't part of the formulas used for labeling:

Wrangham and his colleagues have since shown that cooking unlaces microscopic structures that bind energy in foods, reducing the work our gut would otherwise have to do. It effectively outsources digestion to ovens and frying pans. Wrangham found that mice fed raw peanuts, for instance, lost significantly more weight than mice fed the equivalent amount of roasted peanut butter. The same effect holds true for meat: there are many more usable calories in a burger than in steak tartare.

...

Yet the FDA’s methods for creating a nutrition label do not for the most part account for the differences between raw and cooked food, or pureed versus whole, let alone the structure of plant versus animal cells. A steak is a steak, as far as the FDA is concerned.

The article also explains more research into the linkage between obesity and gut microbes -- we can make both mice and people obese through fecal transplants. The problem is, each person may be able to extract different amounts of energy out of the same food, leading to people having enough calories to subsist well before they feel satiated.

Here's a quick and dirty table to help you get a rough idea of the calorie content of what you are cooking. Keep in mind that this will only give you an estimation of your meal's calorie count.

--------------------------------------
| Food Component | Calories per gram |
|----------------|-------------------|
|       fat      |         9         |
|     alcohol    |         7         |
|     protein    |         4         |
|  carbohydrates |         4         |
--------------------------------------

The difference between raw and cooked forms depends on the type of food and how you cook it. Some food takes on water, some release water, some change their chemical composure entirely.

Truth be told the differences are very minor.

To use your example:

*raw*     chicken whole egg contains 151 calories per 100g
*poached* chicken egg has 147 per 100g

This is (relatively) nothing.

Perhaps a little more powerful than you need, but there are web services which can reference USDA maintained databases of thousands of ingredients in cooked and raw form, thus negating the need to deduce anything. I use a la calc (they have a time unlimited free trial) as they provide a UK service but they do have the latest USDA database too.

You could download the USDA nutrition databases yourself from the USDA website but I tried that and, to be frank, it's all gibberish to me. The website I mentioned has a good interface that flashes up the data of every ingredient they have (they claim they have 12,000+).

This question is impossible to answer.

There are two things going on: the first is that it takes less of your own calories to process cooked foods, of all sorts. This is (I think) always going to be true because the heat applied during cooking breaks down bonds between the food molecules, so your stomach won't have to.

So, eating a raw food will require more net calories of work to process it than eating it's exactly-the-same cooked counterpart. This 'net calories' conversation is one arm of the question.

Where you run into troubles: food has water in it. How are you going measuring your cooked and raw food to compare? One cup of squash that has all the fluid cooked out of it is more calories than one cup of raw squash. One oz of squash that has all the fluid cooked out of it has more calories than one oz of raw squash.

Finally, there's the what-you-cook-it-in factor: cooking something in butter adds available calories, although not as much as the whole butter amount, since some is left in the pan, drips off the sides, etc.

All that said, getting super exact is impossible, sorry!

I believe the only type of food that gains calories from cooking is starches, and it would vary depending on the type of starch.