Chocolate Calculator
Chocolate is roughly a third fat by weight, which is where most of its energy sits.
The formula
kcal = 4 * protein + 4 * carbohydrate + 9 * fat + 7 * alcohol
The Atwater factors
Four calories per gram of protein and carbohydrate, nine per gram of fat, seven for alcohol. Wilbur Atwater established them in the 1890s by burning food in a calorimeter and correcting for what the body fails to absorb, and they have survived remarkably well for numbers that old.
Fat is the one that moves the total
At more than twice the calories per gram of anything else, a dressing or a frying oil dominates a dish in a way its volume does not suggest. Two tablespoons of olive oil on a salad is around 240 kcal — more than the leaves, the tomatoes and the cucumber put together several times over.
Where the factors fail
They are averages over mixed diets and they overstate some foods considerably. Whole nuts release perhaps 70 to 80 % of their calculated energy because the cell walls resist digestion; fibre is counted as carbohydrate but yields far less. The label on a packet is an estimate built on an estimate, which is worth remembering before treating it to the calorie.
Recipes are starting points
Ovens run hot or cold by 20 degrees, flour absorbs water differently by season and brand, and the same joint of meat cooks at a different rate depending on its shape rather than its weight. Everything here is a planning figure that gets you close enough to start, and the thermometer, the scales and your own notes from last time are what finish the job.