How to measure Fat grams to calories?
Calories are really a measure of heat.
You may recall from high-school biology that plants use light energy in the process of photosynthesis to produce carbohydrates, fats and proteins. Thus, food contains energy which is released and converted to other forms of energy when the chemical bonds between molecules are broken.
Typically, to measure the energy content of food, a method called 'direct calorimetry' is used.
The food, normally about 1 gram of it, is placed in a sealed chamber called a 'Bomb calorimeter' that has water surrounding it. The piece of food is then burned (oxidized) by an electrical current. As the food burns, the water temperature rises due to the heat. This rise in water temperature reflects the energy content of the food.
Not all fats in our diet has the same energy content. Some fatty acids have about 8.6 kcals/g whereas some longer chain fatty acids may have 9.6 kcal/g.
However, for the sake of simplicity, we generally say that 1 gram of fat contains 9 calories (carbohydrates and protein contain 4 calories).
Therefore, if the food you are wondering about says it contains 8 grams of fat for example, then you know it represents about 72 calories of energy, i.e.,
8g x 9kcal/g = 72 kcal
If it has 14 grams of fat, then it contains 126 calories (14 x 9 = 126), and so on...
Ok you could read all that. Or you could just multiply the number of grams you have by 9. All fats, multiplied by 9 gives you that amount in calories. However, if you want to find the amount of calories in one gram of protein of carbohydrates then multiply grams by 4.