Fish are mostly protein, so they contain about 17 kilo joules (about 4 kilo calories, or 4 dietary Calories) per gram.

Using joules is one way to measure how much a food can replenish our bodies, but we usually don't use joules as the unit. We use the unit of calorie (one calorie equals 4.18 joules) for food. The number of calories depends on the amount of food and the type of food, so different types of fish will have different calories. For instance, three ounces of halibut has 94 calories, three ounces of wild salmon has 156 calories, and three ounces of cod has 89 calories.

However, it is important to know that calories are not the only way to decide if what we are eating is good for us. How our bodies process food depends greatly on how well we sleep and exercise, how much water we drink, and many other factors.

Fish is a very healthy option because it contains a lot of protein, which helps our bodies grow.

Many fish also contain things called “omega-3 fatty acids”, which is an important nutrient that is good for our heart and brain health. A standing serving of fish is 3 ounces. A standard serving of tuna fish is 73 calories, or 305 joules. A serving of cod fish is 89 calories, or 372 joules. Halibut has 94 calories, or 393 joules. Salmon has 156 calories, or 652 joules.

Different fish have different calories because they vary in how much fat they have on them, and fat has a lot of calories. Therefore, the less fat on the fish, the less calories the fish will have.

The joule count ( or, in the US, Calorie count ) of a food is a measure of how much potential energy is stored in a food. All foods are composed of a few basic components: carbohydrates, proteins, fats, vitamins, minerals, fiber, and water. The first three are required in the largest amounts and are therefore referred to as macronutrients. These are the substances which provide nearly all of the energy which powers humans.

The number of calories in a food is estimated using the quantities of each of the macronutrients and average values of the energy stored in each, 4 Calories/gram (~17 kJ/gram) of protein or carbohydrate and 9 Calories/gram (~37 kJ/gram) for fats. (Side note: the Calories (big-C) listed on food labels are kilo-calories (small c), meaning 1 Calorie = 1,000 calories; and 1 joule = 4.184 calories (small c).)

To determine the joules of food energy in fish then, one needs to know the mass of the carbohydrates, protein, and fat in the fish. These are not fixed quantities. This chart gives the macronutrient breakdowns for various types of seafood, as determined by the US FDA for 3 oz. portions. Most of the items have relatively small fractions of carbohydrates, a large portion of protein, and varying amounts of fats.