Calculating Dietary Energy
Interpret food-label energy values, calculate energy for stated portions and compare daily totals with supplied estimates in classroom examples
What a learner can do afterwards
- Reads and interprets a nutritional information label (kJ and kcal)
- Uses a supplied age/activity guide to select an estimated daily energy requirement and explains why individual needs vary
- Compares the energy content of different diets and identifies surpluses or deficits
1 · Read
Carbohydrates, fats and proteins supply food energy. Labels show energy for a stated amount of food. Read the unit beside each number: kJ means kilojoules and kcal means kilocalories. Food Calories mean kcal, not the smaller calorie used in some science calculations. Both kJ and kcal measure energy.
For the same amount of food, 1 kcal is about 4.2 kJ. Multiply kcal by 4.2 to estimate kJ: 20 × 4.2 = (20 × 42) ÷ 10 = 84 kJ. Divide kJ by 4.2 to estimate kcal. Multiply both numbers by 10 to remove the decimal: 84 ÷ 4.2 = 840 ÷ 42 = 20 kcal. Some exercises use 4 for a rough estimate. Use the question's factor. Approximate agreement checks a calculation; it does not prove a label is accurate.
Check whether a label gives energy per serving, per item or per 100 g. For servings, multiply the energy per serving by the number used. To compare portions, calculate each portion's total before comparing. A chart of energy per serving does not show the total for two servings. The serving size describes the label's amount; it is not a personal eating instruction.
An example food has 60 kcal per serving. Two servings contain 2 × 60 = 120 kcal. Another food has 180 kcal per 100 g. A 200 g portion contains two lots of 100 g, so it has 2 × 180 = 360 kcal. Match the energy value to the stated amount.
Daily energy needs depend on age, body size, growth and activity. The same person usually needs more energy on a more active day. Select the matching age/activity entry in a supplied classroom table. Guides are estimates, not measurements of actual energy use. These exercise figures are not personal eating targets. Growing children should not use them to set calorie limits.
In an exercise, meals and snacks contain 400, 650, 800 and 200 kcal. Add all values in the same unit: 400 + 650 + 800 + 200 = 2050 kcal. The supplied guide is 2150 kcal. The gap is 2150 - 2050 = 100 kcal below it. Above a guide is a model surplus; below it is a model deficit; equal totals match.
Check units and portions, calculate and add energy values, then compare with the supplied model guide. An estimated guide is not a personal eating target.
Lesson, questions and study cards written for this dataset. Classroom energy figures are practice data, not personal dietary recommendations.
Readings and study cards written for this dataset. The linked video is TED-Ed's Sugar: Hiding in plain sight - Robert Lustig. Its reuse terms have not been verified.
More to read
Keep the unit beside the number
Writing 500 without a unit does not tell a reader how much food energy you mean. A food label may use kJ, meaning kilojoules, or kcal, meaning kilocalories. Food Calories mean kcal. Two values in different units can describe the same energy.
For an estimate, 1 kcal is about 4.2 kJ. Thus 150 kcal is about 630 kJ. Reverse the calculation by dividing kJ by 4.2. When an exercise explicitly supplies 4 as its rough factor, use 4 for that exercise. A rough conversion cannot prove that every detail of a label is correct.
Per serving is not per packet
Before using an energy value, read the amount it describes. Suppose a label gives 80 kcal per serving. Three equal servings supply 3 × 80 = 240 kcal. Whether that is the whole packet depends on how many servings the packet contains.
A per-100-g value describes a different amount. If a food has 140 kcal per 100 g, then 200 g contains two lots of 100 g and supplies 280 kcal. The label amount helps with the calculation; it is not an instruction about how much a child should eat.
Include every listed amount
A model day's total needs every energy value listed in the exercise. Meals, snacks and drinks may all contribute. Convert values to the same unit before adding them.
For example, add 300, 650, 750 and 250 kcal. The first two make 950 kcal. Adding 750 gives 1700 kcal. Adding the final 250 gives 1950 kcal. This total describes the listed amounts. Energy alone does not describe all the nutrients in a diet, so a total cannot establish whether a real child's diet meets their needs.
Use a supplied estimate
A body uses energy during rest as well as during activity. Age, body size, growth and activity affect its needs. Comparing the same person on two otherwise similar days helps isolate the effect of activity: a more active day usually requires more energy.
Classroom tables supply estimates so learners can practise reading and comparing data. Select the entry that matches the conditions in the question. Do not treat the result as a measurement of actual energy use or as a personal calorie limit. Adult weight-management calculators are not part of this exercise.
State what the comparison shows
A total can be above, below or equal to a supplied guide. In this lesson, above means a model surplus and below means a model deficit. To find the gap, subtract the smaller value from the larger one and state the direction.
A total of 2250 kcal is 150 kcal above a model guide of 2100 kcal. A total of 1950 kcal is 150 kcal below that guide. These calculations compare the figures supplied. They do not establish an actual person's energy balance or tell that person to change their food intake.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.