How many kilojoules should you eat to lose weight?

Here’s a simple 3-step process to determine the right number for you.

Calculating the number of kilojoules you need to lose weight is a relatively simple three-step process. The Harris-Benedict equation is the most widely used method of calculating your kilojoules needs (and thus your kilojoules needs for weight loss).

Step 1:

Here is the Harris-Benedict equation for women:

BMR = 655.1 + ( 9.563 x weight in kg ) + ( 1.850 x height in cm ) – ( 4.676 x age in years )

You just need to plug in your age, height, and weight. The number you get is the total number of calories you need each day to exist (also known as your basal metabolic rate, BMR). To convert this to kilojoules, multiply the amount by 4.18.

For example, a 28-year-woman who is 165cm and weighs 72kg has a basal metabolic rate of 6 354 kJ.

Since you don’t lie in bed all day, you’ll burn more kilojoules than this. To estimate how many kilojoules you burn during your daily activities, we’ll use the activity factors listed below.

Step 2:

Next, multiply your activity factor by your BMR. For the example we’ll choose an activity factor of 1.5 (common for most people) and multiply that by 6 354 Kilojoules, giving us 9531 Kilojoules. This number is your total kilojoules needs, or roughly the amount of Kilojoules that you need to eat each day to maintain your weight. To lose weight, you need to eat less than this. How much less? That’s the next step.

Step 3:

Determining how many kilojoules to cut each day is where weight loss becomes more of an art than a science, as there are many variables that can impact the kilojoules in Vs kilojoules out equation, including:

1. The type of exercise you are doing. Resistance and interval training will burn more kilojoules after you stop exercising compared to traditional aerobic training.

2. The type of diet you are eating. High-protein diets burn more kilojoules, as protein takes more effort for your body to digest and metabolize.

3. How much weight you have to lose. For simplicity sake we use total body weight instead of your lean body mass (which is your total body weight minus your body fat). Because of this assumption, if you need to lose 10kg or more to reach your goal weight, then your total kilojoules needs, which we calculated above, is probably too high. This is because we treated the kilojoules needs of body fat the same as lean tissue (muscles, bones, and organs), but in reality your body fat has a much lower kilojoules need (close to zero). I explain how to adjust for this below.

4. Your individual metabolism. The Harris-Benedict equation or any equation that estimates your kilojoules needs is just that, an estimate. These equations are based on averages, and you are probably not average. Don’t take the numbers you generate after reading this article as gospel, but use them as a starting point, put them to the test, and adjust from there.

Traditionally, recommendations are made for individuals to subtract 2000-4000 kilojoules from their total Kilojoules needs in order to lose half to 1 kilograms per week. This is because a 2000 kJ daily deficit yields a 14 651 kJ weekly deficit—which is the number of kilojoules you need to burn or lose half a kilogram of fat. However, these reductions are too aggressive initially and cutting too many kilojoules too soon sets you up for long-term weight-loss failure.

If you are weight training, doing interval cardio sessions during the week, and eating a carb-controlled/moderate-protein diet, then you should only subtract 1000 kJ from your total kilojoules needs (or 2000 kJ if you need to lose more than 10 kg to reach your goal weight). You’ll see results that make you happy while being able to eat more. Using our example, the woman would needs to eat 7 531kJ per day to lose weight.

The final step

Put it to the test! At the end of two weeks, see how much weight you have lost. If you aren’t losing at a rate that makes you happy, opt to do more activity before you cut out more kilojoules. If you need to cut out more kilojoules, remove another 1000kJ and put that new kilojoules level to the test for two weeks.