How many kilojoules should you eat a day? Work out your own number
Two adults worked through in kilojoules, a target table for five activity levels, and why the number moves as your weight does.
How many kilojoules should you eat a day?
Your daily kilojoule needs depend on your size, age, sex and how much you move, so there is no single right number. The standard estimate has three steps: work out your basal metabolic rate (BMR) with the Mifflin-St Jeor equation, multiply it by an activity factor, then multiply by 4.184 to turn calories into kilojoules.
For two example adults, that gives very different answers:
- A woman aged 35, 165 cm and 68 kg, lightly active: about 7,912 kJ (1,891 Cal) a day to maintain her weight.
- A man aged 45, 180 cm and 90 kg, moderately active: about 11,706 kJ (2,798 Cal) a day.
These are maintenance figures. Eating less than your maintenance number leads to weight loss over time, and eating more leads to gain. Both are estimates to test against your weight trend over two to three weeks.
Is 8,700 kJ a day right for you?
Probably not exactly. In NSW, food chains with 20 or more outlets in the state or 50 or more nationally must show the average kJ of each standard item and the statement that the average adult daily energy intake is 8,700 kJ (2,079 Cal). That figure is a reference for comparing menu items, not a personal target. More on reading the boards in kilojoules on menus.
The two examples show why. By these estimates, the woman's maintenance need is 788 kJ below 8,700 kJ, and the man's is 3,006 kJ above it. If she ate 8,700 kJ every day she would gain slowly; if he did, he would lose.
Step by step: Mifflin-St Jeor in kilojoules
Mifflin-St Jeor gives BMR in calories (kcal), so calculate it first and convert at the end. The equations are:
- Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5
- Women: 10 × weight in kg + 6.25 × height in cm − 5 × age − 161
Example 1: woman, 35, 165 cm, 68 kg, lightly active.
- BMR: 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031.25 − 175 − 161 = 1,375.25 Cal.
- In kilojoules: 1,375.25 × 4.184 = 5,754 kJ.
- Maintenance: 1,375.25 × 1.375 (light activity) = 1,890.97 Cal, and 1,890.97 × 4.184 = 7,912 kJ.
Example 2: man, 45, 180 cm, 90 kg, moderately active.
- BMR: 10 × 90 + 6.25 × 180 − 5 × 45 + 5 = 900 + 1,125 − 225 + 5 = 1,805 Cal.
- In kilojoules: 1,805 × 4.184 = 7,552 kJ.
- Maintenance: 1,805 × 1.55 (moderate activity) = 2,797.75 Cal, and 2,797.75 × 4.184 = 11,706 kJ.
Mifflin-St Jeor dates from 1990, and a 2005 systematic review compared it with other equations for predicting resting metabolic rate in healthy adults. Like any prediction equation, it gives an estimate, and your real needs can sit above or below it. The full method, in calories, is in how to calculate your TDEE.
Which activity multiplier should you use?
The multiplier scales your BMR up to cover everything you do in a day, from walking to the bus to training. Pick the one that matches a normal week, not your best week.
- Sedentary (1.2): mostly sitting, little walking and no planned exercise.
- Light (1.375): a desk job with some walking, or light exercise on a few days.
- Moderate (1.55): regular exercise on most days, or a job that keeps you on your feet.
- Active (1.725): hard training most days, or a physical job.
- Very active (1.9): a heavy physical job plus training, or training twice a day.
If you are torn between two levels, choose the lower one. You can move up later if you lose faster than planned.
Daily kilojoule targets for maintenance and weight loss
The table runs both examples through all five activity levels. The weight-loss columns subtract 2,000 kJ (478 Cal) a day. By the rule of thumb of about 32,200 kJ per kg of body fat, that predicts about 0.43 kg of loss a week at first (2,000 × 7 = 14,000 kJ; 14,000 ÷ 32,200 = 0.43).
| Activity level | Woman, 68 kg: maintain | Woman: lose | Man, 90 kg: maintain | Man: lose |
|---|---|---|---|---|
| Sedentary (1.2) | 6,905 kJ (1,650 Cal) | 4,905 kJ (1,172 Cal) | 9,063 kJ (2,166 Cal) | 7,063 kJ (1,688 Cal) |
| Light (1.375) | 7,912 kJ (1,891 Cal) | 5,912 kJ (1,413 Cal) | 10,384 kJ (2,482 Cal) | 8,384 kJ (2,004 Cal) |
| Moderate (1.55) | 8,919 kJ (2,132 Cal) | 6,919 kJ (1,654 Cal) | 11,706 kJ (2,798 Cal) | 9,706 kJ (2,320 Cal) |
| Active (1.725) | 9,926 kJ (2,372 Cal) | 7,926 kJ (1,894 Cal) | 13,027 kJ (3,114 Cal) | 11,027 kJ (2,636 Cal) |
| Very active (1.9) | 10,933 kJ (2,613 Cal) | 8,933 kJ (2,135 Cal) | 14,349 kJ (3,430 Cal) | 12,349 kJ (2,951 Cal) |
Activity alone moves the woman's maintenance from 6,905 kJ to 10,933 kJ, a spread of 4,028 kJ. For the sedentary woman, the loss target drops to 4,905 kJ (1,172 Cal). Before going that low, talk to your GP or an Accredited Practising Dietitian. A smaller deficit, or more everyday movement, gives a higher target.
How big a deficit to choose, and what it does week by week, is covered in how many kilojoules to cut to lose weight.
Why your kilojoule needs change as your weight changes
Your number is not fixed. Body weight is part of the equation, so as you lose weight your BMR and maintenance needs fall with it.
Take the man from example 2 after losing 10 kg. At 80 kg his BMR is 10 × 80 + 6.25 × 180 − 5 × 45 + 5 = 1,705 Cal, which is 418 kJ (100 Cal) lower than before. At the same moderate activity level, his maintenance falls from 11,706 kJ to 11,057 kJ, a drop of 649 kJ a day.
This is why a fixed intake produces slower loss over time. A 2011 analysis in The Lancet showed that the simple 3,500-calorie rule (about 7,700 Cal, or 32,200 kJ, per kg) overestimates long-term weight loss, because energy expenditure falls as weight falls. Earlier research found that after weight loss, energy expenditure dropped more than the new body size alone predicted.
The practical answer is to recalculate after every few kilograms and let your weight trend have the final say. If your average weight has not moved in three weeks at a planned deficit, either your real maintenance is lower than the estimate or your intake is higher than your log shows. More in weight loss and metabolic adaptation.
What to do next
- Work out your BMR with Mifflin-St Jeor, multiply by your activity factor, then multiply by 4.184 for kilojoules.
- Treat 8,700 kJ as a menu reference for comparing items, not as your target.
- In Cal.AI, setup asks your goal, pace, activity level, height, weight, age and sex, and sets your daily target with Mifflin-St Jeor. The app shows calories, not kilojoules: multiply its target by 4.184 to see it in kJ, and divide label kJ by 4.184 before you log. The kJ to calories guide has quick shortcuts.
- Weigh in regularly and check the trend after two to three weeks before changing anything.
- Recalculate after every few kilograms lost, or when your activity level changes.
Frequently asked questions
Is 8,700 kJ a day too much?
It depends on you. 8,700 kJ (2,079 Cal) is the average adult daily energy intake used as a reference on NSW menu boards. For the lightly active 68 kg woman in our example, maintenance is about 7,912 kJ, so 8,700 kJ is slightly more than she uses. For the moderately active 90 kg man, it would be a deficit of about 3,006 kJ.
How many kilojoules should a woman eat a day?
There is no single figure. Using Mifflin-St Jeor, a 35-year-old woman who is 165 cm tall and weighs 68 kg needs about 6,905 kJ (1,650 Cal) a day if sedentary and about 8,919 kJ (2,132 Cal) if moderately active. Plug in your own height, weight and age, multiply by your activity factor, then multiply by 4.184.
How do I turn my calorie target into kilojoules?
Multiply by 4.184. A 2,000 Cal target is 8,368 kJ, and a 1,500 Cal target is 6,276 kJ. To go the other way, divide kilojoules by 4.184, or roughly by 4.2. Cal.AI shows calories, so it is usually easiest to convert label kJ figures to calories before logging rather than converting the app's numbers back.
How often should I recalculate my daily kilojoules?
Recalculate whenever your weight changes by a few kilograms or your activity level changes. Weight is part of the equation: for the 90 kg man in our example, losing 10 kg lowers maintenance by about 649 kJ a day. Between recalculations, use your weight trend over two to three weeks to check the estimate against reality.
Sources
- Mifflin MD, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. PubMed
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789. PubMed
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. PubMed
- Leibel RL, Rosenbaum M, Hirsch J. Changes in energy expenditure resulting from altered body weight. N Engl J Med. 1995;332(10):621-628. PubMed
- NSW Food Authority. Fast Choices program: nutritional information menu labelling. NSW Food Authority
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