There’s no single “weight-loss number” of kilojoules that works for everyone. A kJ target that helps one person lose weight steadily might leave another person hungry and stuck, or gaining. The good news is that working out a sensible starting point takes about two minutes.
This guide is general information for healthy adults. If you have a medical condition, are pregnant or breastfeeding, are under 18, or have a history of disordered eating, please get personal advice from a doctor or accredited dietitian.
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Step 1: Estimate Your Maintenance Kilojoules
Your maintenance intake is the number of kilojoules that keeps your weight stable. It depends on your size, age, sex and activity. The easiest way to estimate it is our daily kJ calculator, which uses the Mifflin-St Jeor equation. If a smart scale gives you a BMR in kJ, convert it to Calories here.
As a rough guide, many adults maintain their weight somewhere between 7,000 and 12,000 kJ a day (about 1,700-2,900 Calories).
Step 2: Choose A Sensible Deficit
To lose weight, you eat less energy than you use. Here are common approaches:
| Approach | Daily deficit | Expected loss (early weeks) |
|---|---|---|
| Gentle | ~1,000 kJ (250 kcal) | ~0.25 kg a week |
| Standard | ~2,100 kJ (500 kcal) | ~0.5 kg a week |
| Faster (supervised) | ~3,150 kJ (750 kcal) | ~0.75 kg a week |
For most people the standard deficit is a good balance: noticeable progress without extreme hunger.
Step 3: Check You’re Not Going Too Low
Very low intakes make it hard to get enough protein, fibre, vitamins and minerals, increase muscle loss, and are difficult to sustain. As a general rule, don’t go below about:
- 5,000 kJ (1,200 Calories) a day for women, or
- 6,300 kJ (1,500 Calories) a day for men,
unless you’re following a program supervised by a health professional.
Worked Examples
| Person | Maintenance | Weight-loss target | In Calories |
|---|---|---|---|
| Woman, 30, 165 cm, 75 kg, lightly active | ~8,460 kJ | ~6,370 kJ | ~1,520 kcal |
| Man, 45, 180 cm, 95 kg, moderately active | ~12,030 kJ | ~9,940 kJ | ~2,375 kcal |
| Woman, 60, 160 cm, 68 kg, sedentary | ~6,120 kJ | ~5,020 kJ | ~1,200 kcal |
Why Weight Loss Slows Down
The famous rule, that cutting 2,100 kJ a day loses 0.5 kg a week forever, is a useful starting point but not how bodies work long-term:
- Early loss includes water, especially if you cut carbohydrates, so the first week or two often looks faster.
- Your needs fall as you get lighter. A smaller body uses less energy, so the same intake creates a smaller deficit.
- Your body adapts. People often move a little less and their metabolism adjusts slightly.
Recalculate your target every 4-5 kg, and expect a gradual curve rather than a straight line.
Tips That Make A kJ Deficit Easier
- Prioritise protein. It’s filling and helps preserve muscle. Include a source at every meal.
- Fill up on low-energy-density foods. Vegetables, fruit, legumes and broth-based soups give volume for few kilojoules. See the food kJ chart.
- Watch liquid kilojoules. Soft drinks, juices, milky coffees and alcohol add up fast, a large latte and two glasses of wine can be over 1,800 kJ.
- Weigh energy-dense foods. Oils, nuts, cheese and cereals are easy to over-pour.
- Keep moving. Activity raises your maintenance intake and supports health, even if it’s a slow way to “burn off” food. Try the exercise kJ calculator.
- Sleep. Short sleep is linked to increased appetite.
- Track honestly for a few weeks. It’s the fastest way to learn where your kilojoules come from.
How To Tell If Your Target Is Right
After 2-3 weeks, look at your weight trend (the average of several weigh-ins, not one day):
- Losing about 0.25-1% of body weight a week? Your target is working.
- No change? Reduce by about 500-1,000 kJ a day, or check your tracking accuracy.
- Losing very fast, feeling exhausted or constantly hungry? Increase by 500-1,000 kJ.
Converting A Calorie-Based Plan To kJ
If your plan is written in Calories, multiply by 4.184:
| Calorie target | kJ target |
|---|---|
| 1,200 kcal | 5,021 kJ |
| 1,500 kcal | 6,276 kJ |
| 1,800 kcal | 7,531 kJ |
| 2,000 kcal | 8,368 kJ |
| 2,200 kcal | 9,205 kJ |
Or use the calories to kJ converter for any number.
FAQ
How many kJ a day to lose 0.5 kg a week?
About 2,100 kJ (500 Calories) a day less than your maintenance intake, at least in the early weeks.
Is 6,000 kJ a day enough to lose weight?
For many adults 6,000 kJ (about 1,430 Calories) creates a deficit, but for some smaller, older or sedentary people it may be close to maintenance. Estimate your maintenance first.
Can I eat 4,000 kJ a day?
4,000 kJ (about 960 Calories) is a very low intake and should only be used under medical supervision.
Should I count exercise kilojoules?
Either include your activity in your maintenance estimate or add a portion of exercise kilojoules back, not both. Trackers often overestimate exercise burn.