Losing weight is ultimately physics: if you give your body less energy than it uses, you lose weight. The energy balance decides how much — and your food choices decide whether you stick with it: a reduced-carbohydrate diet and a calorie-reduced Mediterranean mixed diet do better in the long run than low-fat diets. But between this simple principle and everyday practice lie the decisive questions: how big should the calorie deficit be? How do you calculate your energy expenditure? And how do you make sure you lose fat — and not the muscle you worked so hard for? This guide takes you to the answer in three steps, with a formula, an example and the five most common mistakes.
What is a calorie deficit?
A calorie deficit arises when you eat fewer calories than your body uses. It takes the missing energy from its reserves — ideally from fat tissue. What calories actually are and how the energy balance works is explained in our fundamentals article "What actually are calories?" — here we go straight into practice.
Step 1: calculate your basal metabolic rate
Your basal metabolic rate (BMR) is the energy your body uses at complete rest — for organs, brain, breathing and muscle. Several estimation formulas are in common use: the simple rule of thumb based on body weight, Harris-Benedict and Mifflin-St Jeor. We're using Mifflin-St Jeor here; all of them deliver approximate values that you then calibrate against the scale.
- 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: woman, 70 kg, 168 cm, 35 years old: (700) + (1,050) − (175) − 161 = around 1,414 kcal basal metabolic rate.
Step 2: determine your total daily energy expenditure
On top of your BMR comes your activity. Multiply by the appropriate factor (PAL value):
- 1.2: exclusively sitting or lying down (bedridden, for example)
- 1.4 to 1.5: desk job, hardly any movement in your free time
- 1.5 to 1.6: sedentary work with periods of walking and standing
- 1.7 to 1.8: predominantly walking and standing work
- 1.9 to 2.0+: physically heavy work, competitive sport
Important: the PAL value describes your job and daily routine — not your training. You add regular training sessions on top of it (roughly 0.1 to 0.3 depending on volume) instead of simply jumping to the next level. This is exactly where most people misjudge things.
Our example person has an office job with some everyday movement (1.5) and trains three times a week (+0.1): 1,414 × 1.6 = around 2,260 kcal total daily energy expenditure. That is an estimate — the real calibration comes from the scale over two to three weeks.
Step 3: choose the right deficit
Now the key question: how much less? The answer determines whether you lose fat or muscle.
- Moderate (recommended): 300 to 500 kcal below your total expenditure. That works out at roughly 0.3 to 0.5 kg of weight loss per week — sustainable, muscle-sparing and workable in everyday life.
- Aggressive: 500 to 750 kcal. Only sensible for a limited period, with a high protein intake and consistent strength training.
- Crash diets under 1,200 to 1,500 kcal: these quickly lose water and muscle, slow down the metabolism through adaptations and statistically almost always end in the yo-yo effect.
Our example person: 2,260 − 400 = around 1,860 kcal per day as the target value.
Protecting your muscle: the two non-negotiables
A deficit tells your body: break something down. Two measures tell it WHAT to break down — namely fat instead of muscle:
- Keep protein high: 1.8 to 2.7 g per kilogram of body weight. When energy intake is low, the requirement rises considerably — that is the price of your body holding on to muscle despite the deficit. Our guide "How much protein per day?" shows you exactly how to work out your requirement.
- Keep up your strength training: two to three sessions per week with challenging weights signal to your body that the muscle is needed. Why this is so decisive is covered in our article "Strength training for weight loss".
When progress stalls: plateaus and adaptation
After a few weeks, every deficit gets smaller without you changing anything: you weigh less (lower expenditure), you move less without noticing, and your body works more efficiently. That's not an excuse from the scale, it's physiology. You'll find the solutions — from recalculating to diet breaks to a look at measurement errors — in our article "What to do when the scale won't budge?".
The 5 most common mistakes with a calorie deficit
- Starting too big. Start with a 1,000 kcal deficit and you'll last three weeks — and put it all back on. Moderate wins.
- Estimating calories instead of weighing them. Oil, snacks and drinks are chronically underestimated; 2 to 3 weeks of honest tracking calibrates your eye.
- Forgetting liquid calories. Sweet drinks, alcohol and milky coffees can wipe out half your deficit — more on this in our article on sugar and cravings.
- Only cardio, no strength training. Without a strength stimulus, the body preferentially breaks down muscle in a deficit — and your basal metabolic rate drops right along with it.
- Expecting to melt belly fat specifically. Your body decides for itself where it breaks down fat — what works instead is shown in our article "Losing belly fat".
A practical everyday lever against mistakes 2 and 3: prepare your meals. With meal prep you know what's in your food — and hunger-driven decisions in the evening no longer catch you out.
Course tip from update Akademie: Energy balance, macronutrients, behavior change — if you want to support people through weight loss on a solid footing, the Ernährungscoach programme at update Akademie gives you the complete toolkit: scientifically grounded, hands-on and part-time alongside your job.
Frequently asked questions about the calorie deficit
How many calories do I have to save to lose 1 kg of fat?
Around 7,000 kcal. With a daily deficit of 500 kcal, that corresponds to about two weeks — realistically planned rather than at crash speed.
How do I know whether my deficit is right?
From reality: weigh yourself several times a week under the same conditions and average the values. If the weekly average drops by 0.3 to 0.5 kg, it's right. If nothing drops over three weeks, the deficit is smaller than you think.
Do I have to count calories forever?
No. Tracking is a learning tool for a few weeks — after that, a feel for portions, a protein-rich basic structure and habits take over.
Can I build muscle in a calorie deficit?
Yes — losing fat and building muscle are compatible ("body recomposition"). In a deficit, however, building takes considerably longer, and a very high protein intake of 1.8 to 2.7 g/kg is essential so that no muscle is lost. The effect becomes visible fastest in beginners, returners and people who are significantly overweight; more advanced trainees need more patience.
Conclusion
Calculate your basal metabolic rate, assess your activity honestly, subtract 300 to 500 kcal — and your starting value is ready. The rest is craft: keep protein high, keep up the strength training, calibrate with the scale, and meet the plateau with a system instead of panic. That's how you lose fat, keep your muscle — and above all, keep the result.
