How to calculate a calorie deficit correctly for weight loss: where the limits lie

What to expect in this article

1. Why there is no evidence-based deficit figure
2. The documented ranges
3. The lower limit and why it exists
4. What happens to muscle mass and basal metabolic rate
5. Moderate, severe, or crash dieting—the comparison
6. The yo-yo effect and its causes
7. How to determine your deficit
8. When medical support is appropriate
9. What ultimately matters
Frequently asked questions
Sources

Why there is no evidence-based deficit figure

Anyone searching for “calculate calorie deficit” gets almost the same answer everywhere: 500 kilocalories below your needs equals half a kilogram per week. The calculation is intuitive—and it does not appear in the sources used here.

It is noteworthy how consistently the institutions reframe the question. They do not talk about deficit kilocalories, but about kilograms per month, percentages of starting weight, and minimum calorie levels. This is not an evasion, but information about what can reasonably be said in general terms.

Why a blanket figure is problematic

A deficit of 500 kilocalories means something different for someone whose needs are 3,000 kilocalories than for someone whose needs are 1,800. In the first case, 2,500 kilocalories remain; in the second, 1,300—and that is close to the threshold above which, according to the consumer advice center, medical supervision is required.

That is precisely why the documented figures are expressed in relative terms. The consumer advice center writes: Depending on your starting weight, you can lose 500 grams to a maximum of 1 kilogram per week. IQWiG gives a percentage figure—depending on starting weight, doctors recommend losing 5 to 10 percent of your weight within 6 to 12 months.

For you, this means that instead of adopting a number, you determine it based on your requirements, your target rate, and the lower limit. How to do this specifically is explained in Chapter 7.

A second reason is more practical. A deficit cannot be measured directly; it can only be estimated from an estimated requirement and an estimated intake. Two estimates on top of each other do not produce precision, no matter how exact the number between them may look. A rate, by contrast, can be read from the scales.

That is precisely why institutions prefer to discuss outcomes rather than levers. Kilograms per month can be verified. A calorie deficit is an assumption based on two quantities, both of which are uncertain.

The orders of magnitude supported by evidence

Three figures define the framework within which every sensible calculation operates. They come from three different institutions and describe the same subject from different angles.

Sources: Federal Centre for Nutrition (BZfE), 2026 · Institute for Quality and Efficiency in Health Care (IQWiG), 2022 · Consumer advice center, 2025.

Compare the first two figures, and they fit together. Losing one to two kilograms per month amounts to 12 to 24 kilograms over twelve months—for a starting weight of 100 kilograms, the lower half of this range falls exactly within the 5 to 10 percent range cited by IQWiG.

The third figure works in the opposite direction. It says nothing about how quickly you should lose weight, but where self-management ends. A plan that brings you below these values is not an ambitious plan, but one that should be medically supervised.

The interaction of the three figures yields a useful sequence. First, check how much leeway there is between your requirements and the lower limit. Only then decide how much of this leeway to use—not the other way around, by setting a desired rate and seeing what remains.

For people with high requirements, this leeway is large; for people with low requirements, it is small. That is why the same target rate is more demanding for one person than for another—and why one reaches it while the other fails, even though both are doing the same thing.

The lower limit and why it exists

The clearest statement on the lower limit comes from the consumer advice center: For programs lasting several months, or with very low daily energy intakes—less than 1,200 kilocalories for women and less than 1,500 for men—continuous medical supervision is necessary.

The same source is even more explicit at the next level: Diet programs recommending an energy intake below 1,000 kilocalories are barely sustainable because of the intense hunger, and nutrient deficiencies are effectively inevitable.

What happens with extremely one-sided diets

The Verbraucherzentrale is specific about crash diets. The extremely one-sided diet can lead to deficiency symptoms after some time; health damage cannot be ruled out, especially with diets containing very little protein, even if they last only a short time—that is, no more than four weeks.

Then there are the everyday consequences that many people underestimate: During this time, people may find it harder to concentrate, be less capable, and feel tired more often, and they may experience headaches and circulatory problems. This is not a side effect to be accepted—it is why such phases are rarely sustained.

What is remarkable about this list is that all the complaints mentioned work in the same direction. Anyone who is tired and less capable will move less—and thereby reduce precisely the part of energy expenditure that a very large deficit was intended to target. At this point, the calculation works against itself.

For formula diets, the DGE specifies its own time limit: Use lasting more than three weeks should take place under medical supervision. This, too, is a lower limit—only a time limit rather than a calorie limit.

What happens to muscle mass and basal metabolic rate

This is the mechanism that makes every weight loss effort more difficult over time—and IQWiG describes it clearly. A person’s resting metabolic rate depends primarily on the proportion of muscle mass in their body weight. On average, resting metabolism accounts for around 70 percent of total calorie requirements.

And further: Weight loss also leads to a reduction in muscle mass. As a result, the body needs less energy, meaning fewer calories. According to IQWiG, this means that the more weight you lose, the harder it becomes to maintain the weight you have reached or lose even more.

This chain is the real answer to why a calculated deficit does not remain accurate indefinitely. The calculation is not wrong—the baseline value changes. Anyone who continues calculating after three months using the same value as at the beginning is working with a level of need that no longer exists.

A figure with an explicit caveat

Only one figure can be found on how much the basal metabolic rate can decrease—and it must be reproduced with its attribution. The Verbraucherzentrale writes that repeated crash diets can permanently reduce the basal metabolic rate by 10 to 40 percent. It cites no individual source for this, and none of the other institutions examined gives a comparable figure.

That is why it appears here as a statement by the Verbraucherzentrale, not as a consensus. As for the direction, it aligns with the IQWiG mechanism; as for the magnitude, it should not be treated as an established figure. The difference between “documented” and “mentioned by one source” is significant enough here to spell out.

And a note about an obvious potential countermeasure: The DGE, IQWiG, BZfE, and Verbraucherzentrale do not recommend strength training or increased protein intake specifically during weight loss. The DGE even explicitly classifies the evidence on the role of protein in a calorie-restricted diet as unclear. Anyone expecting a clear statement here will not find one in these sources.

This does not mean that exercise has no benefits—only that the rationale is different from preserving muscle. Its effect on weight is documented: According to the IQWiG, after twelve months, physically active participants had lost just under two kilograms more than those who only changed their diet. Their activity consisted of 30 to 45 minutes of mildly strenuous exercise, such as brisk walking, 3 to 5 times per week.

In the same breath, the IQWiG qualifies this: More physical activity alone is generally not enough to lose sufficient weight. Exercise therefore shifts the balance; it does not replace it—and, incidentally, it increases the activity factor you use to calculate your requirements.

Moderate, substantial, or crash—the comparison

The following comparison organizes three approaches according to the same criteria. It does not replace individual advice, but it makes clear what happens at each point.

Information according to the Federal Centre for Nutrition (BZfE), the Institute for Quality and Efficiency in Health Care (IQWiG), and the consumer advice center. The column headings “moderate reduction” and “substantial reduction” are editorial organizational terms, not categories used by the institutions mentioned—none of them specifies a figure for the size of a reduction.

The yo-yo effect and its causes

IQWiG defines it succinctly: Many diets have no long-term success or even cause a yo-yo effect—in that case, weight is higher some time after the diet than before. The DGE describes the same pattern from the perspective of those affected: After the diet, they gain weight again just as quickly, sometimes even more than they had lost, as soon as they return to their old eating habits.

Both descriptions contain the same underlying cause: Eating habits have not changed. The DGE makes this clear with formula diets—because taking them alone does not bring about a change in diet, people must be warned about the risk of a yo-yo effect.

The BZfE adds a newer explanatory approach from research: According to this theory, fat cells remember the overweight state and can more easily be returned to it; molecular changes persist even after substantial weight loss, which could promote the yo-yo effect. The BZfE presents these findings from the studies as a report—it is an approach, not a conclusive explanation.

In practice, this changes little about the consequence, which the DGE states matter-of-factly: Frequent weight fluctuations disrupt metabolism, hormones, and the psyche; therefore, slow weight loss in stages is healthier and more promising in the long term.

Why the phase afterward is more difficult

IQWiG sums it up in one sentence: Permanently maintaining weight loss is usually harder than losing the weight in the first place. This is an uncomfortable message for anyone planning a diet as a project with an end date.

The consumer advice center draws a requirement for programs from this: For long-term success, it is important to continue receiving support after the program ends; a program should be clearly divided into a weight-loss phase, a transition phase, and an aftercare phase. Those who work without a program simply plan these three phases themselves.

In practice, this means that the number guiding you while you lose weight is not the number you continue with afterward. After the reduction phase, your intake rises again to meet your—by then lower—requirements. Consciously managing this transition is the part most plans leave out.

The BZfE puts the conclusion in practical terms: Usually, only a healthier diet maintained over the long term and more exercise lead to weight loss—and once you reach your comfortable weight, you need to maintain it by sticking to your new habits.

How to set your calorie deficit

The established figures can be used to create an approach that does without an invented number. It consists of three perspectives: from above, from below, and over time.

Your total energy requirement sets the upper limit. Our article How Many Calories a Day to Lose Weight explains in detail how to calculate it from your resting energy expenditure and PAL value. This figure is the starting point from which the reduction is subtracted.

The lower limit comes from below: 1,200 kilocalories for women and 1,500 for men. Choose the reduction so that you remain well above it—not just barely above. A figure that comes close to the limit leaves no room for poorly estimated portions.

And over time, the adjustment follows. After four weeks, check whether the rate matches 1 to 2 kilograms per month. If you are significantly below that, the reduction was too small or your needs were overestimated. If you are significantly above it, the reduction was too large—and that is not a success but a reason to make an adjustment.

What the calculation leaves open

A fixed calorie reduction answers the question of how much. It does not answer what you fill the remaining energy with—or whether a carbohydrate-focused or fat-focused selection is easier for you. This question cannot be derived from your energy needs.

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When medical supervision is part of the process

The German Consumer Advice Centre makes a general recommendation that comes before any calculation: Ideally, before starting to lose weight, one should undergo a medical examination to rule out any potential risks.

The guidance becomes more binding at the limits already mentioned—under 1,200 or 1,500 kilocalories per day, respectively, and for formula diets lasting more than three weeks. For weight-loss programs, the German Consumer Advice Centre also gives a guideline of a body mass index above 25 and points out that only doctors specializing in nutrition, nutrition scientists, ecotrophologists, dietitians, and psychologists have appropriate training.

Older people require a separate assessment. Crash diets, fasting cures, and slimming products are particularly not recommended for older people; depending on the starting weight, the German Consumer Advice Centre gives a weight loss of around 0.5 kilograms per month as a realistic goal.

And finally, a situation that requires no number: When food and weight have become an issue that determines everyday life, the question of the right deficit is the wrong question. In that case, the issue belongs in professional support, regardless of what the calculation shows.

This final situation is the only one in this article for which there is no number—and that is not a shortcoming of the sources, but inherent to the issue. Whether food and weight are taking up too much space cannot be determined from kilocalories, but only by how much attention the topic occupies in everyday life. Anyone who asks themselves this question has usually already answered it.

What ultimately matters

The figure most people are looking for does not exist in an evidence-based form. What does exist is a range: capped at the top by a rate of 1 to 2 kilograms per month and at the bottom by 1,200 and 1,500 kilocalories per day, respectively.

Within this range, it is not mathematics but sustainability that determines the outcome. The mechanism described by IQWiG—less weight, less muscle mass, lower requirements—means that every additional loss becomes more difficult over time. A larger deficit will not help; what helps is a figure you can still stick to in the fourth month.

Your specific next step: Calculate what a reduction of 500 kilocalories would actually leave you with based on your needs, and compare the result with 1,200 and 1,500 kilocalories, respectively. If it is just above or below those values, the flat figure is simply wrong for you—and now you know what to use as your guide instead: a rate you can check on the scales and a limit you should not fall below.

Frequently asked questions

Sources

1. Federal Centre for Nutrition (BZfE): The path to a comfortable weight, as of 2026 — bzfe.de
2. Institute for Quality and Efficiency in Health Care (IQWiG): Causes of obesity, as well as weight-loss programs and medications, as of 2022 — gesundheitsinformation.de
3. Consumer Advice Centre: Crash diets, as of 2022, as well as weight-loss programs – what you should consider when choosing, as of 2025 — verbraucherzentrale.de
4. German Society for Nutrition (DGE): Diets and fasting, as well as formula diets (DGEinfo 4/2018) — dge.de
5. German Nutrition Society (DGE): How Does Protein Intake Affect Our Weight? and DGE Protein Guideline — dge.de
6. Federal Centre for Nutrition (BZfE): The Biology Behind the Yo-Yo Effect, as of 2024 — bzfe.de

The figure of 1 to 2 kilograms per month comes from source [1]. The definition of the yo-yo effect, the information on resting metabolic rate, muscle mass, and the approximately 70 percent share of calorie requirements, the 5 to 10 percent in 6 to 12 months, and the nearly two kilograms through physical activity come from source [2]. All information on lower limits (1,200 and 1,500 kilocalories, below 1,000 kilocalories), crash diets, the BMI threshold of 25, suitable healthcare professionals, older people, and the figure of “10 to 40 percent” for basal metabolic rate comes from source [3]; the latter appears there without an individual citation and is expressly attributed to this source in the text. The requirements for structuring a program into weight-loss, transition, and follow-up phases also come from source [3]. The statement that maintaining weight is more difficult than losing it, as well as the amount of exercise in the studies cited, comes from source [2]; the statement on a healthier diet in the long term and maintaining the new habits comes from source [1]. The statements on formula diets, including the three-week limit, and on gradual weight loss in stages come from source [4]. The assessment that the evidence on protein in a calorie-restricted diet is unclear comes from source [5]. The cited research approach concerning fat cells and the yo-yo effect comes from source [6]. None of the six sources specifies a figure for the size of a daily calorie deficit; the article therefore deliberately does not provide one. Product information comes from the mybody®x product pages, accessed on August 5, 2026.

mybody®x Editorial & Expert Team

Energy metabolism Nutritional science Nutrigenetics

This article was created and medically reviewed by the mybody®x editorial and expert team. The team combines expertise in nutrigenetics, microbiome and gut science, blood analysis interpretation, nutritional science, and laboratory diagnostics.

Published on August 5, 2026 · Last updated on August 5, 2026

Medical note: This article is for general information only and does not replace medical advice, diagnosis, or treatment. Energy intake below the stated limits should be medically supervised. In the case of existing medical conditions, during pregnancy and breastfeeding, in older age, and with a history of eating disorders, weight loss should be medically supervised.