Calorie Deficit Explained — How It Works and How Big It Should Be
A calorie deficit is the gap between the energy your body uses and the energy you eat. If you use 2,500 kcal a day and eat 2,000 kcal, you are in a 500 kcal deficit — and your body makes up the difference from its stored energy, mostly body fat. That single idea explains almost all weight loss. It does not matter whether the deficit comes from eating less, moving more, or both; what matters is that the gap exists, that it is sustained, and that it is not so large that it costs you muscle, energy and health along with the fat.
Most people get the idea right and the size wrong. A deficit that is too small produces no visible change, which is discouraging. A deficit that is too large produces fast early results and then stalls, fatigue, hunger and muscle loss. This guide explains how to find your maintenance calories, how to choose a deficit size, and how to adjust it when — not if — your body adapts.
Use the calculators for your own numbers
Start with your maintenance calories using the TDEE Calculator or the Maintenance Calorie Calculator — they apply the Mifflin-St Jeor equation (Mifflin et al., American Journal of Clinical Nutrition, 1990) to your sex, age, height, weight and activity level. Then use the Weight Loss Calculator to see what different deficit sizes mean for your weekly rate of loss. The worked examples below use a 80 kg man, 178 cm, age 35, moderately active, so you can follow the maths and then substitute your own figures.
Related reading: How to Lose Weight Safely, How Fast Should You Lose Weight? Safe Rates, Explained, Weight Loss Plateau — Why It Happens and How to Break Through, Weight Loss Calculator, TDEE Calculator, full guides section
The formula, shown and cited
The maths of a deficit is simple arithmetic. First, basal metabolic rate — the energy you burn at complete rest — from Mifflin-St Jeor:
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
For our example — a man, 80 kg, 178 cm, age 35:
BMR = 10 × 80 + 6.25 × 178 − 5 × 35 + 5 = 800 + 1,112.5 − 175 + 5 = 1,742.5 kcal a day
Total daily energy expenditure multiplies BMR by an activity factor: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extremely active. At moderately active (1.55):
TDEE = 1,743 × 1.55 = 2,701 kcal a day
That is maintenance: eat 2,701 kcal and weight stays roughly steady. A deficit subtracts from that figure. A kilogram of body fat stores roughly 7,700 kcal of energy, so a daily deficit of 500 kcal — 3,500 kcal a week — corresponds to about 0.45 kg of fat a week in theory. In practice the figure is messier, because the body adapts and because weight loss is never pure fat, but the 7,700 kcal rule is a useful starting approximation.
| Daily deficit | Weekly deficit | Expected fat loss per week | Target intake (from 2,701 kcal maintenance) |
|---|---|---|---|
| 250 kcal | 1,750 kcal | ~0.2 kg | 2,451 kcal |
| 500 kcal | 3,500 kcal | ~0.45 kg | 2,201 kcal |
| 750 kcal | 5,250 kcal | ~0.7 kg | 1,951 kcal |
| 1,000 kcal | 7,000 kcal | ~0.9 kg | 1,701 kcal |
Notice what the table does not say: it does not say a 1,000 kcal deficit is twice as good as a 500 kcal one. It is twice as fast, on paper, but it is also twice as hungry, twice as tiring, and far more likely to cost muscle. For a man, the safety floor is 1,500 kcal a day; for a woman, 1,200 kcal a day. Our example at a 1,000 kcal deficit is still above the floor at 1,701 kcal, but a smaller or less active person hits the floor much sooner — a sedentary woman maintaining at 1,800 kcal cannot run a 750 kcal deficit without dropping below 1,200 kcal, and should not try.
How big should your deficit be?
For most people, 300–500 kcal a day is the sweet spot. It is large enough to produce visible change — roughly 0.3–0.5 kg a week — and small enough to sustain for months without constant hunger. It also leaves room to eat enough protein (1.6–2.2 g/kg while dieting, per Morton et al., 2018) and enough food volume to feel full, which is what actually determines whether a diet survives week six.
A 750 kcal deficit suits someone with more weight to lose and a higher maintenance intake, because the same percentage cut leaves more food on the plate. As a rule of thumb, think in percentages rather than absolute calories: a deficit of 15–20% of maintenance is moderate and sustainable; 25% is aggressive and should be time-limited; more than 25% is a crash diet, and it needs medical supervision. Our example's 500 kcal deficit is 18.5% of 2,701 kcal — right in the moderate band. A 1,000 kcal deficit would be 37% — aggressive, and not a good default.
There is one more consideration: the deficit should come from both sides of the equation. Cutting 500 kcal from food alone is harder than cutting 300 kcal from food and adding 200 kcal of activity — a daily walk, for example, burns roughly 200–300 kcal an hour at a brisk pace for most adults, and it preserves muscle in a way that dieting alone does not. Resistance training two or three times a week during a deficit is the single best way to make sure the weight you lose is fat rather than muscle.
Why the maths drifts — and how to adjust
After a few weeks in a deficit, weight loss slows even if you change nothing. This is not failure; it is adaptation. A lighter body burns fewer calories, because there is less of you to move and maintain. Appetite hormones shift, you move less without noticing, and your body becomes more efficient at the same activity. The combined effect is that a 500 kcal deficit can shrink to 200–300 kcal within a couple of months.
The fix is measurement, not willpower. Weigh yourself daily and look at the weekly average, not any single reading. If the average has been flat for two to three weeks, you have three options: cut another 100–200 kcal from food, add 100–200 kcal of activity, or hold steady and accept a slower rate. Do not cut by 500 kcal at a time — that is how a moderate diet becomes a crash diet. A diet break also helps: one to two weeks at maintenance calories, once every eight to twelve weeks, restores some of the hormonal adaptation and makes the next phase of dieting easier. It feels like going backwards; the evidence suggests it is one of the best tools for long-term success.
What a deficit does NOT tell you
A calorie deficit tells you that you are losing weight. It does not tell you what you are losing. Without enough protein and resistance training, a meaningful share of weight loss is muscle — and muscle is what keeps your metabolic rate up and your body functional. It also does not tell you about health: a deficit built on ultra-processed food can still produce weight loss while leaving you undernourished in fibre, vitamins and minerals. And it does not tell you about sustainability: a deficit you cannot live with for three months is a worse plan than a smaller one you can live with for six.
Frequently asked questions
How do I calculate my calorie deficit?
Find your maintenance calories first — the TDEE Calculator does this from your sex, age, height, weight and activity level. Then subtract 300–500 kcal for a moderate deficit. If your maintenance is 2,700 kcal, a 500 kcal deficit means eating about 2,200 kcal a day. Recalculate every few weeks as your weight changes.
Is a 500 calorie deficit safe?
For most healthy adults, yes. A 500 kcal deficit is roughly 15–20% of a typical maintenance intake and produces about 0.45 kg of fat loss a week. It only becomes unsafe if it pushes you below the safety floors — 1,200 kcal a day for women, 1,500 kcal a day for men — or if it is combined with very low protein or extreme exercise. If your maintenance is low, a 300 kcal deficit may be the safer choice.
Why am I not losing weight in a calorie deficit?
Three common reasons: the deficit is smaller than you think (portion sizes, cooking oils, drinks and snacks are routinely underestimated), your maintenance has dropped as you lost weight, or water retention is masking fat loss on the scales. Weigh daily, take the weekly average, track food honestly for a week, and if the average is flat for three weeks, cut another 100–200 kcal or add activity.
Can I lose weight without a calorie deficit?
No — not in any sustained way. Every effective diet works by creating a deficit, whether it does so by counting calories, cutting a food group, shrinking portions, or adding exercise. Diets that claim otherwise are either creating a deficit without saying so, or producing short-term water loss that looks like fat loss. The deficit is the mechanism; the diet is just the method of creating it.
How long should I stay in a calorie deficit?
As long as it takes to reach a healthy weight, in phases. A practical rhythm is eight to twelve weeks in a deficit followed by one to two weeks at maintenance, repeated until you reach your goal. Once you are there, return to maintenance calories and focus on holding the weight — maintenance is a separate skill from losing, and it deserves the same attention.
Does exercise count toward the deficit?
Yes, but count it conservatively. Activity trackers and gym machines routinely overestimate calorie burn by 20–30%. If you burn an estimated 300 kcal on a walk, treat it as 200 kcal when planning your food. And remember that exercise is worth doing during a deficit mainly because it preserves muscle — the calorie burn is a bonus, not the main event.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.