Calorie Deficit Calculator
Enter your TDEE (from the Calorie Calculator), your goal weight, and preferred rate of loss to see your daily calorie target and estimated time to reach your goal.
Reviewed by Richard Ross · Last updated April 2026
How Calorie Deficit Calculator works
Calorie deficit and fat loss
Body fat contains approximately 7,700 kilocalories per kilogram. To lose 0.5 kg of fat per week requires a daily deficit of 7,700 × 0.5 ÷ 7 ≈ 550 kcal. This is the theoretical value; actual fat loss also involves changes in water retention, glycogen stores, and metabolic adaptation, so real-world results vary. The direction of the relationship is reliable — sustained deficit leads to weight loss.
TDEE and where to find it
Total Daily Energy Expenditure (TDEE) is the total number of calories your body burns in a day, including activity. Use the Calorie Calculator on this site to estimate your TDEE from your age, sex, height, weight, and activity level. Note that TDEE is an estimate — if your actual weight does not change on a given calorie intake, that intake is your maintenance level.
Rate of loss and muscle preservation
Faster rates of loss (above 1 kg/week) increase the proportion of weight lost from muscle rather than fat, particularly without adequate protein and resistance training. A rate of 0.5–0.75 kg/week is generally considered the optimal range for fat loss while preserving muscle mass. Slower rates (0.25–0.5 kg/week) are more sustainable for most people and cause less metabolic adaptation.
Metabolic adaptation
The body adapts to sustained calorie deficits by reducing energy expenditure — a process sometimes called metabolic adaptation or "starvation mode." This is why the theoretical timeline based on a fixed deficit tends to underestimate actual time to goal in practice. Periodic diet breaks (returning to maintenance for 1–2 weeks) can help manage adaptation and improve long-term adherence.
Frequently asked questions
How many calories should I eat to lose 1 kg per week?
Losing 1 kg of fat per week requires a daily deficit of approximately 1,100 kcal (7,700 kcal/kg ÷ 7 days). For most people, this is a very aggressive target — eating only 1,000–1,200 kcal/day — which is hard to sustain and may lead to muscle loss and nutritional deficiencies. A target of 0.5 kg/week (550 kcal/day deficit) is more practical and sustainable.
What is the minimum number of calories I should eat?
The NHS and British Dietetic Association recommend not going below 1,200 kcal/day for women or 1,500 kcal/day for men without medical supervision. Very low calorie diets (VLCDs) below 800 kcal/day carry health risks and should only be undertaken under medical supervision.
Why is my weight loss slower than the calculation predicts?
Several factors slow actual weight loss: metabolic adaptation (the body reduces energy expenditure in response to deficit), water retention fluctuations, digestive system contents varying day to day, and individual variation in calorie absorption. Over weeks and months, the trend should follow the prediction, but daily and weekly fluctuations of 1–2 kg from water alone are normal.
Should I eat back exercise calories?
If your TDEE was calculated using an activity factor that accounts for your exercise, no. If your TDEE is sedentary baseline (and exercise is additional), then partially eating back exercise calories helps prevent an excessive deficit. Many people who "eat back" all exercise calories find it harder to lose weight because calorie burn estimates (from fitness trackers) tend to be overestimated.
How large a calorie deficit is safe?
A deficit of 500-750 kcal/day is generally considered safe for most adults, producing 0.5-0.75kg of fat loss per week. Larger deficits (750-1,000 kcal/day) accelerate loss but increase the risk of muscle loss, nutrient deficiencies, hormonal disruption, and rebound eating. Below about 1,200 kcal/day for women and 1,400 kcal/day for men, nutrient adequacy becomes difficult without supplementation. NICE recommends supervised VLCDs (800 kcal/day) only for people with obesity with associated health conditions.
Does metabolism slow down in a calorie deficit?
Yes — this is called metabolic adaptation. As you lose weight, your basal metabolic rate decreases because a smaller body requires less energy. Additional adaptive thermogenesis (beyond what weight loss would predict) also occurs: the body reduces non-exercise activity thermogenesis (NEAT — fidgeting, posture, walking). After significant weight loss, total daily energy expenditure may be 15-20% below what the same-sized person who never dieted would have. This "metabolic adaptation" is a key reason weight loss slows over time and maintaining loss is difficult.
How do I break a weight loss plateau?
Plateaus occur because as weight decreases, calorie requirements also decrease — so the same deficit that produced loss early on may now represent maintenance. To break a plateau: recalculate your TDEE based on current weight, reduce intake by another 100-200 kcal/day, or increase activity (adding 30 minutes of walking burns approximately 150 kcal/day). Diet breaks (eating at maintenance for 1-2 weeks) can reduce adaptive thermogenesis. Re-tracking food intake often reveals inadvertent increases in calorie consumption as discipline wanes.
Is it better to create a calorie deficit through diet or exercise?
Diet is generally more efficient for creating a calorie deficit — it is easier to not eat 500 calories than to burn 500 calories through exercise (which requires roughly 45-60 minutes of moderate effort). However, exercise preserves muscle mass during a deficit (particularly resistance training), improves insulin sensitivity, and provides cardiovascular and mental health benefits. The optimal approach combines moderate dietary restriction with regular exercise — neither extreme restriction alone nor exercise alone without dietary control tends to work long-term.
What is the difference between fat loss and weight loss?
Weight loss includes fat, water, glycogen (stored carbohydrate), and muscle. Fat loss specifically targets adipose tissue. Early rapid weight loss at the start of a diet is largely water and glycogen depletion, not fat. A kilogram of pure fat contains approximately 7,700 kcal. To lose 1kg of actual fat per week requires a 7,700 kcal deficit (1,100 kcal/day) — an aggressive target. Moderate deficits of 500 kcal/day lose approximately 0.5kg of fat per week, plus some water fluctuation.
How does calorie deficit interact with muscle building?
In a calorie deficit, the body preferentially uses stored energy for fuel, making it difficult to build muscle (which requires a net positive energy balance for synthesis). However, beginners to resistance training, people returning after a break, and those with significant body fat can achieve "body recomposition" — losing fat and gaining muscle simultaneously — even in a slight deficit, provided protein intake is high (2g/kg+) and progressive overload training is consistent. More advanced trainees generally need a surplus to build meaningful muscle.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.