What Is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a day. Your Total Daily Energy Expenditure (TDEE) represents the total calories you burn — accounting for your resting metabolic rate and all physical activity. When your calorie intake falls below this number, your body turns to stored fat for energy, resulting in gradual weight loss.
This is the fundamental mechanism behind every science-based weight-loss approach. No matter the dietary philosophy — low-carb, intermittent fasting, Mediterranean, or calorie counting — the underlying driver of fat loss is always a sustained calorie deficit.
How to Calculate a Calorie Deficit
The formula is straightforward: Daily Calorie Target = TDEE − Daily Deficit. Your daily deficit is determined by your target weekly loss rate. Since 1 kg of body fat contains approximately 7,700 kcal of stored energy, a daily deficit of 550 kcal produces roughly 0.5 kg of fat loss per week (550 × 7 = 3,850 kcal ≈ half a kilogram).
Before using this calculator, you need your TDEE. If you don't have it yet, use our TDEE Calculator first — it takes your age, sex, height, weight, and activity level to give you a personalised maintenance calorie figure. Your BMR Calculator result is the floor below which you should never eat.
What Is a Safe Calorie Deficit?
A deficit of 250–500 kcal per day is widely considered the sustainable, safe range for most people. This produces 0.25–0.5 kg of weight loss per week — slow enough to preserve muscle mass and avoid metabolic adaptation. A deficit of 500–750 kcal (0.5–0.75 kg/week) is appropriate for individuals with a higher body fat percentage. Deficits exceeding 1,000 kcal per day are associated with muscle loss, nutrient deficiencies, fatigue, and metabolic slowdown — even when short-term weight loss appears rapid.
How to Use Your Calorie Target
Your daily calorie target is the number you aim to hit each day through food and drink. To use it effectively:
- Track your food intake using a nutrition app for 2–4 weeks to build awareness.
- Prioritise protein (1.6–2.2 g/kg of body weight) to preserve muscle. Use the Protein Calculator to find your exact target.
- Fill remaining calories with fibre-rich carbohydrates and healthy fats. The Macro Calculator can distribute your calorie target across all three macronutrients.
- Weigh yourself at the same time each morning (after waking, before eating) and track a weekly average — not individual daily readings.
- Recalculate your TDEE and deficit every 4–6 weeks as your weight changes.
Why Does a Calorie Deficit Sometimes Stop Working?
As you lose weight, your TDEE decreases — your body is lighter and requires fewer calories. Additionally, prolonged restriction triggers adaptive thermogenesis: the body reduces its energy expenditure beyond what body weight alone would predict. This is the physiological basis of the "weight loss plateau" many people experience after 8–12 weeks.
The solution is not to reduce calories further indefinitely. Periodic diet breaks (eating at maintenance for 1–2 weeks) and refeed days (eating at TDEE once per week) can partially reverse metabolic adaptation. Resistance training throughout your cut is the most important factor for preserving metabolic rate and muscle mass during a deficit.
Calorie Deficit vs. Calorie Restriction: What's the Difference?
A calorie deficit is a precise, calculated shortfall relative to your personal TDEE. "Calorie restriction" often refers to arbitrary cuts without reference to individual maintenance needs. A 1,200 kcal diet is a deficit for a sedentary 50 kg woman but extreme under-eating for an active 90 kg man whose TDEE is 3,200 kcal. This calculator personalises the deficit to your specific TDEE and goal — it does not prescribe a generic low-calorie diet.
Scientific basis: Hall KD et al. "Quantification of the effect of energy imbalance on bodyweight." The Lancet. 2011;378(9793):826–837. This study validated the 7,700 kcal/kg fat tissue approximation used in this calculator.
For a deeper understanding of how calories, protein, carbs, and fat work together to support your goals, read our complete Nutrition Guide — covering macronutrients, hydration, and goal-specific calorie strategies in one place.