Your body burns a certain number of calories every day just to exist - breathing, circulating blood, maintaining body temperature, thinking. This is your total daily energy expenditure (TDEE). Add physical activity and digestion, and you get your full daily calorie burn.
A calorie deficit means eating fewer calories than that number. The gap has to come from somewhere, and the body primarily fills it by burning stored body fat.
How a Calorie Deficit Causes Weight Loss
Fat cells store energy in the form of triglycerides. When calorie intake falls short of expenditure, the body releases stored triglycerides, breaks them down into fatty acids and glycerol, and burns them for energy. This is fat loss.
One pound (0.45kg) of fat contains approximately 3,500 calories. A consistent 500-calorie daily deficit produces roughly 0.5kg of weight loss per week. This is an approximation - the relationship isn't perfectly linear because the body adapts - but it's a reliable framework.
A 1,000-calorie daily deficit theoretically produces 1kg of weight loss per week. In practice, sustained deficits this large are difficult to maintain and trigger significant hunger, metabolic adaptation, and muscle loss.
How to Calculate Your Calorie Deficit
Step 1: Estimate your TDEE
Your TDEE includes:
- Basal metabolic rate (BMR) - calories burned at rest
- Thermic effect of food - calories burned digesting food (roughly 10% of intake)
- Physical activity - calories burned moving
Online TDEE calculators give a reasonable estimate. For a sedentary 70kg adult, TDEE is roughly 1,800-2,000 calories. For an active adult of the same weight, it's 2,400-2,800.
Step 2: Subtract to create a deficit
A moderate deficit is 300-500 calories below TDEE. This is enough to produce steady fat loss while minimising hunger, metabolic adaptation, and muscle loss.
A 200-300 calorie deficit is smaller but more sustainable long-term with less noticeable hunger. Useful for maintaining weight loss after reaching a goal.
What Makes a Safe Calorie Deficit?
Deficits below 1,000-1,200 calories per day (for women) or 1,500 calories per day (for men) trigger significant muscle loss alongside fat loss, cause nutritional deficiencies, and produce the strongest metabolic adaptation responses - where the body lowers energy expenditure to compensate.
Crash diets with very large deficits produce fast initial results and then plateau sharply as the body adapts. They also produce high rates of weight regain after stopping.
The most effective deficits are moderate (300-500 calories), sustained for months rather than weeks, and paired with adequate protein intake (1.6-2.0g/kg) to preserve muscle mass.
Why You Might Be in a Deficit and Not Losing Weight
Metabolic adaptation means the TDEE decreases as you lose weight. A deficit calculated at your starting weight becomes smaller as you become lighter. This is normal and explains why weight loss slows over time even with consistent eating.
Weighing daily and tracking weekly averages (rather than daily numbers) is the most reliable way to confirm whether a deficit is active.
Key Takeaway
A calorie deficit is the fundamental mechanism behind all fat loss. The quality of food, macronutrient ratios, and meal timing all matter - but they all operate within the calorie balance framework. Sustainable weight loss requires a consistent but moderate deficit, not the largest possible one.