What Is a Calorie Deficit and How Big Should It Be?
What Is a Calorie Deficit?
A calorie deficit occurs when you consistently consume fewer calories than your body burns (your TDEE). This energy shortfall forces your body to draw on stored energy reserves — primarily body fat — to meet its metabolic needs. Over time, sustained caloric restriction with adequate protein and resistance training results in fat loss while preserving lean muscle mass.
The 3,500 Calorie Rule
The traditional estimate is that one pound (0.45kg) of body fat contains approximately 3,500 calories. Therefore, a daily deficit of 500 calories should theoretically produce 0.45kg of fat loss per week. This is a useful rule of thumb for planning, but real-world fat loss is more complex: metabolic adaptation, glycogen and water changes, muscle gain, and individual variation all affect observed weight loss.
How Big Should Your Calorie Deficit Be?
| Deficit Size | Daily Calories Below TDEE | Weekly Fat Loss | Best For |
|---|---|---|---|
| Mild | 250 kcal | ~0.25 kg | Long-term, minimal muscle risk |
| Moderate | 500 kcal | ~0.5 kg | Standard approach, recommended for most |
| Aggressive | 750 kcal | ~0.75 kg | Short-term, higher muscle loss risk |
| Maximum | 1,000 kcal | ~1 kg | Short-term only, medical supervision advised |
The most evidence-supported approach for healthy adults is a 500 calorie daily deficit. This produces meaningful progress (approximately 2kg per month), is sustainable long-term, and minimises muscle loss when combined with adequate protein intake and resistance training.
The Problem with Very Large Deficits
Deficits above 750–1,000 calories per day trigger progressive metabolic adaptation: your body reduces BMR, decreases NEAT, lowers thyroid output, and increases hunger hormones (ghrelin). A 2016 study of The Biggest Loser contestants found that six years after the competition, their metabolic rates were still significantly lower than expected for their body size — a consequence of the extreme deficits used during the show.
The sweet spot: A 20–25% calorie deficit below your TDEE balances meaningful fat loss with muscle preservation and sustainable metabolic rate. This equates to approximately 400–600 calories below maintenance for most adults.
Frequently Asked Questions
References & Sources
- Hall, K.D. et al. (2016). Persistent metabolic adaptation 6 years after 'The Biggest Loser' competition. Obesity, 24(8), 1612–1619.
- Dulloo, A.G. et al. (1997). Postreduction lean body mass, resting metabolic rate. American Journal of Clinical Nutrition, 65(3), 717–723.
- Stiegler, P. & Cunliffe, A. (2006). The role of diet and exercise for the maintenance of fat-free mass. Sports Medicine, 36(3), 239–262.