What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a given period. When this happens, your body meets its energy needs by drawing on stored energy — primarily body fat, but also some muscle mass if protein intake and resistance training are insufficient.
The commonly cited estimate is that 1 kg of body fat stores approximately 7,700 calories (3,500 kcal per pound). So a deficit of 500 calories per day would theoretically produce approximately 0.5 kg of fat loss per week. In practice, the relationship is less linear — the body adapts metabolically to sustained deficits — but it's a useful working model for planning.
How to calculate your calorie deficit: step by step
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Sedentary (desk job, little exercise): × 1.2
Lightly active (light exercise 1–3 days/week): × 1.375
Moderately active (moderate exercise 3–5 days/week): × 1.55
Very active (hard exercise 6–7 days/week): × 1.725
Extremely active (very hard exercise + physical job): × 1.9
Mild deficit (250 cal/day): ~0.25 kg loss per week. Best for people close to goal weight or those who want to minimise muscle loss.
Moderate deficit (500 cal/day): ~0.5 kg loss per week. The most common recommendation — effective and sustainable for most people.
Larger deficit (750+ cal/day): Faster loss but higher risk of muscle loss, fatigue, and metabolic adaptation. Not recommended without medical oversight.
Example: TDEE = 2,200 calories. Deficit = 500 calories. Daily target = 1,700 calories.
Important: Never eat below your BMR without medical supervision. For most women, a practical floor is 1,200 calories; for most men, 1,500 calories.
Calculate your exact calorie deficit target — including safe rate and protein needs.
Calorie Deficit Calculator →Worked example
Example: Sarah, 35F, 70kg, 165cm, moderately active
Step 1 — BMR: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 700 + 1031.25 − 175 − 161 = 1,395 calories
Step 2 — TDEE: 1,395 × 1.55 (moderately active) = 2,162 calories
Step 3 — Deficit: 500 calories for ~0.5 kg/week loss
Step 4 — Daily target: 2,162 − 500 = 1,662 calories per day
Protein target: 70 kg × 1.8g = 126g protein per day to preserve muscle
How big should your calorie deficit be?
| Deficit size | Expected fat loss | Best for |
|---|---|---|
| 250 cal/day | ~0.25 kg/week | Near goal weight, minimising muscle loss |
| 500 cal/day | ~0.5 kg/week | Most people — best balance of speed and sustainability |
| 750 cal/day | ~0.75 kg/week | Higher starting body fat, short-term aggressive phase |
| 1000+ cal/day | ~1 kg/week | Medical supervision only — high muscle loss risk |
Protein: the most important variable in a deficit
During a calorie deficit, the body needs adequate protein to preserve lean muscle mass. Without sufficient protein, a significant proportion of weight lost comes from muscle rather than fat — worsening body composition even if the scale moves.
Target 1.6–2.2g of protein per kg of bodyweight while in a deficit. For someone weighing 70kg, this means 112–154g of protein per day. This is higher than standard dietary recommendations (0.8g/kg) precisely because of the muscle-protective role of protein during calorie restriction.
Protein keeps you fuller for longer
Protein has a higher thermic effect than fat or carbohydrates (your body burns more calories digesting it), and it is the most satiating macronutrient. High-protein eating in a deficit is associated with greater fat loss, less muscle loss, and better adherence than lower-protein approaches.
Common calorie deficit mistakes
- Underestimating calorie intake: Research consistently shows people underestimate how much they eat by 20–40%. Weighing food with a kitchen scale — rather than estimating portions — typically reveals the source of a stall.
- Overestimating exercise calories burned: Fitness trackers and cardio machines notoriously overestimate calorie burn. "Eating back" exercise calories based on these estimates often eliminates the deficit entirely.
- Setting the deficit too aggressively: Deficits larger than 500–750 calories per day increase muscle loss, cause fatigue, and are hard to sustain. Slower, consistent progress beats fast starts followed by rebounding.
- Not adjusting over time: As you lose weight, your TDEE decreases (a lighter body burns fewer calories). Recalculate your TDEE every 4–6 weeks or when weight loss stalls for 2+ weeks.
- Insufficient protein: Low protein intake in a deficit leads to disproportionate muscle loss — making you lighter but not leaner. Prioritise protein at every meal.
- Ignoring sleep and stress: Poor sleep and chronic stress elevate cortisol, which promotes muscle breakdown and fat storage — working against the deficit. Fat loss is a whole-body process.
How long to stay in a calorie deficit
Extended continuous calorie restriction leads to metabolic adaptation — the body lowers TDEE in response to sustained undereating, making the same deficit produce less fat loss over time. This is why results often slow after the initial weeks.
Most people do best with structured deficit periods of 8–12 weeks, followed by a maintenance phase of 4–6 weeks at TDEE. This allows metabolic rate to partially recover before the next deficit phase. The total fat loss across alternating deficit and maintenance phases often exceeds what continuous dieting achieves.