A calorie deficit is the only mechanism that produces fat loss. Every diet on earth, keto, paleo, carnivore, intermittent fasting, works by creating one.
Numbers in this article are in kg. The calculator accepts both kg and lb. Quick conversion: 1 kg ≈ 2.2 lb; 1 kg of body fat ≈ 7,700 cal (1 lb ≈ 3,500 cal).
A calorie deficit means eating fewer calories than your body burns each day. The size of the deficit determines how fast you lose, but bigger isn't always better, go too aggressive and you lose muscle along with fat.
Skip the math: use the free Virtus Athlete Calorie Deficit Calculator to get your daily deficit, calorie target, protein range, and goal date in seconds. Same formulas explained below.
How to Calculate Your Deficit
Two steps:
Step 1: Find Your TDEE
Your TDEE (Total Daily Energy Expenditure) is what you burn per day at your current weight. The formula:
Mifflin-St Jeor BMR:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Multiply by activity:
- Sedentary (desk + no exercise): × 1.2
- Light (1-3 sessions/week): × 1.375
- Moderate (3-5 sessions/week): × 1.55
- Very active (6-7 sessions/week): × 1.725
Result = TDEE.
(For the full TDEE walkthrough, see the calorie calculator article.)
Step 2: Subtract Your Deficit
| Goal | Deficit Size | Expected Fat Loss (kg) | Expected Fat Loss (lb) |
|---|---|---|---|
| Slow & sustainable | TDEE − 250 | ~0.25 kg/week | ~0.5 lb/week |
| Standard cut | TDEE − 500 | ~0.5 kg/week | ~1.1 lb/week |
| Aggressive | TDEE − 750 | ~0.75 kg/week | ~1.6 lb/week |
| Extreme | TDEE − 1000 | ~1 kg/week | ~2.2 lb/week |
The math: 1 kg of body fat = ~7700 calories. A 500 cal/day deficit = 3500/week = roughly 0.5 kg/week of fat loss. Or drop your stats into the free calorie deficit calculator and skip the arithmetic.
The Sweet Spot: 0.5-1% of Bodyweight per Week
Pure number-of-calories deficits don't account for body size. A 500 cal deficit is huge for a 60 kg person and small for a 100 kg person.
Better target: lose 0.5-1% of your body weight per week.
| Bodyweight | 0.5%/week loss | 1%/week loss |
|---|---|---|
| 60 kg (132 lb) | 0.3 kg (0.7 lb) | 0.6 kg (1.3 lb) |
| 80 kg (176 lb) | 0.4 kg (0.9 lb) | 0.8 kg (1.8 lb) |
| 100 kg (220 lb) | 0.5 kg (1.1 lb) | 1.0 kg (2.2 lb) |
Slower than 0.5%/week → too small a deficit, you'll get bored. Faster than 1%/week → too big a deficit, you'll lose muscle.
Adjust Based on Real Results
The calculator is a starting point. Real-world feedback decides the final number.
- Eat your calculated calories for 2 weeks.
- Take a 7-day weight average each week.
- Compare week 1 vs week 2 average.
- Lost 0.5-1% of bodyweight? Stay where you are.
- Lost less? Drop another 100-150 calories.
- Lost more (or losing strength)? Add 100-150 calories.
After 5-10 kg of weight loss, recalculate your TDEE. As body mass drops, BMR drops with it. A target that worked at 95 kg will overshoot at 85 kg.
Why Bigger Deficits Backfire
A 1500-calorie/day deficit feels efficient on paper. In practice, it leads to:
- Muscle loss. Without enough food, your body breaks down muscle for energy. You end up smaller, not leaner.
- Hunger spikes that break adherence. A diet you can't sustain produces zero results.
- Hormonal disruption. Testosterone, thyroid, and leptin drop sharply on aggressive deficits, slowing metabolism.
- Weakness in training. Your lifts go down, which reduces muscle retention further.
Most successful fat-loss phases use a 300-600 calorie deficit, not 1000+.
Protein Is Non-Negotiable in a Deficit
The single most important variable in a cut is protein intake. Adequate protein (1.6-2.2 g/kg bodyweight) lets you lose mostly fat, not muscle.
| Bodyweight | Daily Protein |
|---|---|
| 60 kg (132 lb) | 96-132 g |
| 70 kg (154 lb) | 112-154 g |
| 80 kg (176 lb) | 128-176 g |
| 90 kg (198 lb) | 144-198 g |
| 100 kg (220 lb) | 160-220 g |
Below 1.6 g/kg, muscle loss accelerates regardless of how good the rest of your diet looks.
Looking for a workout tracker?
If you want to make real progress and build discipline in the gym, use Virtus Athlete. Free on iOS and Android.
