What Is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a given day. Your body must make up the energy shortfall from stored sources — primarily body fat. Over time, a sustained deficit leads to weight loss.
The math is straightforward: 1 pound of body fat contains approximately 3,500 calories of stored energy. A daily deficit of 500 calories therefore produces roughly 1 pound of fat loss per week (500 × 7 = 3,500). This "3,500 calorie rule" is a useful approximation, though real-world results vary based on body composition, hormones, and metabolic adaptation.
How to Calculate Your Calorie Deficit
The calorie deficit formula is: Daily calories = TDEE − deficit target
Your TDEE (Total Daily Energy Expenditure) is the total number of calories your body burns in a day, including all physical activity. This calculator estimates your TDEE using the Mifflin-St Jeor equation — the most validated formula for estimating BMR (Basal Metabolic Rate) — then multiplies by an activity factor.
Step 1 — BMR (Mifflin-St Jeor):
Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
Step 2 — TDEE: BMR × activity factor
Step 3 — Target: TDEE − 500 = 1 lb/week loss target
Safe Calorie Deficit: How Low Is Too Low?
The safest range for a calorie deficit is 250–500 calories below your TDEE. This produces 0.5–1 lb of weekly fat loss, which is sustainable and preserves muscle mass.
| Deficit Level | Cal/Day Cut | Weekly Loss | Risk Level |
|---|---|---|---|
| Mild | 250 cal | ~0.5 lb | Very low |
| Moderate | 500 cal | ~1 lb | Low |
| Aggressive | 750–1,000 cal | 1.5–2 lbs | Moderate |
| Very aggressive | >1,000 cal | >2 lbs | High — not recommended |
Why Your Deficit May Stall Over Time
As you lose weight, your TDEE decreases because your body weighs less and requires fewer calories to function. Additionally, the body undergoes metabolic adaptation (sometimes called "adaptive thermogenesis") — a survival mechanism where metabolism slows beyond what weight loss alone would predict. This is why calories need to be recalculated every 10–15 lbs lost.
Preserving muscle through adequate protein intake (0.7–1g per pound of body weight) and resistance training helps minimize metabolic adaptation and keeps your TDEE higher throughout the weight loss process.
Calorie Deficit vs. Intermittent Fasting
Intermittent fasting (IF) works for weight loss primarily by creating a calorie deficit — it does not have metabolic magic beyond that. When people lose weight on IF, it is because restricting eating windows makes it easier to eat fewer calories overall.
Both approaches work. The best method is the one you can sustain. Tracking calories with a set daily target gives more control and predictability; time-restricted eating can be easier for people who prefer not to count calories. Research shows similar weight loss outcomes when total calorie intake is matched.