Source Check and Safety Boundaries
Source-reviewed on 2026-05-31. The calculator uses Mifflin-St Jeor for BMR, an activity multiplier for TDEE, and a deficit target for planning. NIDDK's Body Weight Planner is the stronger reference when you need dynamic metabolic modeling over time; Calorique keeps the daily target easy to understand and updates the estimate as your weight changes.
- • CDC frames gradual, steady weight loss around 1-2 pounds per week for many adults.
- • NIDDK emphasizes personalized calorie and activity planning rather than one fixed deficit for everyone.
- • Use qualified guidance for pregnancy, breastfeeding, diabetes medication, GLP-1 medication, eating-disorder history, underweight status, bariatric surgery, kidney disease, rapid unexplained weight loss, or concerning symptoms.
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 common shortcut is straightforward: a daily deficit of 500 calories equals 3,500 calories per week, which is often used as a rough 1-pound-per-week planning estimate. Real-world results vary because scale weight includes water, glycogen, digestion, lean tissue, activity changes, adherence, 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?
A practical starting range for many adults is 250-500 calories below your TDEE. This often maps to about 0.5-1 lb of weekly weight loss, but the safest target depends on body size, current weight, training load, hunger, sleep, medication, and medical history.
| 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, resistance training, and sleep can help reduce lean-mass loss during a deficit. Use the Protein Calculator and Macro Calculator if the daily calorie target needs to become a complete nutrition plan.
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.