A calorie deficit = eating fewer calories than you burn. A deficit of 300–500 calories below your TDEE produces 0.5–1 lb of fat loss per week while preserving muscle. Calculate your TDEE first, subtract 300–500, and track your intake accurately — most people underestimate how much they eat by 20–40%.
Fat loss has exactly one non-negotiable requirement: a sustained calorie deficit. Every diet that has ever worked — keto, intermittent fasting, low-fat, high-protein — has worked because it helped people eat fewer calories than they burned. The mechanism is always the same. The packaging changes.
Understanding the calorie deficit lets you cut through every diet trend and design a sustainable approach that actually fits your life.
What Is a Calorie Deficit?
Your body burns a certain number of calories every day just to stay alive and function — your Basal Metabolic Rate (BMR). Add the calories burned through movement and exercise and you get your Total Daily Energy Expenditure (TDEE).
When you eat fewer calories than your TDEE, your body must find energy from somewhere else — it draws on stored fat. This is a calorie deficit, and it's the direct mechanism of fat loss. When you eat more than your TDEE, the surplus is stored — primarily as fat.
3,500 calories ≈ 1 lb of fat. A daily deficit of 500 calories produces roughly 1 lb of fat loss per week.
How Large Should Your Deficit Be?
Conservative Cut
~0.5 lb/week. Best for athletes or anyone who wants to minimize muscle loss and maintain performance. Slow but very sustainable.
Standard Cut (Recommended)
~1 lb/week. The sweet spot for most people — fast enough to see progress, moderate enough to preserve muscle and hormonal health.
Aggressive Cut
~1.5 lbs/week. Acceptable for short periods (4–6 weeks) with high protein intake. Increases risk of muscle loss, fatigue, and rebound.
Very Aggressive / Crash Diet
Not recommended. Significantly increases muscle loss, metabolic adaptation, nutrient deficiencies, and rebound weight gain.
How to Calculate Your Calorie Deficit
Step 1: Find your TDEE — use our free TDEE calculator. It uses the Mifflin-St Jeor equation with your activity level to estimate how many calories you burn per day.
Step 2: Subtract 300–500 calories. If your TDEE is 2,400 calories, your fat loss target is 1,900–2,100 calories per day.
Step 3: Set your protein first. Aim for 0.8–1g of protein per pound of bodyweight to protect muscle during your cut. Fill the remaining calories with carbs and fats based on preference.
Step 4: Track your actual intake. Use ChromaFit's AI food scanner to log meals from a photo — no manual entry, no barcode needed.
The hidden variable: Research consistently shows people underestimate their food intake by 20–40%. If you're not losing weight in your "deficit," the most likely explanation is inaccurate tracking — not a broken metabolism.
How to Lose Fat Without Losing Muscle
The two biggest levers for muscle preservation during a cut are protein and training:
- Protein: Increase to 0.9–1.2g/lb bodyweight during a deficit (higher than maintenance). Protein is the primary defense against muscle catabolism when calories are restricted.
- Resistance training: Continuing to lift weights sends a signal to your body that muscle is needed. Without that signal, your body has no reason to maintain it during a calorie deficit.
- Moderate deficit: Deficits larger than 500–750 calories significantly increase muscle loss. Slower fat loss is better fat loss when you care about body composition.
- Sleep: Poor sleep increases cortisol, which accelerates muscle breakdown and fat storage. 7–9 hours is a fat loss tool, not just a health recommendation.
Why You Stop Losing Weight in a Deficit
Fat loss plateaus are common and have predictable causes:
- Underestimating intake: The #1 cause. Restaurant meals, cooking oils, and snacks are chronically underlogged. AI food scanning catches these gaps better than manual entry.
- Metabolic adaptation: After 6–8+ weeks of deficit, your body reduces TDEE by 5–15% through reduced movement and metabolic efficiency. The fix: recalculate your TDEE based on current weight and activity, and reduce intake by another 100–200 calories if needed.
- Water weight masking fat loss: Glycogen, sodium, hormones, and stress all cause water retention that can mask weeks of real fat loss on the scale. Use a 2-week average and track body measurements, not just weight.
- Incorrect TDEE estimate: Activity multipliers are notoriously inaccurate. If you're not losing weight at your calculated target, reduce intake by 100 calories and monitor for 2 weeks before concluding.
Tracking Your Progress Beyond the Scale
The scale only tells part of the story. Pair it with:
- Body measurements: Waist, hips, chest — these change independently of scale weight
- Body fat percentage: Use our free body fat calculator monthly
- Physique photos: AI physique scoring every 4 weeks tracks definition improvements by muscle group — the most direct measure of how your body composition is changing
Frequently Asked Questions
What is a calorie deficit?
A calorie deficit occurs when you eat fewer calories than your body burns. Your body makes up the difference by drawing on stored energy — primarily fat — resulting in fat loss. A deficit of 3,500 calories corresponds to approximately 1 lb of fat.
How large should my calorie deficit be?
A deficit of 300–500 calories below your TDEE is optimal for most people — producing 0.5–1 lb of fat loss per week while preserving muscle mass. Deficits larger than 750 calories significantly increase muscle loss and metabolic adaptation.
How do I calculate my calorie deficit?
Calculate your TDEE (Total Daily Energy Expenditure) using our free TDEE calculator, then subtract 300–500 calories. That's your daily calorie target for fat loss.
Why am I not losing weight in a calorie deficit?
The most common reasons are underestimating food intake (most people underestimate by 20–40%), water weight masking fat loss, or an overestimated TDEE. Track your actual intake accurately for 2 weeks and compare to your real weight trend before concluding your approach isn't working.
Track Your Deficit Without the Hassle
Log any meal with a photo — no manual entry, no barcode. ChromaFit's AI food scanner makes hitting your calorie target effortless.