What Is a Calorie Deficit — and How Big Should Yours Be?

A calorie deficit is the gap between the calories your body burns and the calories you feed it. When the number on the way out exceeds the number on the way in, your body pulls the difference from stored energy — mostly fat, with a small contribution from glycogen and a smaller one from muscle protein if the deficit is aggressive. That is the entire mechanism behind weight loss. Every diet that has ever produced real results works because it created a consistent calorie deficit.

How big should the deficit be?

The size of the deficit determines two things: how fast you lose weight, and how much muscle you keep. The standard heuristic is 3,500 kcal ≈ 1 pound of fat, which means a 500 kcal/day deficit produces about a pound of fat loss per week. That is the rate most lifters do well at.

• 300 kcal/day deficit: roughly 0.5 lb/week. Best when you are already lean, returning from a break, or trying to preserve performance during a competition block.
• 500 kcal/day deficit: roughly 1 lb/week. The sweet spot for most people — fast enough to see progress, small enough to keep training volume high.
• 750 kcal/day deficit: roughly 1.5 lb/week. Faster but harder to sustain. Use short cycles, not the default.
• 1,000+ kcal/day deficit: too aggressive for anyone who trains. Expect muscle loss, fatigue, hunger, and metabolic adaptation within 2 to 3 weeks.

How to find your real deficit number

The formula is simple in principle and messy in practice:

1. Calculate your TDEE with the Mifflin-St Jeor equation (10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 for men or −161 for women), then multiply by an activity factor between 1.4 and 1.75 based on weekly workouts, daily steps, and job activity.
2. Subtract 300 to 500 kcal. That is your daily calorie target.
3. Log your weight for two to three weeks. Compare the trend to what the math predicted. If you are losing faster than expected, eat 50 to 100 kcal more. If slower, eat 50 to 100 less.

The recalibration step is the part most calculators skip, and it is the part that actually works. Predictive equations are off by ~10% on average and up to 30% in outliers — see Hall et al. (2011). The trend-aware trackers (MacroFactor, CalStory) outperform one-shot calculators because they adjust the target as your weight changes, instead of staying frozen at your onboarding number.

Why aggressive deficits backfire

Three things happen when you cut too hard:

1. Muscle loss. A 2010 meta-analysis by Helms, Fitschen, and Aragon found that aggressive deficits (≥ 1,000 kcal/day) resulted in significantly more lean mass loss than moderate ones in resistance-trained athletes. A 500 kcal deficit preserves muscle when paired with adequate protein and continued strength training.

2. Metabolic adaptation. Resting metabolic rate drops disproportionately to the deficit. A 2016 study in The American Journal of Clinical Nutrition showed that metabolic adaptation after extreme weight loss can persist for years — which is why so many crash dieters regain the weight.

3. Hormonal and performance hits. Testosterone drops, thyroid function slows, training volume becomes impossible to sustain, and mood takes a beating. None of these are conducive to keeping the weight off.

How to eat in a deficit without losing muscle

Four rules that survive every intervention trial:

1. Keep protein high. 1.6 to 2.2 g per kilogram of bodyweight is the range with evidence behind it. Protein is the most satiating macro and the one your body uses to preserve muscle during a cut.
2. Lift heavy. Strength training sends the signal to your body that your muscle is load-bearing and needs to be kept. Cardio-only cuts lose more lean mass.
3. Prioritize volume foods. Vegetables, lean proteins, broth-based soups, and eggs keep you full on fewer calories than calorie-dense foods (nuts, oils, cheese) do.
4. Track the trend, not the scale. Daily weight bounces 1 to 2 kg from water and sodium alone. A 7-day rolling average tells you whether the deficit is actually working.

Where CalStory fits

CalStory computes your TDEE with Mifflin-St Jeor during onboarding, sets your daily calorie target 300 to 500 below maintenance, and recalibrates the number every week as you log your weight. The calorie ring on the dashboard shows what you have left for the day, the macro pills show whether you are hitting protein, and the Progress page plots the trend so you can see the deficit working before the mirror tells you.

Frequently asked questions