How large a deficit should I use to lose fat without losing muscle?

Research supports 300-500 calories below TDEE for most people. This yields 0.5-1 lb weekly loss. Deficits exceeding 25% of TDEE accelerate muscle protein breakdown — a 150 lb person should not exceed ~500-600 calorie daily deficit. Protein intake of 0.8-1g per lb of body weight is the most important muscle-preservation factor.

Why did I stop losing weight on the same deficit?

Three reasons: (1) Adaptive thermogenesis — metabolism slows by 5-15% during sustained restriction. (2) Weight loss itself reduces TDEE — lighter bodies burn fewer calories. (3) Food tracking drift — portion accuracy decreases over time. Recalculate your deficit based on current weight and consider a 1-2 week diet break at maintenance.

Is it safe to eat 1200 calories a day?

1200 calories is a minimum threshold commonly cited, but adequacy depends on body size. A 130 lb, 5 foot 4 inch lightly active woman might have a TDEE around 1700 calories — a 500 calorie deficit brings her to 1200, which is adequate. A larger person eating 1200 calories would be in an extreme deficit (40%+) risking muscle loss, fatigue, and nutrient deficiencies.

Methodology: Calorie Deficit Calculator

Scope

Calculates the daily calorie deficit required to hit a target bodyweight by a target date, using the 3500 kcal/lb rule of thumb.

Formula

required_daily_deficit = (target_fat_loss_kg * 7700) / days. Target calories = TDEE - required_daily_deficit.

Coefficients

Parameter	Value	Note
Energy per kg fat	`~7700 kcal`
Safe weekly loss	`0.5–1.0% bodyweight`

Data sources

Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542-546. — PMID 13594881. Origin of the 3500 kcal/lb (~7700 kcal/kg) heuristic.
Hall KD. What is the required energy deficit per unit weight loss? Int J Obes (Lond). 2008;32(3):573-576. — PMID 17848938. Quantifies why the 3500 rule over-predicts long-run loss.
Thomas DM, Martin CK, Heymsfield S, et al. A simple model predicting individual weight change in humans. J Biol Dyn. 2011;5(6):579-599. — PMID 24522441. Dynamic weight-loss model that refines Wishnofsky's static rule.
Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes. Int J Sport Nutr Exerc Metab. 2014;24(2):127-138. — PMID 24092765. Supports the 'weight lost is primarily fat' assumption when protein and resistance training are adequate.

Assumptions

Weight lost is primarily fat (high-protein + resistance training reinforces this).
TDEE input is current, not pre-diet.

Approximation range

The 7700 kcal/kg rule over-predicts long-run fat loss by 10–30% because metabolic adaptation drops TDEE over a multi-month cut.

For cuts longer than ~12 weeks, use the tool's output as a starting deficit and recalibrate monthly from the scale trend.

Limitations

Doesn't model water, glycogen, or bowel-content fluctuations that dominate week-1 scale readings.
Very aggressive targets (>1% BW/week) are flagged but not blocked.

Reproducibility

Goal -5 kg in 12 weeks (84 days). Required = 5*7700/84 = 458 kcal/day deficit. With TDEE 2800, daily target 2342 kcal.

Change log

2026-04-24: methodology page first published.

Body Recomposition Planner — Plan body-fat reduction pace, deficit targets, and protein needs around a timeline.
Hybrid Training Planner — Hybrid training planner: build a weekly schedule that balances running and lifting with interference warnings and recovery guidance.

FAQ

How large a deficit should I use to lose fat without losing muscle?: Research supports 300-500 calories below TDEE for most people. This yields 0.5-1 lb weekly loss. Deficits exceeding 25% of TDEE accelerate muscle protein breakdown — a 150 lb person should not exceed ~500-600 calorie daily deficit. Protein intake of 0.8-1g per lb of body weight is the most important muscle-preservation factor.
Why did I stop losing weight on the same deficit?: Three reasons: (1) Adaptive thermogenesis — metabolism slows by 5-15% during sustained restriction. (2) Weight loss itself reduces TDEE — lighter bodies burn fewer calories. (3) Food tracking drift — portion accuracy decreases over time. Recalculate your deficit based on current weight and consider a 1-2 week diet break at maintenance.
Is it safe to eat 1200 calories a day?: 1200 calories is a minimum threshold commonly cited, but adequacy depends on body size. A 130 lb, 5 foot 4 inch lightly active woman might have a TDEE around 1700 calories — a 500 calorie deficit brings her to 1200, which is adequate. A larger person eating 1200 calories would be in an extreme deficit (40%+) risking muscle loss, fatigue, and nutrient deficiencies.