Health & Wellness

Calorie Deficit Calculator

Build a sustainable deficit that hits your goal date safely.

The internet is full of "eat 1,200 calories" advice, and the internet is wrong. Aggressive deficits backfire: they cause muscle loss, metabolic adaptation, and rebound weight gain. This calculator uses the Mifflin-St Jeor equation — the most accurate formula validated by the American Dietetic Association — to find a sustainable deficit that hits your goal date without wrecking your metabolism.

Your body and goal

Your daily calorie target
0 kcal/day

Enter your details to see a sustainable calorie target.

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How this calculator works

The math, in plain English

We use the Mifflin-St Jeor equation, published in 1990 and validated as the most accurate formula for estimating Resting Metabolic Rate (RMR) in healthy adults. It is the formula the American Dietetic Association recommends.

The formulas

Men: RMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5

Women: RMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

TDEE (Total Daily Energy Expenditure) = RMR × activity factor. Daily calorie target = TDEE − deficit. One pound of fat = 3,500 calories, so a 500-calorie daily deficit loses 1 lb/week.

A worked example
32-year-old male, 185 lbs, 5'10", moderate activity. RMR: ~1,780. TDEE: 1,780 × 1.55 = 2,759. Goal: 20 lbs in 20 weeks = 1 lb/week = 500 cal/day deficit. Target: 2,259 kcal/day. That is a sustainable deficit — not the 1,200-calorie starvation diets on TikTok.

Why aggressive deficits backfire

Three reasons: (1) Muscle loss — deficits above 1% of body weight per week cause muscle catabolism, which lowers your metabolic rate. (2) Metabolic adaptation — your body detects the deficit and reduces RMR by 10-20% within weeks. (3) Rebound — 95% of aggressive dieters regain the weight within 2 years because their lowered metabolism cannot sustain the new weight. Slow loss (0.5-1% body weight per week) preserves muscle and minimizes adaptation.

Minimums and warnings

Women should not eat below 1,200 kcal/day; men should not eat below 1,500 kcal/day without medical supervision. Below these levels, you risk nutrient deficiencies, gallstones, hormonal disruption (especially in women), and electrolyte imbalances. If your calculated target is below your floor, choose "Conservative" or extend your timeline.

FAQ

Common questions

Is "calories in, calories out" really all that matters?
For weight loss, yes — the First Law of Thermodynamics is inescapable. But the quality of those calories determines hunger, energy, muscle preservation, and adherence. 1,500 calories of chicken and vegetables will produce very different results than 1,500 calories of soda and chips. Track calories for weight; track protein and fiber for body composition and satiety.
Should I eat back exercise calories?
Generally no — the activity factor in TDEE already accounts for exercise. If you do an unusually hard workout (2+ hours), add 100-200 calories. Fitness trackers overestimate exercise burn by 20-90%, so "eating back" often creates a surplus. Better: set your activity level conservatively and let the deficit work.
What about macronutrients — protein, carbs, fat?
For weight loss, protein matters most. Aim for 0.7-1.0 grams of protein per pound of body weight (130-185g for a 185-lb person). Protein preserves muscle, increases satiety, and has the highest thermic effect (20-30% of protein calories are burned digesting it). Carbs and fat can split the remaining calories based on preference.
Why did my weight loss stall after 3 weeks?
Two common reasons: (1) Water weight fluctuation — initial rapid loss is often water, then the scale stalls while fat loss continues. Weigh daily and average weekly. (2) Metabolic adaptation — your RMR drops 10-15% during a deficit. After 6-8 weeks, take a 1-week "diet break" at maintenance to reset leptin and thyroid hormones.
How do I track calories accurately?
Use a food scale ($15 on Amazon) and an app like Cronometer or MacroFactor. Volume estimates (cups, spoons) are 30-50% off. Restaurant meals are notoriously undercounted — a "600-calorie" restaurant burrito is often 1,000+. Track consistently for 2 weeks, identify patterns, then relax into a sustainable rhythm.
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Disclaimer: This calculator is provided for educational and informational purposes only. It does not constitute financial, tax, legal, medical, or professional advice. Results depend on the accuracy of the inputs you provide and the assumptions documented above. Always consult a qualified professional before making decisions based on these calculations.