How to Calculate Your Calorie Deficit
Enter your current stats — sex, age, height, weight, and activity level. The calculator uses the Mifflin-St Jeor equation to compute your Basal Metabolic Rate (BMR): the calories your body needs at complete rest. BMR is then multiplied by an activity factor (1.2 for sedentary to 1.9 for very active) to get your TDEE — Total Daily Energy Expenditure — the number of calories you burn per day at your current activity level.
Enter your goal weight and target weekly loss rate (typically 0.25–1 kg per week). The tool calculates the required daily calorie deficit: for 0.5 kg/week loss, a ~550 kcal/day deficit is needed (since 1 kg of fat ≈ 7,700 kcal). Your daily calorie target is TDEE minus this deficit. The estimated weeks to goal assumes a consistent deficit maintained throughout.
Review the safety warnings — the tool flags if your calculated target falls below the minimum safe intake (1,200 kcal/day for women, 1,500 kcal/day for men) and warns against deficits over 1,000 kcal/day. These thresholds exist because extreme deficits increase muscle loss, metabolic adaptation, and nutrient deficiency risk. For best results, pair the calorie target with adequate protein intake (1.6–2.4 g per kg of body weight) and progressive resistance training.
Why Use a Calorie Deficit Calculator?
- Science-based formula — uses Mifflin-St Jeor, the most validated BMR equation (within ±10% for most people)
- TDEE calculation — accounts for your actual activity level, not just resting metabolism
- Goal timeline — shows estimated weeks to reach target weight based on your chosen deficit
- Safety guardrails — warns when calculated targets fall below minimum safe intake thresholds
- Macro guidance — suggests daily protein targets to preserve muscle during weight loss
- Avoids guessing — replaces rough estimates with a personalized, formula-based calorie target
Frequently Asked Questions
What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a given day (your TDEE). When this happens, your body draws on stored energy — primarily body fat — to meet its needs. A deficit of approximately 7,700 kcal is needed to lose 1 kg of body fat (or 3,500 kcal per pound). So a daily deficit of 550 kcal results in roughly 0.5 kg of fat loss per week. The deficit must be maintained consistently — a single day's deficit has negligible effect; the cumulative deficit over weeks determines total fat lost.
How large should my calorie deficit be?
Most evidence-based guidelines recommend a deficit of 500–750 kcal/day for sustainable fat loss, producing 0.5–0.75 kg per week. This rate preserves muscle mass, keeps energy levels manageable, and is realistically sustainable for months. Deficits above 1,000 kcal/day increase the risk of muscle loss (the body breaks down lean tissue for energy), trigger hormonal adaptations that slow metabolism, and make it extremely difficult to meet micronutrient needs. The American College of Sports Medicine recommends staying above 1,200 kcal/day for women and 1,500 kcal/day for men without medical supervision.
What is the Mifflin-St Jeor formula?
The Mifflin-St Jeor equation, published in 1990, is the most accurate widely-used formula for estimating Basal Metabolic Rate (BMR). For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161. A 2005 meta-analysis found it was more accurate than the older Harris-Benedict equation for most people. Multiply the result by an activity factor (1.2–1.9) to get TDEE.
Will I lose muscle on a calorie deficit?
Some muscle loss during a calorie deficit is inevitable, but it can be minimized substantially. The most effective strategies are: (1) Keep protein high — aim for 1.6–2.4 g of protein per kg of body weight per day. High protein intake provides amino acids for muscle protein synthesis and is highly satiating. (2) Do resistance training — progressive strength training signals the body to preserve and rebuild muscle tissue even during a caloric deficit. (3) Keep the deficit moderate — deficits over 1,000 kcal/day accelerate muscle loss because the body needs more energy from lean tissue. (4) Avoid very low calorie diets — below 1,200 kcal/day, preserving muscle becomes extremely difficult.
How accurate is the calorie calculator?
The Mifflin-St Jeor formula is accurate to within approximately ±10% for most people, meaning a calculated TDEE of 2,000 kcal/day could reflect a true TDEE of 1,800–2,200 kcal/day. This range exists because the formula cannot account for individual metabolic variation, body composition (muscle vs. fat), genetics, and hormonal factors. The activity multipliers are also estimates. In practice: start with the calculated target, track your weight for 2–3 weeks, and adjust by 100–200 kcal if you are not losing at the expected rate. Real-world data from your own body is always more accurate than any formula.
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