Calorie Deficit Calculator

Calculate your optimal calorie deficit for safe, sustainable weight loss

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How this calculator works

This calorie deficit calculator uses the Mifflin-St Jeor equation (1990), recommended by the Academy of Nutrition and Dietetics as the most accurate predictive equation for estimating Basal Metabolic Rate (BMR) in healthy individuals.

The formula calculates your BMR based on your age, sex, height, and weight:

Men: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5

Women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161

Your BMR is then multiplied by an activity factor (1.2 to 1.9) to estimate your Total Daily Energy Expenditure (TDEE). A calorie deficit is created by eating below your TDEE. One kilogram of body fat contains approximately 7,700 calories, so a daily deficit of 550 calories results in approximately 0.5 kg of weight loss per week.

According to research published in the Journal of the Academy of Nutrition and Dietetics (2014), a moderate deficit of 500-750 calories per day is associated with the best long-term adherence and sustainable weight loss outcomes. Very large deficits (above 1,000 kcal/day) may increase the risk of muscle loss, nutritional deficiencies, and metabolic adaptation.

The calculator enforces a minimum of 1,200 kcal/day, as intake below this level is generally not recommended without medical supervision (Academy of Nutrition and Dietetics, 2016).

Frequently asked questions

What is a calorie deficit and how does it cause weight loss?

A calorie deficit occurs when you consume fewer calories than your body burns. Your body then uses stored energy (primarily body fat) to make up the difference. One kilogram of body fat stores approximately 7,700 calories, so a consistent daily deficit of 550 calories leads to roughly 0.5 kg (1.1 lbs) of weight loss per week. This is a fundamental principle of thermodynamics applied to human metabolism.

How big should my calorie deficit be for safe weight loss?

Most health organizations recommend a moderate deficit of 500-750 calories per day, which produces 0.5-0.75 kg (1-1.5 lbs) of weight loss per week. Research from the National Institutes of Health shows that deficits larger than 1,000 calories per day may lead to muscle loss, nutritional deficiencies, metabolic slowdown, and are harder to maintain long-term.

What is the minimum number of calories I should eat per day?

The Academy of Nutrition and Dietetics recommends that women consume no fewer than 1,200 calories per day and men no fewer than 1,500 calories per day without medical supervision. Very low-calorie diets (800 calories or fewer) should only be followed under direct medical supervision. This calculator enforces a 1,200-calorie minimum.

Will eating too few calories slow my metabolism?

Yes, this is known as metabolic adaptation or "adaptive thermogenesis." When you drastically reduce calories, your body lowers its metabolic rate to conserve energy. A 2016 study published in Obesity found that participants who lost weight rapidly experienced significant metabolic slowdown that persisted for years. Moderate deficits (500-750 kcal/day) minimize this effect.

How accurate is the calorie deficit calculator?

The Mifflin-St Jeor equation is the most validated formula for estimating energy needs, accurate within approximately 10% for most healthy adults (Frankenfield et al., 2005). Individual variation exists due to genetics, muscle mass, hormones, and medications. Use these numbers as a starting point, monitor your progress for 2-3 weeks, and adjust by 100-200 calories if needed.

Should I eat back the calories I burn during exercise?

If you selected an activity level that includes exercise, your TDEE already accounts for those calories. If you exercise more than your selected level, eating back 50-75% of extra exercise calories is reasonable — fitness trackers tend to overestimate calories burned by 20-50%.

How do I prevent muscle loss while in a calorie deficit?

Research shows three key strategies: (1) consume adequate protein — 1.6 to 2.2 grams per kilogram of body weight per day; (2) perform resistance training at least 2-3 times per week; and (3) keep your deficit moderate (no more than 25% below TDEE). A 2020 meta-analysis in the British Journal of Sports Medicine confirmed that high protein intake combined with resistance training preserves lean mass during weight loss.