Weight Loss Calculator

The Formula

This calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR), multiplied by an activity factor to give your Total Daily Energy Expenditure (TDEE). A calorie deficit is then subtracted to create the conditions for fat loss.

Mifflin-St Jeor BMR

Validated as the most accurate BMR formula for the general population by the Academy of Nutrition and Dietetics. It accounts for weight, height, age and sex:

Calorie Deficit for Fat Loss

A deficit of 500 kcal/day produces roughly 0.5 kg of fat loss per week — consistent with the widely cited 7,700 kcal per kg of fat figure. Hall et al. (2012) demonstrated that the relationship between deficit size and weight loss is not perfectly linear due to metabolic adaptation, but a moderate deficit (200–500 kcal) reliably drives fat loss while minimising muscle loss and hormonal disruption. Aggressive deficits above 750 kcal increase the risk of lean mass loss even with adequate protein. This calculator floors calorie intake at 1,200 kcal as an absolute minimum.

High Protein During a Deficit

Protein is set at 2.2g/kg bodyweight — elevated above the maintenance recommendation to protect muscle tissue. Helms et al. (2014) found that lean athletes in a deficit benefited from intakes at or above 2.3g/kg of lean body mass. The ISSN Position Stand (2022) supports 1.8–2.7g/kg for athletes in energy restriction. Higher protein also increases diet-induced thermogenesis and satiety, making adherence easier during a cut.

Macronutrient Split

After protein is allocated, fat is set at 25% of total calories to maintain hormonal health — falling below 20% total fat has been associated with suppressed testosterone in males (Hamalainen et al., 1984). Remaining calories go to carbohydrates to fuel training sessions and preserve performance during the cut.