About This Calculator
What we built, why we built it, and the peer-reviewed science behind every calculation.
Our goal
Most online calculators give you a number with no explanation. We wanted to build something different: a tool that shows you how the calculation works, why each formula was chosen, and what to actually do with the result.
This site is built and maintained by an independent developer with a background in software engineering and a long-standing interest in evidence-based nutrition. All formulas are sourced from peer-reviewed research and applied exactly as published. We do not modify, "tune", or optimise formulas for marketing purposes.
Methodology
BMR calculation
The Mifflin–St Jeor equation (1990) is used as the default for BMR estimation. A 2005 meta-analysis published in the Journal of the American Dietetic Association evaluated several equations against measured resting metabolic rate in 200+ subjects and found Mifflin–St Jeor to be the most accurate for the general non-athletic population, predicting within 10% of actual values in roughly 82% of cases.
When body fat percentage is provided, the calculator switches automatically to the Katch–McArdle formula (1996), which is based on lean body mass. This eliminates the systematic overestimation of BMR in higher-body-fat individuals and underestimation in lean/muscular individuals that occurs when total weight is used.
TDEE calculation
TDEE is calculated as BMR × activity multiplier. The multipliers used (1.2, 1.375, 1.55, 1.725, 1.9) are the Harris–Benedict activity factors, which remain the most widely cited and validated values in nutrition science despite predating more recent equations. The categories have been reworded for clarity but the numeric values are unchanged from the original publication.
Macro calculation
Protein targets are set at 0.85g per lb of bodyweight (1.87g/kg), which falls within the range of 1.6–2.2g/kg recommended by the International Society of Sports Nutrition for active individuals seeking to preserve or gain muscle mass. For users not engaged in resistance training, this may be slightly above minimum requirements but is never harmful and provides a meaningful buffer.
After protein is allocated, remaining calories are split approximately 35% fat and 65% carbohydrate. This ratio is not a rigid prescription — it reflects a balanced starting point. Individual adjustments based on preference, training type, and insulin sensitivity are entirely appropriate.
BMI calculation
BMI is calculated as weight (kg) ÷ height (m)², using the WHO standard classification: Under 18.5 (Underweight), 18.5–24.9 (Normal), 25–29.9 (Overweight), 30+ (Obese). We include a disclaimer with every BMI result because this measure does not distinguish between fat and muscle mass and is a poor individual health metric for athletic populations.
Ideal weight
The Devine formula (1974) is used for the ideal weight estimate. This formula was originally developed for medical drug dosing calculations — not as a fitness target. It is included for reference only and should not be treated as a goal. Many healthy, fit individuals fall significantly above their "ideal" Devine weight.
Accuracy and limitations
All BMR equations are statistical estimates derived from population studies. Individual metabolic rate varies due to genetics, thyroid function, gut microbiome, sleep quality, stress hormones, and factors not captured by any equation. The typical margin of error for Mifflin–St Jeor is ±10% — meaning a calculated BMR of 1,800 kcal could reflect a true BMR anywhere from approximately 1,620 to 1,980 kcal.
The most reliable approach is to:
- Use this calculator to establish a starting point.
- Track actual food intake carefully (using a food scale and app like Cronometer or MyFitnessPal) for 2–3 weeks.
- Compare expected vs. actual weight change and adjust calorie targets by 100–200 kcal accordingly.
- Recalculate every 4–8 weeks as body weight changes.
This iterative, data-driven approach consistently outperforms relying solely on any calculator's initial output.
Sources & references
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990) A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2):241–247.
- Frankenfield D, Roth-Yousey L, Compher C (2005) Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. Journal of the American Dietetic Association, 105(5):775–789.
- McArdle WD, Katch FI, Katch VL (1996) Exercise Physiology: Energy, Nutrition, and Human Performance. 4th ed. Williams & Wilkins. (Katch–McArdle formula source.)
- Harris JA, Benedict FG (1919) A biometric study of human basal metabolism. Proceedings of the National Academy of Sciences, 4(12):370–373.
- Morton RW, Murphy KT, McKellar SR et al. (2018) A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength. British Journal of Sports Medicine, 52(6):376–384.
- Devine BJ (1974) Gentamicin therapy. Drug Intelligence and Clinical Pharmacy, 8:650–655. (Ideal weight formula source.)
- World Health Organization (2000) Obesity: Preventing and managing the global epidemic. WHO Technical Report Series 894. (BMI classification.)
Contact
Found a bug? Have a suggestion? Disagree with something in our methodology? We genuinely want to hear it.
Email: n1043m.n1ght@gmail.com
We aim to respond within 2–3 business days. We do not accept sponsored content, paid placements, or affiliate arrangements that would compromise our calculation accuracy.