Our Formulas
Every IndexBody calculator uses a specific, peer-reviewed formula. This page documents each one with its source, limitations, and why we chose it over alternatives.
Last Updated: May 2026. We review formula selections annually against emerging research. If a superior validated formula becomes available, we update our calculators and document the change here.
BMI — Body Mass Index
Formula: BMI = weight (kg) / height (m)2
Source: World Health Organization, 1995. Updated classification system 2000.
Why we use it: Universal adoption across healthcare systems. Despite well-documented limitations (cannot distinguish muscle from fat, does not capture fat distribution), it remains the standard population-level screening tool endorsed by WHO, CDC, and NHS.
Limitation note: We explicitly note BMI limitations on every BMI page and recommend pairing with our body fat calculator.
BMR — Basal Metabolic Rate
Formula: Mifflin-St Jeor Equation (1990)
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Source: Mifflin, M.D. et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2), 241–247.
Why Mifflin-St Jeor over Harris-Benedict: Multiple validation studies have confirmed Mifflin-St Jeor outperforms the revised Harris-Benedict equation by approximately 5% accuracy in diverse populations. The American Dietetic Association recommends Mifflin-St Jeor as the preferred formula for clinical use.
We also display Harris-Benedict estimates on our BMR calculator page for comparison.
TDEE — Total Daily Energy Expenditure
Method: Mifflin-St Jeor BMR × Activity Multiplier (Katch-McArdle activity factors)
Activity Multipliers: Sedentary 1.2 · Lightly Active 1.375 · Moderately Active 1.55 · Very Active 1.725 · Extremely Active 1.9
Source: McArdle, W.D., Katch, F.I., & Katch, V.L. (2010). Exercise Physiology. Lippincott Williams & Wilkins.
Body Fat — US Navy Circumference Method
Formula (Men): BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Formula (Women): BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
Source: Hodgdon, J.A. & Beckett, M.B. (1984). Prediction of percent body fat for US Navy men and women. Naval Health Research Center, San Diego.
Accuracy: Standard error of approximately 3–4 percentage points compared to hydrostatic weighing.
Body Fat — Jackson-Pollock Skinfold (3-Site)
Formula: Siri density equation applied to JP3 sum-of-skinfolds regression
Source: Jackson, A.S. & Pollock, M.L. (1978). Generalized equations for predicting body density of men. British Journal of Nutrition, 40(3), 497–504.
Ideal Body Weight
We display four validated IBW formulas simultaneously: Hamwi (1964), Devine (1974), Robinson (1983), and Miller (1983).
Source: Pai, M.P. & Paloucek, F.P. (2000). The origin of the “ideal” body weight equations. Annals of Pharmacotherapy, 34(9), 1066–1069.
Lean Body Mass — Boer Formula
Formula (Men): LBM = (0.407 × kg) + (0.267 × cm) − 19.2
Formula (Women): LBM = (0.252 × kg) + (0.473 × cm) − 48.3
Source: Boer, P. (1984). Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology, 247(4), F632–F636.
FFMI — Fat-Free Mass Index
Formula: FFMI = LBM / height(m)2; Normalized FFMI = FFMI + 6.1 × (1.8 − height in m)
Source: Kouri, E.M. et al. (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine, 5(4), 223–228.
Protein Recommendations
Source: Morton, R.W. et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains. British Journal of Sports Medicine, 52(6), 376–384.
Ranges: 0.8g/kg (RDA minimum) through 2.4g/kg (aggressive fat loss). All ranges are within the published evidence and explicitly labeled as such in the calculator.
Sleep — 90-Minute Cycle Method
Source: Dement, W.C. & Kleitman, N. (1957). Cyclic variations in EEG during sleep. Electroencephalography and Clinical Neurophysiology, 9(4), 673–690.
The 90-minute average sleep cycle duration and 14-minute average sleep onset latency are well-established in sleep science literature.
Water Intake
Source: National Academies of Sciences, Engineering, and Medicine. (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
Our formula uses 30–35ml/kg body weight as the baseline, consistent with NAS recommendations.
Waist-to-Height Ratio
Threshold: WHtR < 0.5 = healthy (universal boundary endorsed by Ashwell & Hsieh, 2005).
Source: Ashwell, M., Gunn, P., & Gibson, S. (2012). Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors. Obesity Reviews, 13(3), 275–286.