Our Formulas
Every IndexBody calculator uses a specific, peer-reviewed formula. This page documents each one with its source, its limitations, and why we chose it over alternatives.
Last updated: May 2026. We review our formula selections annually against emerging research. If a better-validated formula becomes available, we update the relevant calculator and document the change here.
None of the calculators or figures on this page are personalized medical guidance. They apply published, population-level formulas to the numbers you enter; individual results can vary, and none of these tools diagnose, treat, or predict a specific health outcome. See our medical disclaimer · Privacy · Terms for details.
BMI — Body Mass Index
Formula: BMI = weight (kg) / height (m)2
Source: World Health Organization, 1995. Classification system updated in 2000.
What it measures: BMI is a simple ratio of weight to height, used as a population-level screening tool to flag potential weight-related health risk categories. It does not directly measure body fat percentage or body composition.
Why we use it: BMI has universal adoption across healthcare systems. Despite well-documented limitations (it cannot distinguish muscle from fat and does not capture where fat is distributed on the body), it remains the standard population-level screening tool referenced by the WHO, CDC, and NHS.
Limitation note: We explicitly note BMI's limitations on every BMI page and recommend pairing it with our body fat calculator for a fuller picture.
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
What it measures: BMR is an estimate of the energy your body needs at complete rest to maintain basic functions like breathing, circulation, and cell repair — it does not include the energy used for movement, digestion, or exercise.
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 found that Mifflin-St Jeor tends to outperform the revised Harris-Benedict equation for accuracy across diverse populations. The Academy of Nutrition and Dietetics (formerly the American Dietetic Association) lists Mifflin-St Jeor as a preferred formula for clinical use.
We also display Harris-Benedict estimates on our BMR calculator page for comparison.
TDEE — Total Daily Energy Expenditure
What it measures: TDEE estimates the total number of calories you burn in a day, combining your BMR with the energy used for daily activity and exercise.
Method: Mifflin-St Jeor BMR × an activity multiplier.
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
What it measures: An estimate of body fat percentage derived from circumference measurements (waist, neck, and for women, hip), rather than a direct measurement of fat mass.
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: Studies comparing this method against hydrostatic weighing report a standard error of roughly 3–4 percentage points, meaning individual results can vary meaningfully from more precise clinical methods.
Body Fat — Jackson-Pollock Skinfold (3-Site)
What it measures: An estimate of body density from skinfold thickness at three sites, then converted to an estimated body fat percentage.
Formula: Siri density equation applied to the 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). These formulas estimate a population-average reference weight for a given height and were originally developed for drug-dosing calculations, not as personal weight targets.
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
What it measures: An estimate of the mass of your body that is not fat — including muscle, bone, organs, and water.
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
What it measures: FFMI expresses your lean body mass relative to your height, similar to how BMI expresses total weight relative to height. It is used mainly to compare muscularity across different heights.
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.
Our displayed ranges run from 0.8 g/kg (the RDA minimum for sedentary adults) up to 2.4 g/kg (an upper range sometimes used during aggressive fat loss to help preserve muscle). All ranges fall within the published evidence and are labeled accordingly 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.
Our sleep calculator uses a 90-minute average sleep cycle length and a 14-minute average sleep onset latency, both well-established estimates in sleep science literature. Actual cycle length varies from person to person, so treat the suggested times as a starting point rather than a guarantee of waking up refreshed.
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 30–35 ml/kg of body weight as the baseline, consistent with the ranges referenced in that report. Actual fluid needs vary with climate, activity level, and individual physiology.
Waist-to-Height Ratio
Threshold: A WHtR below 0.5 is generally considered a healthy reference range, a boundary popularized 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.