Clinical Physiology & Mathematics

Scientific Formulas & Equations

Explore the validated physiological equations, metabolic algorithms, and clinical math powering our health calculators.

1. Body Mass Index & Body Fat Clinical Equations

Metric Standard

Quetelet Index (Metric)

BMI = weight (kg) / [height (m)]²

Standard epidemiological formula established by Adolphe Quetelet and universally endorsed by the WHO.

Imperial Standard

Quetelet Index (Imperial)

BMI = [weight (lbs) × 703] / [height (in)]²

Applies the 703 conversion multiplier to standardize US customary units (pounds & inches) to kg/m².

Body Composition

Deurenberg Body Fat Formula

BF% = (1.20 × BMI) + (0.23 × Age) − (10.8 × Sex) − 5.4

Where Sex = 1 for males and 0 for females. Adjusts for age-related muscular loss and gender adipose variance.

2. Energy Expenditure & Metabolic Equations

Metabolic Gold Standard

Mifflin-St Jeor (Men)

BMR = (10 × wt_kg) + (6.25 × ht_cm) − (5 × age) + 5

Calculates resting energy expenditure with highest correlation to indirect calorimetry in adult males.

Metabolic Gold Standard

Mifflin-St Jeor (Women)

BMR = (10 × wt_kg) + (6.25 × ht_cm) − (5 × age) − 161

Adjusted baseline constant (-161) calibrated for female physiological body composition.

Daily Expenditure

Total Daily Energy (TDEE)

TDEE = BMR × Activity Multiplier (1.2 – 1.9)

Multiplies BMR by physical activity: Sedentary (1.2), Light (1.375), Moderate (1.55), Very Active (1.725), Super Active (1.9).

3. Ovulation & Fertility Biological Rhythm Math

Biological Rhythm

Ogino-Knaus Ovulation Formula

Ovulation Day = Cycle Length − 14 Days

Based on the physiological constant that the post-ovulatory luteal phase remains consistently 14 days in healthy cycles.

Conception Window

Fertile Window Interval

Window = [Ovulation − 5 days] to [Ovulation + 1 day]

Reflects the 5-day maximum sperm survivability in fertile cervical mucus plus the 24-hour viability of the released ovum.

4. Obstetrical Gestation & Due Date Formulas

Obstetrical Standard

Naegele's Rule (Adjusted)

EDD = LMP + 280 days + [Cycle Length − 28 days]

Adds 280 days (40 completed gestational weeks) to the first day of the Last Menstrual Period, adjusted for cycle variance.

Gestational Age

Gestational Timeline Progress

Weeks Pregnant = Math.floor([Today − LMP] / 7 days)

Tracks exact gestational weeks and days, dividing pregnancy into 1st (1–13 wks), 2nd (14–27 wks), and 3rd (28–40+ wks) trimesters.