This document details the physiological models, mathematical equations, and health guidelines powering the calculators on this hub. All formulas execute locally in your browser.
1. Body Mass Index (BMI)
Imperial: BMI = [weight (lbs) / (height (in))²] × 703
The Body Mass Index (BMI) is a simple ratio developed by Adolphe Quetelet in the 19th century. It acts as an epidemiological screen to correlate general population weight distributions with statistical wellness outcomes.
We use the standard categories defined by the World Health Organization (WHO) and the National Institutes of Health (NIH):
- Underweight: BMI < 18.5
- Healthy Weight: 18.5 ≤ BMI < 25.0
- Overweight: 25.0 ≤ BMI < 30.0
- Obesity: BMI ≥ 30.0
Source: World Health Organization (WHO) physical status assessment guidelines.
2. Basal Metabolic Rate (BMR)
To estimate the caloric baseline required to support core metabolic systems at rest, our calculator uses the Mifflin-St Jeor equation. Peer-reviewed studies indicate this equation offers a high level of predictive accuracy for standard modern populations:
Female: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age (y) - 161
Source: Mifflin MD, St Jeor ST, et al. "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition, 1990.
3. Total Daily Energy Expenditure (TDEE)
TDEE is calculated by multiplying your BMR by an activity factor representing physical activity levels:
- Sedentary: BMR × 1.2 (little to no exercise, desk job)
- Lightly Active: BMR × 1.375 (light exercise 1–3 days/week)
- Moderately Active: BMR × 1.55 (moderate exercise 3–5 days/week)
- Very Active: BMR × 1.725 (hard exercise 6–7 days/week)
- Extremely Active: BMR × 1.9 (highly active physical job or intense daily training)
Source: Harris-Benedict model adaptations for daily physical activity levels.
4. Macronutrient Calorie Conversions
Calories are converted into specific grams of protein, carbohydrates, and fats based on standard dietary thermal conversions:
- Protein: 4 calories per gram (kcal/g)
- Carbohydrates: 4 calories per gram (kcal/g)
- Fats: 9 calories per gram (kcal/g)
Source: Atwater system energy values for macronutrient conversions.
5. Sleep Cycles
A healthy human sleep cycle alternates between Non-REM and REM (Rapid Eye Movement) stages. For healthy adults, this complete loop takes approximately 90 minutes.
The sleep calculator projects optimal awakening opportunities at the boundaries of these cycles (specifically at 3, 4, 5, or 6 complete cycles), factoring in your custom sleep latency:
Source: National Sleep Foundation clinical recommendations on sleep architecture.
6. Pregnancy Timeline
A standard human pregnancy is estimated to last approximately 280 days (40 weeks) from the first day of your last menstrual period (LMP). Our calculator adjusts this model for variations in menstrual cycles:
Source: Naegele's rule adjustments for estimated due dates (EDD) and American College of Obstetricians and Gynecologists (ACOG) standard gestational tables.