Every HealthyLifeStyles calculator uses an established, published equation — no proprietary “black boxes.” Below is the exact method behind each one, with a link to its primary source so you can check our work.

Estimates, not diagnoses. These formulas are population models. They give useful ballpark figures for healthy adults but can’t account for everything about an individual. Use them for education and direction, and consult a qualified professional for medical decisions.

Body & weight

BMI: weight (kg) ÷ height (m)²; healthy range 18.5–24.9. WHO — Body mass index.
BMR: Mifflin–St Jeor equation (sex-specific). Mifflin MD, St Jeor ST, et al., 1990.
TDEE: BMR × an activity factor (1.2 sedentary → 1.9 very active). NIDDK Body Weight Planner.
Body fat %: U.S. Navy circumference method (neck, waist, and hip for women). Hodgdon & Beckett, U.S. Navy.
Lean body mass: Boer formula from weight and height. Boer P., 1984.
Ideal weight: Healthy-BMI weight range plus the Devine formula. Devine BJ, 1974.
Waist-to-height ratio: waist ÷ height; keep below 0.5. Ashwell M, et al., systematic review.
Waist-to-hip ratio: waist ÷ hip; risk thresholds by sex. WHO waist–hip ratio report.

Nutrition

Calorie target: TDEE minus a deficit; a safe pace is 0.5–1 kg (1–2 lb)/week. CDC — Losing Weight.
Protein: 1.2–2.2 g per kg of body weight depending on goal and training. ISSN Position Stand: Protein, 2017.
Macros: Protein from g/kg, fat ~0.8–1 g/kg, remaining calories as carbohydrate. ISSN / Dietary Guidelines for Americans.
Water intake: ~35 ml per kg of body weight, adjusted for activity. EFSA — Dietary reference values for water.
Alcohol calories: Ethanol ≈ 7 kcal/g, by standard-drink sizes. NIAAA — What is a standard drink?.

Fitness

Calories burned: METs × weight (kg) × time (hours). Compendium of Physical Activities (Ainsworth, 2011).
One-rep max: Epley and Brzycki equations from reps × weight. Epley 1985 / Brzycki 1993.
VO₂ max (estimate): 15.3 × (max HR ÷ resting HR). Uth N, et al., 2004.
Steps to calories: Stride length (from height) × steps → distance × MET energy. Compendium of Physical Activities.

Heart & vitals

Max heart rate: 208 − 0.7 × age (Tanaka); the simpler 220 − age is also shown. Tanaka H, et al., 2001.
Target HR zones: Karvonen method using heart-rate reserve (max − resting). Karvonen / ACSM guidelines.
Blood pressure categories: ACC/AHA 2017 category thresholds. Whelton PK, et al., 2017 (ACC/AHA).

Women's health

Due date: Naegele’s rule: last period + 280 days (or conception + 266). ACOG — Estimating the due date.
Ovulation & fertile window: Ovulation ≈ next period − 14 days; fertile window the preceding ~6 days. ACOG — Fertility awareness.
Pregnancy weight gain: Recommended ranges by pre-pregnancy BMI. Institute of Medicine (IOM) guidelines.

Sleep & metabolic

Sleep / nap timing: Built around ~90-minute sleep cycles plus ~15 min to fall asleep. Sleep Foundation — Sleep cycles.
Caffeine timing: First-order decay with a ~5-hour half-life. Institute of Medicine — Caffeine pharmacology.
Metabolic age: Your BMR vs the average BMR for your age — an illustrative estimate, not a clinical measure. Based on the Mifflin–St Jeor BMR equation.

Units & rounding

All math runs in metric base units; imperial inputs are converted consistently both ways. Results are rounded for readability, so small differences from other calculators usually come down to rounding or a different activity-factor / formula choice.

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Formulas and their sources are checked by our medical review board. Read how we work in our editorial policy.