Methodology

How we calculate, cited.

Every tool on TheHealthTools.com follows a published clinical formula from a recognized health authority. This page lists each tool, the equation it implements, the original source, and the relevant limitations.

Update process. When a guideline body publishes a relevant update (KDIGO, ACC/AHA, WHO, ACOG), we revise the affected calculator and update the log below. We don't employ in-house clinicians — see our editorial process for the full statement on what this site is and isn't.

Calculators and citations

BMI Calculator

BMI = weight(kg) / height(m)²

Source: World Health Organization. Body mass index — BMI. Updated 2024.

Limitations: WHO classification thresholds (Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30) apply to adults aged 18+. BMI is a screening tool, not a diagnostic measure of body fat or health.

BMR Calculator

Mifflin-St Jeor (1990): Male: 10·kg + 6.25·cm − 5·age + 5; Female: 10·kg + 6.25·cm − 5·age − 161

Source: Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.

Limitations: TDEE multipliers (1.2–1.9) follow the standard activity factor convention. Predictive equations carry an SE of roughly ±10% in healthy adults.

Calorie Calculator

TDEE = BMR × activity factor; weight goal calories = TDEE ± 500 kcal/day for ±0.45 kg/week

Source: Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837.

Limitations: The 3,500 kcal-per-pound rule oversimplifies long-term weight change. Real-world deficits should be paired with strength training and tracked over weeks, not days.

Macro Calculator

Protein 4 kcal/g, Carbohydrates 4 kcal/g, Fat 9 kcal/g; preset splits per goal

Source: Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press, 2005.

Limitations: Macro splits are presets — individual needs vary by training status and clinical context.

GFR Calculator

CKD-EPI 2021 race-free: eGFR = 142 × min(Cr/κ, 1)^α × max(Cr/κ, 1)^(−1.200) × 0.9938^age × (1.012 if female)

Source: Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737-1749.

Limitations: The 2021 equation explicitly removes the race coefficient that the 2009 equation included. Endorsed by the National Kidney Foundation and the American Society of Nephrology.

Blood Pressure Calculator

ACC/AHA 2017 staging — Normal <120/<80; Elevated 120–129/<80; Stage 1 130–139/80–89; Stage 2 ≥140/≥90; Crisis >180/>120

Source: Whelton PK, Carey RM, et al. 2017 ACC/AHA Guideline for the Prevention, Detection, Evaluation, and Management of High Blood Pressure in Adults. Hypertension. 2018;71(6):e13-e115.

Limitations: Single-reading classification is a screening tool. Diagnosis requires multiple measurements and clinical context.

Body Fat Calculator

US Navy circumference method (Hodgdon & Beckett, 1984). Men: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76; Women: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387

Source: Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, 1984.

Limitations: Circumference methods are estimates; gold standards include DEXA and hydrostatic weighing. Accuracy ~3–4% on average against DEXA in adult populations.

Ideal Weight Calculator

Devine (1974), Robinson (1983), Miller (1983), Hamwi (1964) — height-based formulas originally developed for medication dosing

Source: Pai MP, Paloucek FP. The origin of the 'ideal' body weight equations. Ann Pharmacother. 2000;34(9):1066-1069.

Limitations: These formulas pre-date modern body composition science and do not account for muscle mass or frame size. Use alongside BMI and body fat percentage.

Water Intake Calculator

Base 33 mL × kg/day + 350 mL per 30 minutes of activity, ×1.15 for hot climate

Source: Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2005.

Limitations: Standard estimates. Individual needs vary with kidney function, medications, pregnancy, and breastfeeding.

Pregnancy Due Date Calculator

Naegele's rule: Due date = LMP + 280 days, adjusted for cycle length variation. Conception: + 266 days. IVF (day-5 transfer): + 261 days.

Source: American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol. 2017;129(5):e150-e154.

Limitations: Due-date estimates are most accurate when combined with first-trimester ultrasound. Naegele assumes a regular 28-day cycle.

Update log

We log every formula or content change here. Most recent update: April 15, 2026.