Health Calculators That You Can Trust

BMI formula: weight (kg) ÷ height² (m²)

WHO Categories:

• Under 18.5 — Underweight
• 18.5–24.9 — Normal weight
• 25.0–29.9 — Overweight
• 30.0+ — Obese

Important limitations: BMI does not account for muscle mass, bone density, age, or sex. Athletes with high muscle mass may register as "overweight" while having healthy body composition. BMI is a screening tool, not a diagnosis. Consult a healthcare provider for personal assessment.

This calculator uses the Mifflin-St Jeor equation, considered the most accurate BMR formula by the American Dietetic Association:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5

Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Your TDEE = BMR × Activity Multiplier. Weight loss requires a 500 calorie/day deficit (about 1 lb/week). Aggressive cuts above 1000/day are not sustainable for most people.

Neck: Measure just below the larynx, with tape angled slightly downward.

Waist: For men, measure at navel level. For women, measure at the narrowest point.

Hips (women only): Measure at the widest point of the buttocks.

Accuracy: The US Navy method is accurate within ±3% for most people, comparable to skinfold calipers and far cheaper than DEXA scans.

Categories for men: Essential 2-5%, Athletes 6-13%, Fitness 14-17%, Average 18-24%, Obese 25%+

Categories for women: Essential 10-13%, Athletes 14-20%, Fitness 21-24%, Average 25-31%, Obese 32%+

Base water need: 35ml per kg of body weight (per Institute of Medicine guidelines).

Additional water needed for:

• Exercise: +350-1000ml depending on intensity and duration
• Hot weather: +200-500ml in temperatures above 25°C
• Pregnancy: +300ml/day
• Breastfeeding: +700ml/day

About 20% of daily water comes from food. The rest should come from beverages. Water, herbal tea, and milk all count. Coffee and alcohol have mild diuretic effects but still contribute to hydration.

Devine (1974): Developed for medication dosing. Tends to underestimate for tall people.

Robinson (1983): Modified Devine, accounts for ethnicity better.

Hamwi (1964): The oldest formula, used in clinical nutrition.

Miller (1983): Generally produces the highest estimates.

All formulas use height as the primary input. None account for muscle mass, frame size, or age. They are starting points, not goals.

Naegele's Rule: Due date = First day of last period + 280 days (40 weeks).

From conception: Due date = Conception date + 266 days (38 weeks).

Only about 4% of babies are born on their actual due date. Normal delivery is anywhere from 37-42 weeks. First trimester: 1-12 weeks. Second: 13-27 weeks. Third: 28-40 weeks.

For accuracy, ultrasound dating in the first trimester is the most reliable method. This calculator is for planning purposes.

Ovulation typically occurs 14 days before your next period (not 14 days after the last one — common mistake).

The fertile window is approximately 6 days: 5 days before ovulation + ovulation day itself. Sperm can survive in the female body for up to 5 days; an egg lives ~24 hours after release.

This calculator works best for women with regular cycles. If cycles vary by more than 7 days, consider using ovulation tests or tracking basal body temperature.

• Normal: Less than 120/80 mmHg
• Elevated: 120-129 systolic AND less than 80 diastolic
• Stage 1 Hypertension: 130-139 systolic OR 80-89 diastolic
• Stage 2 Hypertension: 140+ systolic OR 90+ diastolic
• Hypertensive Crisis: Over 180 systolic and/or over 120 diastolic — seek emergency care

A single reading does not diagnose hypertension. Doctors typically require 2+ elevated readings on separate occasions. Home BP monitors should be validated and used per their instructions for accuracy.

Basic (220 - age): Simple, widely used estimate. Less accurate for trained athletes and people over 50.

Karvonen: Uses your resting heart rate for personalization. More accurate, especially for fitness training. Requires knowing your resting HR (best measured immediately after waking).

Zone purposes: Zone 2 is best for fat burning and long endurance. Zone 3-4 builds cardiovascular fitness. Zone 5 is for short intervals only. Most training should be in Zone 2.

One complete sleep cycle takes about 90 minutes and includes light sleep, deep sleep, and REM sleep. Waking at the end of a cycle (in light sleep) feels natural and refreshed. Waking mid-cycle (in deep sleep or REM) leaves you groggy and disoriented.

Adults need 4-6 complete cycles per night:

• 4 cycles = 6 hours (minimum)
• 5 cycles = 7.5 hours (sweet spot)
• 6 cycles = 9 hours (recovery)

It typically takes 10-20 minutes to fall asleep, so plan accordingly.