Health Calculators: The Complete Guide to BMR, TDEE, Body Fat & Water Intake
Evidence-based formulas for the six core health metrics — what they measure, which formula is most accurate, healthy ranges by age and gender, and free calculators.
Free Health Calculators
How the 6 Health Metrics Connect
Each health metric tells you something different about your body. Together they give a complete picture that no single number can provide.
| Metric | What It Measures | Use It To |
|---|---|---|
| BMR | Calories burned at complete rest | Set a minimum calorie floor; never eat below it |
| TDEE | Total daily calorie expenditure | Set weight loss / maintenance / gain targets |
| Body Fat % | Fat as % of total body weight | Track body composition changes; more accurate than BMI |
| BMI | Weight relative to height | Quick population-level screen; not accurate for muscular individuals |
| Water Intake | Daily hydration target | Prevent dehydration-related fatigue, headaches, poor performance |
| Ideal Weight | Healthy weight range for height | Set realistic weight goals using 4 formula-based ranges |
BMR — Basal Metabolic Rate
BMR is the calories your body burns at complete rest — keeping your heart beating, lungs breathing, and organs functioning. It accounts for 60–75% of your total daily calorie burn.
Three formulas, compared:
Mifflin-St Jeor (most accurate, ±10%):
Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
Katch-McArdle (best if you know body fat %):
370 + (21.6 × lean body mass in kg)
→ Use the BMR Calculator · Read the full guide
TDEE — Total Daily Energy Expenditure
TDEE = BMR × Activity Multiplier. It's the number you actually set your calorie intake against.
| Activity Level | Multiplier | Example |
|---|---|---|
| Sedentary | 1.2 | Desk job, no exercise |
| Lightly Active | 1.375 | Exercise 1–3 days/week |
| Moderately Active | 1.55 | Exercise 3–5 days/week |
| Very Active | 1.725 | Exercise 6–7 days/week |
| Extra Active | 1.9 | Physical job + daily training |
→ Use the TDEE Calculator · Read the full guide
Body Fat Percentage
Body fat % is a better health indicator than BMI because it distinguishes fat from muscle. The US Navy method uses circumference measurements — no body fat scale needed.
→ Use the Body Fat Calculator · Read the full guide
Water Intake
Daily water needs depend on weight, activity, climate, and diet. The baseline: 35ml × body weight in kg. Add 500ml per hour of exercise. Pale yellow urine = well hydrated.
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Frequently Asked Questions
What is BMR and why does it matter?
BMR (Basal Metabolic Rate) is the calories your body burns at complete rest — just to keep organs functioning. It represents 60–75% of your total daily calorie burn. Knowing your BMR lets you set accurate calorie targets for weight loss, maintenance, or muscle gain.
Which BMR formula is most accurate — Mifflin-St Jeor or Harris-Benedict?
The Mifflin-St Jeor equation (1990) is validated as the most accurate for most adults, with a mean error of ±10%. The Harris-Benedict equation (1919, revised 1984) is older and tends to overestimate by 5%. If you know your body fat %, Katch-McArdle is the most accurate of all three.
What is the difference between BMR and TDEE?
BMR is calories burned at rest. TDEE (Total Daily Energy Expenditure) multiplies BMR by an activity factor (1.2–1.9) to account for movement, digestion, and exercise. TDEE is the number you actually use to plan your diet — eat below it to lose weight, above it to gain.
What is a healthy body fat percentage?
For men: Essential 2–5%, Athletic 6–13%, Fitness 14–17%, Average 18–24%, Obese 25%+. For women: Essential 10–13%, Athletic 14–20%, Fitness 21–24%, Average 25–31%, Obese 32%+. Body fat is a better health predictor than BMI alone because it accounts for muscle mass.
How much water should I drink per day?
The general baseline is 35ml per kg of body weight. A 70kg person needs ~2.45 litres. Add 500ml per hour of moderate exercise, more in hot climates. The colour of urine is the most practical indicator — pale yellow means well hydrated; dark yellow means drink more.
What is BMI and is it accurate?
BMI (Body Mass Index) = weight(kg) ÷ height(m)². It's a population-level screening tool, not a precise individual health measure. It doesn't distinguish muscle from fat, so muscular people are often misclassified as overweight. Use BMI alongside body fat % and waist-to-height ratio for a more complete picture.