Free Health Calculators

Evidence-based health calculators for BMI, calorie needs, water intake, and body composition. All formulas sourced from the CDC, NIH, and peer-reviewed research.

BMI Calculator

Calculate your Body Mass Index using height and weight. See your BMI category (Underweight, Normal, Overweight, Obese) and healthy weight range.

BMR & TDEE Calculator

Calculate your Basal Metabolic Rate and Total Daily Energy Expenditure using the Mifflin-St Jeor formula. Includes calorie targets for weight loss, maintenance, and gain.

Water Intake Calculator

Find your personalized daily water intake recommendation based on body weight, activity level, and climate conditions.

Health Calculator (BMI + Calories + Body Fat)

All-in-one health assessment: BMI, daily calorie needs (Harris-Benedict), and body fat percentage (US Navy method).

Tip Calculator

Calculate the right tip amount based on service quality and split the bill among any number of people.

About Our Health Calculators

Our health calculators use evidence-based formulas from recognized medical and public health organizations. The BMI calculator follows CDC guidelines. The BMR calculator uses the Mifflin-St Jeor equation, consistently rated as the most accurate for most adults by peer-reviewed research. The body fat calculator uses the US Navy method, validated in multiple clinical studies.

These tools are designed for adults. Children and teenagers should use age-specific calculators provided by pediatricians, as standard BMI categories don't apply to growing bodies.

Important disclaimer: Our health calculators are educational tools, not medical devices. Results should not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making significant changes to your diet, exercise routine, or health regimen.

Henry, the site's founder, checks every health calculator against CDC and NIH guidance, so that all formulas, category thresholds, and interpretive guidance reflect current public health guidelines.

Health Calculator FAQs

What is a healthy BMI?

The CDC classifies BMI as: Underweight = below 18.5, Normal weight = 18.5–24.9, Overweight = 25.0–29.9, Obese = 30.0 and above. However, BMI is a screening tool only. It doesn't directly measure body fat and can misclassify muscular athletes as overweight. A healthcare provider should interpret BMI in context with other health indicators.

What is BMR and how is it calculated?

BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to maintain basic functions — breathing, circulation, cell repair. The most accurate formula for most adults is Mifflin-St Jeor: For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161.

How much water should I drink per day?

The Institute of Medicine recommends about 3.7 liters (125 oz) per day for men and 2.7 liters (91 oz) for women from all beverages and foods combined. As a rule of thumb, 0.5 oz per pound of body weight is a common starting point, adjusted for activity level, climate, and individual factors. About 20% of daily water intake comes from food.

How do I calculate body fat percentage?

The most accessible method without equipment is the US Navy body fat formula, which uses neck, waist (and hips for women), and height measurements. More accurate methods include DEXA scans (most accurate), hydrostatic weighing, and Bod Pod. Calipers are also widely used. Our Health Calculator uses the Navy method, which is accurate within 3% for most people.

How many calories should I eat to lose weight?

A safe rate of weight loss is 1–2 pounds per week, which requires a daily calorie deficit of 500–1,000 calories. To find your target: calculate your TDEE (maintenance calories), then subtract 500 for 1 lb/week loss or 1,000 for 2 lbs/week. Never go below 1,200 calories/day (women) or 1,500 (men) without medical supervision.

What's the difference between BMR and TDEE?

BMR is how many calories you burn at complete rest. TDEE (Total Daily Energy Expenditure) is your BMR multiplied by an activity factor that accounts for exercise and daily movement. TDEE is your maintenance calorie level — eat at TDEE to maintain weight, below TDEE to lose, above to gain. Most people's TDEE is 1.4–1.9× their BMR.

Is BMI accurate for everyone?

BMI is less accurate for: athletes (muscle weighs more than fat, so BMI overstates obesity risk), elderly adults (BMI understates body fat as muscle mass declines with age), and people of Asian descent (health risks appear at lower BMI thresholds). The CDC recommends using BMI alongside waist circumference and other health metrics for a complete picture.