BMI Calculator

Calculate your Body Mass Index (BMI) quickly and easily with this free, no-signup BMI calculator. Get instant results and health category classifications for both metric and imperial measurements.

BMI Categories:

  • Underweight = <18.5
  • Normal weight = 18.5–24.9
  • Overweight = 25–29.9
  • Obesity = BMI of 30 or greater

For more information, visit NIH - Assessing Your Weight and Health Risk

About

Unit System Selection

Choose between metric (kg and cm) or imperial (pounds, feet, and inches) units

BMI Calculation

Instantly calculate BMI based on input weight and height

BMI Category Classification

Determine weight category (underweight, normal weight, overweight, or obesity) based on calculated BMI

Result Copying

Copy BMI result and category to clipboard with one click

Input Validation

Ensure accurate calculations with input validation for weight and height

Reference

Body Mass Index (BMI) is a simple, widely used method for estimating body fat content and assessing potential health risks associated with weight. Developed by Adolphe Quetelet in the 1830s, BMI is calculated by dividing a person's weight by the square of their height. While BMI is a useful screening tool, it does have limitations. It doesn't account for factors such as muscle mass, bone density, age, or gender. For more comprehensive health assessments, healthcare professionals often use BMI in conjunction with other measurements like waist circumference, skinfold thickness, or more advanced body composition analysis techniques.

The World Health Organization (WHO) and the National Institutes of Health (NIH) use BMI to define weight status categories:
- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 29.9
- Obesity: BMI 30 or greater

It's important to note that these categories may not be appropriate for all populations, particularly athletes, children, or older adults. Always consult with a healthcare provider for personalized health advice.

Calculation

The BMI calculation uses the following formulas:

For metric units:
BMI = weight (kg) / (height (m))²

For imperial units:
BMI = (weight (lbs) / (height (inches))²) * 703

The calculator first validates the input to ensure it's within reasonable ranges:
- Weight: 20-500 kg or 44-1100 lbs
- Height: 50-300 cm or 1-9 ft (plus 0-11 inches)

After calculating the BMI, the result is rounded to one decimal place. The BMI value is then used to determine the weight category based on the WHO/NIH classifications.

Frequently Asked Questions

Is BMI an accurate measure of body fat?
BMI is a useful screening tool, but it doesn't directly measure body fat and may not be accurate for all body types.
How often should I calculate my BMI?
It's generally recommended to check your BMI annually or when there are significant changes in your weight or height.
Can athletes rely on BMI for health assessment?
Athletes may have higher BMIs due to increased muscle mass, so BMI may not accurately reflect their body composition or health status.
Are there alternatives to BMI for assessing body composition?
Yes, alternatives include waist circumference measurements, skinfold thickness tests, and bioelectrical impedance analysis.
How does BMI differ for children and teens?
BMI for children and teens uses age and sex-specific percentiles, as their bodies are still developing.