Free BMI Calculator - Body Mass Index
Calculate your Body Mass Index (BMI) instantly. Check if you're underweight, normal, overweight, or obese. Supports both metric (kg/cm) and imperial (lbs/ft) units.
BMI Categories (WHO Standards)
| Category | BMI Range |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 16.9 |
| Mild Thinness | 17 - 18.4 |
| Normal | 18.5 - 24.9 |
| Overweight | 25 - 29.9 |
| Obese Class I | 30 - 34.9 |
| Obese Class II | 35 - 39.9 |
| Obese Class III | ≥ 40 |
What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a simple, widely-used screening tool that measures body fat based on your height and weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI helps healthcare professionals quickly assess whether a person is underweight, normal weight, overweight, or obese. The World Health Organization (WHO) and Centers for Disease Control (CDC) use BMI as a standard measure for population health statistics.
Our free online BMI calculator instantly computes your body mass index using either metric (kilograms and centimeters) or imperial (pounds and feet/inches) measurements. No signup required - just enter your height and weight to get immediate results with personalized health recommendations.
BMI Formula - How to Calculate BMI
The BMI formula divides your weight by the square of your height. Here are both metric and imperial formulas:
Metric: BMI = Weight (kg) ÷ Height² (m²)
Imperial: BMI = Weight (lbs) × 703 ÷ Height² (inches²)
Example: A person weighing 70 kg with height 1.75 m has BMI = 70 ÷ (1.75 × 1.75) = 22.9
Health Risks of Being Overweight or Obese
Having a BMI of 25 or higher increases your risk of several serious health conditions:
Cardiovascular Disease
High blood pressure, coronary heart disease, stroke, and heart attack risk increases significantly with higher BMI.
Type 2 Diabetes
Excess body fat, especially around the abdomen, increases insulin resistance and diabetes risk.
Certain Cancers
Obesity is linked to increased risk of breast, colon, kidney, liver, and pancreatic cancers.
Joint Problems
Excess weight puts stress on joints, leading to osteoarthritis, especially in knees and hips.
Health Risks of Being Underweight
A BMI below 18.5 also carries health risks that shouldn't be ignored:
Malnutrition
Insufficient nutrient intake can lead to vitamin deficiencies, anemia, and weakened bodily functions.
Weakened Immune System
Underweight individuals are more susceptible to infections and illnesses.
Osteoporosis
Low body weight is associated with decreased bone density and higher fracture risk.
Fertility Issues
Being underweight can disrupt hormonal balance and affect reproductive health.
Limitations of BMI - What BMI Doesn't Tell You
While BMI is a useful screening tool, it has important limitations:
• Muscle vs. Fat: Athletes and bodybuilders may have high BMI due to muscle mass, not fat. Muscle weighs more than fat.
• Age Differences: Older adults tend to have more body fat than younger adults at the same BMI.
• Gender Differences: Women typically have more body fat than men at the same BMI level.
• Ethnic Variations: Different ethnic groups may have different body compositions at the same BMI.
• Fat Distribution: BMI doesn't account for where fat is stored. Abdominal fat is more dangerous than fat in other areas.
For a complete health assessment, consult with a healthcare provider who can consider BMI along with other factors like waist circumference, blood pressure, cholesterol, and blood sugar levels.
Frequently Asked Questions About BMI
What is a healthy BMI for adults?
A healthy BMI for adults is between 18.5 and 24.9. This range is associated with the lowest risk of weight-related health problems. However, optimal BMI can vary based on individual factors like age, muscle mass, and overall health.
How accurate is BMI as a health indicator?
BMI is a useful screening tool but not a diagnostic measure. It accurately identifies weight categories for most people but may misclassify athletes, elderly individuals, or those with unusual body compositions. It's best used alongside other health assessments.
Is BMI different for men and women?
The BMI formula is the same for both genders, but interpretation may differ. Women naturally have higher body fat percentages than men. A BMI of 22 might represent different body compositions in men versus women.
Does BMI apply to children and teenagers?
For children and teens (ages 2-20), BMI is calculated the same way but interpreted using age and sex-specific percentiles. A child's BMI is compared to other children of the same age and gender rather than using fixed adult categories.
What should I do if my BMI is too high or too low?
If your BMI falls outside the normal range, consult a healthcare provider. They can assess your overall health, consider contributing factors, and recommend appropriate lifestyle changes, dietary modifications, or medical interventions if needed.
How often should I check my BMI?
For most adults, checking BMI a few times per year is sufficient. If you're actively trying to lose or gain weight, monthly tracking can help monitor progress. Remember that short-term weight fluctuations are normal due to water retention and other factors.