BMI Calculator
Calculate your Body Mass Index (BMI) to determine if you're at a healthy weight. Our BMI calculator provides instant results and personalized insights.
Category: Fitness & Health
BMI Categories
Underweight
BMI: < 18.5
Normal
BMI: 18.5-24.9
Overweight
BMI: 25-29.9
Obesity I
BMI: 30-34.9
Obesity II
BMI: 35-39.9
Obesity III
BMI: > 40
BMI Health Risks by Category
| BMI Category | BMI Range | Associated Health Risks |
|---|---|---|
| Underweight | <18.5 |
|
| Normal weight | 18.5-24.9 |
|
| Overweight | 25-29.9 |
|
| Obesity (Class 1) | 30-34.9 |
|
| Obesity (Class 2) | 35-39.9 |
|
| Extreme Obesity (Class 3) | >40 |
|
BMI Distribution by Age Group in the US
- Underweight
- Normal weight
- Overweight
- Obese
Source: Data adapted from the National Health and Nutrition Examination Survey (NHANES), 2017-2020
What is BMI?
Body Mass Index (BMI) is a widely used screening tool developed by Belgian mathematician Adolphe Quetelet in the 1830s. It provides a simple numeric measure of a person's weight relative to height, calculated by dividing weight in kilograms by the square of height in meters (kg/m²).
The BMI Formula:
For metric: BMI = weight(kg) / [height(m)]²
For imperial: BMI = [weight(lbs) × 703] / [height(in)]²
BMI is used by healthcare professionals worldwide as an initial assessment tool to categorize individuals as underweight, normal weight, overweight, or obese. The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) have established standard BMI ranges that are associated with different health outcomes.
BMI Categories
BMI Limitations
While BMI is a useful screening tool for the general population, it has several important limitations:
- It may overestimate body fat in athletes and others with muscular builds.
- It may underestimate body fat in older persons and those who have lost muscle.
- It doesn't account for factors like frame size, muscle composition, or distribution of fat.
- It doesn't differentiate between fat, muscle, and bone mass.
- It may not be appropriate for all ethnic groups. For example, Asian populations may have higher health risks at lower BMI values.
- It doesn't measure body fat directly or account for where fat is stored (central obesity is a greater health risk).
Health Implications
A high BMI can be an indicator of high body fatness. Being overweight or obese increases your risk for many diseases and health conditions, including:
- Cardiovascular disease: High blood pressure, abnormal blood lipids, and atherosclerosis (hardening of arteries)
- Metabolic disorders: Type 2 diabetes, insulin resistance, and metabolic syndrome
- Joint problems: Osteoarthritis, limited mobility, and increased risk of fractures
- Sleep disorders: Sleep apnea and respiratory problems
- Certain cancers: Including endometrial, breast, colon, kidney, gallbladder, and liver cancer
- Psychological effects: Depression, anxiety, and reduced quality of life
Similarly, being underweight can lead to health issues like malnutrition, vitamin deficiencies, osteoporosis, decreased immune function, and in women, irregular periods and fertility complications.
Beyond BMI: Other Health Measures
For a more comprehensive health assessment, healthcare providers often use BMI in conjunction with:
- Waist circumference: Measures abdominal fat, which is a better predictor of health risks
- Waist-to-hip ratio: Indicates how fat is distributed
- Body composition analysis: Measures percentages of fat, muscle, water, and bone
- Blood tests: Check cholesterol levels, blood sugar, and other biomarkers
- Family health history: Identifies genetic predispositions to certain conditions
- Personal health behaviors: Diet, physical activity, smoking, and alcohol consumption
Keep in mind that BMI is just one indicator of health. For a complete picture of your health, consult with a healthcare provider.
Frequently Asked Questions
What is BMI?
Body Mass Index (BMI) is a measure of body fat based on height and weight that applies to adult men and women. It's calculated by dividing a person's weight in kilograms by the square of height in meters (kg/m²).
What are the BMI categories?
BMI categories are: Underweight (below 18.5), Normal weight (18.5-24.9), Overweight (25-29.9), Obesity Class I (30-34.9), Obesity Class II (35-39.9), and Extreme Obesity Class III (40 and above).
Is BMI accurate for everyone?
BMI is a useful screening tool, but it does have limitations. It may overestimate body fat in athletes and those with muscular builds, and underestimate body fat in older persons or those who have lost muscle mass. It also doesn't account for factors like bone density, muscle mass, or ethnic differences.
How often should I calculate my BMI?
It's reasonable to calculate your BMI once every few months or when there's a significant change in your weight. More frequent tracking may be appropriate if you're actively trying to reach a healthy weight under medical supervision.