BMI Calculator — Body Mass Index
Calculate your Body Mass Index (BMI) to check if your weight is healthy. Supports metric and imperial units. See also Ideal Weight Calculator and Calorie Calculator.
BMI Categories Chart
What is BMI (Body Mass Index)?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It provides a simple screening tool to categorize individuals into weight categories: underweight, normal weight, overweight, and obese. BMI was devised by Adolphe Quetelet in the 1830s and is used worldwide by healthcare professionals as a quick assessment of body fat levels. While BMI does not directly measure body fat, research shows it correlates moderately with more direct measures of body fat like skinfold thickness and underwater weighing.
BMI Formula — How to Calculate BMI
Metric Formula (kg and meters)
Imperial Formula (pounds and inches)
Example
Weight: 70 kg, Height: 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625
BMI = 22.9 (Normal Weight)
BMI Chart — Weight Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Malnutrition risk, osteoporosis |
| 18.5 – 24.9 | Normal Weight | Low risk — healthy range |
| 25.0 – 29.9 | Overweight | Increased risk of heart disease, diabetes |
| 30.0 – 34.9 | Obese Class I | High risk of heart disease, diabetes |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| ≥ 40.0 | Obese Class III | Extremely high risk — morbid obesity |
BMI Prime and Ponderal Index
BMI Prime is the ratio of your BMI to the upper limit of normal (25). A BMI Prime of 1.0 means you are at the upper boundary of normal weight. Values below 1.0 indicate normal or underweight; above 1.0 indicates overweight or obese.
Ponderal Index (also called Corpulence Index) uses height cubed instead of squared: PI = weight / height³. It is considered more accurate for very tall or very short individuals because it scales more proportionally with body size.
Limitations of BMI
BMI is a useful screening tool but has important limitations. It does not distinguish between muscle mass and fat mass — a muscular athlete may have a high BMI but low body fat. It does not account for age, sex, ethnicity, or body composition. BMI may underestimate body fat in older adults who have lost muscle mass, and overestimate it in athletes. For a more complete health assessment, combine BMI with waist circumference, body fat percentage, and other health markers. BMI is not recommended as the sole diagnostic tool for individuals — it is best used for population-level health screening.
Frequently Asked Questions
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered normal/healthy weight. This range is associated with the lowest risk of weight-related health problems.
Is BMI accurate for athletes?
No. Athletes with high muscle mass often have elevated BMI values that classify them as overweight or obese, even though they have low body fat. For athletes, body fat percentage or DEXA scans are more accurate measures.
Does BMI differ for men and women?
The BMI formula and categories are the same for both sexes. However, women naturally have more body fat than men at the same BMI. Some health organizations suggest slightly different risk thresholds by sex.
How is BMI calculated for children?
For children and teens (ages 2-20), BMI is calculated the same way but interpreted using age- and sex-specific percentile charts from the CDC. A child's BMI percentile shows how their BMI compares to other children of the same age and sex.
What should I do if my BMI is too high or too low?
Consult a healthcare professional. They can assess your overall health, body composition, and risk factors to provide personalized advice. BMI is a starting point, not a diagnosis.