BMI Calculator
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BMI Calculator – Content Guide
What Is BMI?
A common tool for screening for healthy body weight versus body height is called “body mass index” (BMI). It is measured as kg/m² (body weight/height squared). Although BMI is not a direct measurement of body fat, it is a good proxy for body fat percentage in the majority of adults and can be used to identify those who may have a risk of developing health problems due to excess body weight.
BMI Table for Adults (WHO Classification)
The World Health Organization (WHO) uses the following BMI ranges for adults aged 20 and over:
Classification | BMI Range (kg/m²) |
Severe Thinness | < 16 |
Moderate Thinness | 16 – 17 |
Mild Thinness | 17 – 18.5 |
Normal (Healthy Weight) | 18.5 – 25 |
Overweight | 25 – 30 |
Obese Class I | 30 – 35 |
Obese Class II | 35 – 40 |
Obese Class III (Morbid Obesity) | > 40 |
BMI Table for Children & Teens (Age 2–20)
For children and teenagers, the Centers for Disease Control and Prevention (CDC) uses BMI-for-age percentile charts rather than fixed cut-off values, because body fatness changes with age and differs between boys and girls:
Category | Percentile Range |
Underweight | < 5th percentile |
Healthy Weight | 5th – 85th percentile |
At Risk of Overweight | 85th – 95th percentile |
Overweight / Obese | > 95th percentile |
BMI Formula
Metric (SI) Units
BMI = weight (kg) ÷ height² (m)
Example: A person who is 1.78 m tall and weighs 70 kg has a BMI of 70 ÷ (1.78²) = 22.1 kg/m².
US Customary Units
BMI = 703 × weight (lbs) ÷ height² (inches)
Example: A person who is 5’10” (70 in) and weighs 160 lbs has a BMI of 703 × 160 ÷ 70² = 23.0 kg/m².
BMI Prime
BMI Prime is the proportion of an individual’s BMI to the maximum of the normal BMI range (25 kg/m²). A BMI Prime of 1.0 indicates a BMI that is on the normal maximum level. Below 0.74 = underweight; 0.74 – 1.0 = normal weight; above 1.0 = overweight; and above 1.2 = obese. BMI Prime is an easy-to-use tool to determine at a glance how far an individual’s BMI is from the normal range.
Ponderal Index
The Ponderal Index (PI) is a similar index to BMI, except that it raises height to the third power: PI = weight (kg) ÷ height³ (m). This is because BMI is a more reliable indicator of body fatness in people that are very tall or very short. The average Ponderal Index is in the range of 11-14 kg/m³.
Risks of Being Overweight
Carrying excess weight is associated with a number of serious health conditions, including:
- High blood pressure (hypertension)
- High LDL (“bad”) cholesterol and low HDL (“good”) cholesterol
- Type 2 diabetes
- Coronary heart disease and stroke
- Gallbladder disease and gallstones
- Osteoarthritis (joint disease)
- Sleep apnea and breathing difficulties
- Certain cancers (breast, colon, kidney, endometrial, liver)
- Depression, anxiety, and other mental health conditions
Risks of Being Underweight
Being underweight also poses health risks:
- Malnutrition and vitamin deficiencies
- Anaemia (reduced ability to carry oxygen in the blood)
- Osteoporosis (weakened bones)
- Reduced immune function
- Growth and developmental issues in children and teens
- Hormonal imbalances and reproductive complications in women
Limitations of BMI
BMI is a useful screening tool, but it has important limitations:
- It does not measure body fat directly. A muscular athlete may have a high BMI without excess fat.
- Age and sex affect body composition. Older adults typically carry more fat at the same BMI than younger adults; women generally carry more fat than men at the same BMI.
- Ethnicity matters. Health risks associated with excess body fat can begin at lower BMI values in some ethnic groups (e.g., South Asian populations).
- It cannot distinguish fat from muscle or bone. Always use BMI alongside other assessments such as waist circumference, body fat percentage, and medical evaluation.
Frequently Asked Questions
But what is a healthy BMI?
For most adults, a BMI range of 18.5 to 24.9 kg/m² is considered a healthy (normal) weight. A score less than 18.5 is considered underweight, and a score at or above 25 is considered overweight or obese.
Are the BMIs the same for men and women?
Women tend to have a higher body fat percentage at the same BMI because of their physiological makeup, but the same BMI scale is used for both men and women. BMI remains a valuable screening tool for both men and women but must always be used in conjunction with other health measures.
What is the formula to find BMI by hand?
In metric units: weight in kilograms / height in meters squared. For example, if you weigh 70 kg and are 1.75 m tall: 70 ÷ (1.75 × 1.75) = 22.9 kg/m². In US units: weight in pounds x 703 / height² in inches.
What is my healthy weight according to my height?
BMI for a healthy weight is 18.5 – 25. These BMIs can be used to determine healthy weight ranges, which will be the square of your height in meters. This is done automatically in the results section of our calculator.
Is BMI different for different ages?
The BMI formula does not vary by age, but body composition does. The amount of body fat older adults have at the same BMI is greater than that of younger adults. BMI is used in a different way for children and teens (ages 2-20) with age- and sex-specific percentile charts.