BMI Calculator

What is BMI?

Body Mass Index (BMI) is a simple numerical value derived from a person’s height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used by the World Health Organization (WHO) and healthcare professionals as a quick screening tool to categorize individuals into weight status categories.

BMI does not measure body fat directly. Instead, it provides a rough estimate of whether a person’s weight falls within a healthy range relative to their height. Despite its simplicity, it remains one of the most commonly used metrics in public health and clinical settings worldwide.

How to Use This Calculator

This calculator supports two unit systems. Use the toggle at the top to switch:

• cm / kg — Enter height in centimeters and weight in kilograms. Default for most countries.
• ft·in / lb — Enter height in feet and inches, and weight in pounds. Default for English (US) users.

The calculator automatically converts imperial values to metric internally before applying the BMI formula.

The BMI Formula

BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters:

$BMI = \frac{weight\ (kg)}{height\ (m)^2}$

Example: A person who is 175 cm tall and weighs 70 kg:

$BMI = \frac{70}{1.75^2} = \frac{70}{3.0625} = 22.86$

This falls within the “Normal” weight range.

WHO Classification Table

The World Health Organization defines the following BMI categories for adults (ages 20 and older):

Classification	BMI Range
Underweight	Less than 18.5
Normal weight	18.5 — 24.9
Overweight	25.0 — 29.9
Obese	30.0 or higher

These thresholds are based on epidemiological data linking BMI ranges to health risks. A BMI below 18.5 is associated with malnutrition and weakened immune function, while a BMI of 30 or higher is associated with increased risk of cardiovascular disease, type 2 diabetes, and certain cancers.

Limitations of BMI

While BMI is a useful screening tool, it has several well-known limitations:

Does not distinguish between muscle and fat. Athletes and individuals with high muscle mass may have a BMI in the “Overweight” or “Obese” range despite having low body fat. For example, many professional rugby players and bodybuilders register BMIs above 30.

Does not account for body fat distribution. Where fat is stored matters. Visceral fat (around the organs) is more dangerous than subcutaneous fat (under the skin). Two people with the same BMI can have very different health risk profiles depending on where their fat is located.

Age and sex differences. Women typically have more body fat than men at the same BMI. Older adults tend to have more body fat than younger adults at the same BMI. The standard thresholds do not adjust for these differences.

Ethnic variations. Research has shown that health risks associated with BMI vary across ethnic groups. For example, Asian populations may face elevated health risks at lower BMI values (a BMI of 23 or higher is sometimes considered overweight in Asian health guidelines), while some Pacific Islander populations may have lower risk at higher BMIs.

Not suitable for children. BMI for children and teens is interpreted differently, using age- and sex-specific percentile charts rather than the fixed thresholds shown above.

For a more comprehensive assessment of body composition and health risk, consider complementary measurements such as waist circumference, waist-to-hip ratio, body fat percentage (via DEXA scan or bioelectrical impedance), or consultation with a healthcare professional.

How accurate is BMI?

BMI is a population-level screening tool, not a diagnostic instrument. It correlates reasonably well with body fat percentage at the population level but can be inaccurate for individuals. It is best used as a starting point rather than a definitive health assessment.

What is a healthy BMI?

According to the WHO, a BMI between 18.5 and 24.9 is considered “Normal” weight. However, optimal BMI can vary depending on age, sex, ethnicity, and individual health factors. A BMI in the normal range does not guarantee good health, and a BMI outside it does not necessarily indicate poor health.

How often should I check my BMI?

There is no strict rule. Checking once or twice a year is sufficient for most adults. If you are actively trying to lose or gain weight, monthly tracking can help you monitor trends. Focus on the overall direction rather than day-to-day fluctuations.

Can BMI be used during pregnancy?

Pre-pregnancy BMI is used by healthcare providers to assess weight gain recommendations during pregnancy. However, BMI should not be calculated or interpreted during pregnancy as a measure of weight status, since weight gain during pregnancy is expected and necessary.

What should I do if my BMI is outside the normal range?

A single number does not define your health. If your BMI is outside the normal range, consider speaking with a healthcare professional who can assess your overall health, including diet, physical activity, family history, and other risk factors. Do not make drastic dietary changes based on BMI alone.