BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a simple numerical measure that relates a person's weight to their height. It is calculated by dividing weight in kilograms by height in metres squared (kg/m²). The result places a person into a broad weight category that is associated with health risk levels. BMI was developed in the 1800s by Belgian mathematician Adolphe Quetelet and has been used in public health research ever since.

Despite its age and simplicity, BMI remains one of the most widely used screening tools in medicine worldwide because it requires only two measurements — weight and height — and correlates reasonably well with body fat percentage in large populations. It is used to identify individuals who may be at risk of weight-related health conditions, and to track population-level trends in overweight and obesity.

BMI Categories Explained

The World Health Organization (WHO) defines the following BMI categories for adults aged 18 and over: Under 18.5 is classified as underweight, which can be associated with nutritional deficiencies, reduced immune function, and in severe cases osteoporosis or organ damage. 18.5 to 24.9 is the normal or healthy weight range, associated with the lowest risk of weight-related health conditions. 25.0 to 29.9 is classified as overweight, associated with a moderately increased risk of conditions like type 2 diabetes, high blood pressure, and cardiovascular disease. 30.0 and above is classified as obese, with class I at 30–34.9, class II at 35–39.9, and class III (sometimes called severe or morbid obesity) at 40 and above.

How to Calculate BMI

In metric units: BMI = weight (kg) ÷ height (m)². For example, a person weighing 75 kg and 1.75 m tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5, which falls in the healthy range. In imperial units: BMI = (weight in pounds × 703) ÷ height in inches². Our calculator handles both unit systems automatically — just select the one you prefer and enter your measurements.

Important Limitations of BMI

BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals. It does not distinguish between muscle mass and fat mass, which means that a muscular athlete with very low body fat can have a BMI in the overweight range, while someone with average weight but high body fat (sometimes called "skinny fat") may register a healthy BMI despite having elevated health risks.

BMI also does not account for where fat is distributed on the body. Visceral fat — fat stored around the abdominal organs — is associated with greater health risks than subcutaneous fat stored under the skin, and two people with identical BMIs can have very different fat distributions and health risk profiles. For this reason, waist circumference is often used alongside BMI as a complementary measure.

BMI thresholds were developed primarily from data on European populations and may not apply equally to all ethnic groups. Research has suggested that people of Asian descent may have elevated health risks at lower BMI values, and some health organisations recommend lower thresholds for Asian populations.

Frequently Asked Questions

Is BMI accurate for athletes and bodybuilders?

Not reliably. Muscle tissue is significantly denser than fat tissue, so athletes with high muscle mass often have BMIs in the overweight or even obese category despite having low body fat and excellent cardiovascular health. For physically active individuals, body fat percentage — measured by DEXA scan, hydrostatic weighing, or skinfold calipers — is a more meaningful measure than BMI.

Does BMI apply to children and teenagers?

Not directly. Children and teenagers have different body composition as they grow, so BMI must be assessed against age- and sex-specific growth charts rather than the adult thresholds. Paediatric BMI is expressed as a percentile relative to other children of the same age and sex — a child above the 95th percentile is generally considered obese, and below the 5th percentile is considered underweight.

What is a healthy BMI for older adults?

Some research suggests that slightly higher BMIs (in the range of 25–27) may be associated with better health outcomes in adults over 65, partly because some extra weight can provide a buffer against illness-related weight loss. However, the relationship is complex and individual health conditions matter more than a single number. Always discuss weight management with a healthcare professional.

How often should I check my BMI?

BMI is most useful as an occasional check, not something to monitor daily or even weekly. Weight fluctuates by 1–3 kg over the course of a day due to hydration, food, and other factors. A monthly check, or checks every few months while working toward a health goal, is sufficient for most purposes.

Beyond BMI: Other Ways to Assess Healthy Weight

Waist circumference is a simple and meaningful supplementary measure. A waist measurement above 88 cm (women) or 102 cm (men) is associated with increased cardiovascular and metabolic risk regardless of BMI. The waist-to-height ratio (waist circumference divided by height, with a healthy target of under 0.5) is another simple metric. For a thorough assessment, body composition analysis using DEXA scanning or bioelectrical impedance provides information on fat mass, lean mass, and bone density that BMI cannot.