BMI Calculator

Body Mass Index (BMI) is a numerical value derived from a person's height and weight that provides a rapid, low-cost indicator of whether an individual's weight falls within a range associated with good health. The measure was originally developed by Belgian mathematician Adolphe Quetelet in the 1830s and 1840s, when it was known as the Quetelet Index. It was later adopted by the World Health Organization and popularised under the name BMI in the 1970s by physiologist Ancel Keys, who identified it as a practical population-level screening tool.

The formula is straightforward: BMI = weight in kilograms divided by the square of height in metres. Written as an equation: BMI = kg / m². For a worked example, take a person weighing 70 kg with a height of 1.75 m. First, square the height: 1.75 × 1.75 = 3.0625. Then divide the weight by that figure: 70 ÷ 3.0625 = 22.9. A BMI of 22.9 falls comfortably in the healthy weight range.

If you prefer imperial units, the formula is adjusted to: BMI = (weight in pounds / height in inches²) × 703. The constant 703 accounts for the unit conversion between the imperial and metric systems. Both formulas produce the same result, so you can use whichever set of units you find more intuitive. Our calculator handles both automatically.

Health authorities around the world use four primary BMI classifications for adults. Underweight is defined as a BMI below 18.5. People in this range may be at risk of nutritional deficiencies, weakened immune function, anaemia, and reduced bone density. In some cases, very low BMI can indicate an underlying medical condition or an eating disorder that warrants prompt clinical attention.

A BMI between 18.5 and 24.9 is considered normal or healthy weight. Research consistently associates this range with the lowest risk of weight-related diseases, including type 2 diabetes, cardiovascular disease, and certain cancers. Overweight is defined as a BMI between 25 and 29.9. At this level, the risk of developing high blood pressure, high cholesterol, sleep apnoea, and metabolic syndrome becomes measurably elevated compared to the healthy range.

Obesity is subdivided into three classes. Class I obesity covers a BMI of 30–34.9, where health risks are significant and lifestyle intervention is typically recommended. Class II (severe obesity) runs from 35 to 39.9; individuals here often benefit from structured weight management programmes and close medical monitoring. Class III obesity — sometimes called morbid or extreme obesity — is defined as a BMI of 40 or above. At this level, the risk of life-threatening conditions including heart disease, type 2 diabetes, and certain cancers is substantially increased, and medical or surgical intervention may be considered.

BMI is a useful population-level screening tool, but it has well-documented shortcomings when applied to individuals. Perhaps its most significant limitation is that it cannot distinguish between fat mass and lean mass. A professional rugby player or bodybuilder may have a BMI of 28 or 29 — technically overweight — while carrying very little body fat and being in excellent metabolic health. Conversely, a sedentary person with a BMI of 24 may have a high proportion of body fat despite appearing within the healthy range.

Age and sex also affect how BMI maps to body fat. Women naturally carry more essential fat than men due to hormonal and reproductive physiology, so at any given BMI, women will typically have a higher body fat percentage. Older adults tend to lose muscle mass and gain fat over time — a process called sarcopenic obesity — meaning their BMI can remain stable while their body composition worsens. Ethnic background further complicates interpretation: studies show that people of South and East Asian descent face elevated metabolic risk at lower BMI values than those of European descent, leading some authorities to recommend adjusted thresholds.

Another blind spot is that BMI ignores where fat is stored on the body. Visceral fat — the fat packed around internal organs in the abdominal cavity — is far more metabolically harmful than subcutaneous fat stored beneath the skin elsewhere. Two people with identical BMIs can have very different distributions of fat and therefore very different health profiles. Complementary measures that address these gaps include waist circumference, waist-to-hip ratio, and body fat percentage testing.

The standard adult BMI classification thresholds do not apply to children and teenagers because the relationship between height, weight, and body fat changes continuously during development. A child's BMI naturally rises and falls at different ages as their body grows; interpreting a single number without accounting for age and sex would be misleading.

For children and adolescents aged 2–19, healthcare providers instead use age- and sex-specific BMI percentile charts, most commonly the CDC growth charts used in the United States. These charts compare a child's BMI to reference data from thousands of children of the same age and sex. A result below the 5th percentile indicates underweight, which may signal inadequate nutrition or an underlying medical issue. The 5th to 84th percentile range is considered healthy. A child at the 85th to 94th percentile is classified as overweight, while the 95th percentile and above indicates obesity.

Because children go through growth spurts and periods of rapid physical change, a single BMI reading is less informative than tracking the trend over time. Paediatricians assess BMI percentile alongside physical development milestones, dietary history, and family health background. Parents who are concerned about a child's growth trajectory should consult their GP or paediatrician rather than relying on adult BMI thresholds.

If your BMI falls outside the 18.5–24.9 range, the most important first step is to speak with your GP or a registered healthcare professional. BMI is a screening tool, not a diagnostic one — it signals that further investigation may be worthwhile, but it cannot on its own tell you whether you have a health problem. A doctor will consider your BMI alongside blood pressure readings, cholesterol levels, blood glucose, waist circumference, family history, and lifestyle factors to form a complete picture.

For those in the overweight or obese range, research consistently shows that even modest weight loss — as little as 5–10% of body weight — produces meaningful improvements in blood pressure, blood sugar, and cholesterol. Gradual, sustainable lifestyle changes tend to be far more effective long-term than rapid weight loss diets. Aiming for a calorie deficit of 500 calories per day through a combination of diet and increased physical activity typically produces safe, steady progress of around 0.5–1 kg per week.

For those in the underweight category, a gradual calorie surplus combined with adequate protein intake can help restore healthy weight without promoting excessive fat gain. In both directions, working with a registered dietitian can be particularly valuable for structuring an approach that is nutritionally complete and realistic to maintain. Remember that BMI is one data point among many — a broader picture of health markers gives a far more actionable and accurate view of your wellbeing.