Calculate Your Body Mass Index
Body Mass Index, almost always shortened to BMI, is a simple numerical measure that uses your height and weight to estimate whether you fall within a healthy weight range. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s, though it was not widely adopted by the medical community as a screening tool until much later in the twentieth century. Despite its age and some well known limitations, BMI remains one of the most commonly used initial health screening tools worldwide.
The reason BMI has stuck around for so long despite criticism is straightforward: it is fast, free and requires only two measurements that anyone can take without any equipment beyond a scale and a tape measure. In large scale population studies and public health work, a quick screening tool that gives a rough indication of weight status is genuinely useful even if it is imperfect at the individual level.
For most adults between 18 and 65, BMI gives a reasonable indication of body fatness. It is not a diagnostic tool and it cannot tell you exactly how much body fat you carry or where it is distributed on your body. But it serves as a useful starting point for conversations about weight related health risks with a healthcare provider.
The calculation is straightforward. You take your weight in kilograms and divide it by your height in metres squared. So a person who weighs 70 kilograms and is 1.75 metres tall has a BMI of 70 divided by 1.75 squared, which equals 70 divided by 3.0625, which comes to approximately 22.9. That falls comfortably in the normal weight range.
In imperial units, you multiply your weight in pounds by 703 and then divide by your height in inches squared. The multiplier of 703 is simply a conversion factor that makes the final result land in the same numerical range as the metric calculation.
The World Health Organisation uses the following standard BMI categories for adults. These are the same categories used in most countries for clinical and research purposes.
A BMI below 18.5 falls in the underweight category. Being underweight carries its own health risks including nutritional deficiencies, a weakened immune system, bone density loss and in severe cases organ damage. Underweight status should be investigated by a doctor to rule out underlying causes.
A BMI between 18.5 and 24.9 is classified as normal or healthy weight. People in this range generally have the lowest risk of weight related chronic disease, though other lifestyle factors like diet quality, physical activity levels, sleep and stress management all contribute significantly to overall health outcomes.
A BMI between 25 and 29.9 falls in the overweight category. Being overweight increases the risk of several conditions including type 2 diabetes, high blood pressure, heart disease, joint problems and certain cancers. The risk is elevated but not dramatically so compared to the obese categories.
A BMI of 30 or above is classified as obese. This is further subdivided into Class 1 obesity from 30 to 34.9, Class 2 obesity from 35 to 39.9 and Class 3 or severe obesity at 40 and above. Each step up significantly increases health risks. However it is worth emphasising that BMI alone does not determine anyone's health, and many people with high BMI are metabolically healthy while some people with normal BMI have poor metabolic health.
For all its convenience, BMI has some significant limitations that are worth understanding so you interpret your result in the right context.
BMI cannot distinguish between fat mass and muscle mass. A heavily muscled athlete might have a BMI that technically puts them in the overweight or even obese category despite having very low body fat and excellent cardiovascular fitness. A professional rugby player or a competitive bodybuilder will almost certainly have a BMI that suggests they are overweight by clinical standards, which is clearly misleading.
BMI also does not account for fat distribution. Research consistently shows that where fat is stored on the body matters as much as how much fat there is. Visceral fat stored deep in the abdomen around the organs is associated with much higher metabolic and cardiovascular risk than subcutaneous fat stored just under the skin around the hips and thighs. Two people with identical BMI values can have very different health profiles depending on their fat distribution patterns.
Age affects the interpretation of BMI somewhat. Older adults tend to have more body fat at the same BMI compared to younger adults because muscle mass naturally decreases with age. So a BMI of 22 in a 25 year old and a BMI of 22 in a 65 year old represent different actual body compositions.
Ethnicity also plays a role. Research has found that people of Asian descent tend to have higher health risks at lower BMI values compared to people of European descent. Some health organisations in Asia use lower BMI cutoffs for overweight and obesity classifications to reflect this. Conversely, people of Polynesian or some African descent tend to have higher lean body mass at the same BMI, which means standard cutoffs may overestimate health risk for these groups.
If you want a more accurate picture of your body composition, there are several options beyond BMI. Waist circumference is one of the simplest and most clinically meaningful measurements. A waist measurement above 94 centimetres for men or 80 centimetres for women is associated with significantly increased cardiovascular and metabolic risk. This simple tape measure test captures visceral fat risk better than BMI does.
Waist to height ratio is another useful metric that research suggests may be more predictive of health outcomes than BMI. The simple rule of thumb is that your waist circumference should be less than half your height. So a person who is 180 centimetres tall should ideally have a waist circumference below 90 centimetres.
DEXA scanning, bioelectrical impedance analysis and underwater weighing are all methods that directly measure body fat percentage and lean mass. These give much more detailed information but are more expensive and less accessible than a simple BMI calculation.
If your BMI falls outside the healthy range and you are considering working towards a healthier weight, it helps to set goals that are realistic and gradual. Research on long term weight management consistently shows that modest weight loss of 5 to 10 percent of body weight produces meaningful improvements in blood pressure, blood sugar, cholesterol and other metabolic markers even if it does not bring you to a normal BMI.
Sustainable weight loss happens slowly. A rate of 0.5 to 1 kilogram per week is considered reasonable and achievable through a moderate calorie deficit combined with increased physical activity. Faster weight loss typically involves muscle loss alongside fat loss and is harder to maintain long term.
Working with a healthcare provider or a registered dietitian rather than relying solely on BMI targets gives you a much more personalised and effective approach to managing your weight and overall health.
Enter your weight and height in either metric or imperial units. The calculator instantly shows your BMI, your weight category and the healthy weight range for your height. Use it as a general starting point for thinking about your health rather than as a definitive judgement of your weight status.