BMI Calculator

What is BMI?

Body Mass Index (BMI) is a numerical value derived from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and remains the most widely used screening tool for assessing body weight relative to height at a population level.

BMI does not directly measure body fat, but research shows it correlates reasonably well with direct measures of body fat for most adults. Clinicians and public health agencies use it as an inexpensive, non-invasive first step to identify individuals who may be at risk for weight-related health conditions.

A BMI between 18.5 and 24.9 is considered normal weight for most adults. Values below this range suggest underweight, while values above suggest overweight or obesity — each associated with distinct health risks.

The BMI formula

BMI is calculated the same way regardless of age or sex:

Where:

• Weight — measured in kilograms
• Height² — your height in metres, squared

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

In imperial units, the formula uses a correction factor: BMI = (weight in lbs ÷ height² in inches) × 703.

BMI categories explained

The World Health Organization (WHO) defines four standard BMI categories for adults aged 18 and over. These thresholds were determined through large-scale epidemiological studies linking BMI ranges to health outcomes.

Note that some health organisations use slightly different cut-offs for certain ethnic groups. The WHO recommends lower thresholds for Asian populations (overweight at BMI ≥ 23, obesity at BMI ≥ 27.5) because metabolic risk starts earlier at lower BMI values.

Healthy weight ranges by height

The table below shows the healthy weight range (BMI 18.5–24.9) for common heights. Use it as a quick reference, but remember that ideal weight depends on many individual factors.

Limitations of BMI

BMI is a useful population-level screening tool, but it has important limitations when applied to individuals. It cannot distinguish between fat mass and lean mass — meaning a muscular athlete and a sedentary person of the same height and weight will have identical BMIs, despite very different body compositions and health profiles.

For a more complete picture, clinicians often combine BMI with waist circumference, waist-to-hip ratio, body fat percentage measurements, and blood markers such as cholesterol, blood glucose, and blood pressure.

BMI vs. other body composition measures

Practical tips for a healthy weight

• Focus on body composition, not just the number. Strength training builds muscle which increases BMI but improves metabolic health. The scale and BMI can go up while your health improves.
• Measure waist circumference too. A waist above 94 cm (men) or 80 cm (women) is an independent risk factor for cardiovascular disease, regardless of BMI.
• Weigh yourself consistently. If tracking weight, use the same time of day (morning, after bathroom) to reduce daily fluctuation noise. Weekly averages are more meaningful than daily readings.
• Aim for sustainable changes. Research consistently shows that gradual weight loss (0.5–1 kg/week) is easier to maintain than aggressive dieting, which often leads to rebound weight gain.
• Prioritise sleep and stress management. Poor sleep raises ghrelin (the hunger hormone) and lowers leptin (the satiety hormone), making it significantly harder to maintain a healthy weight.
• Consult a healthcare professional. BMI is a screening tool, not a diagnosis. If your BMI falls outside the normal range, a doctor can order appropriate tests and tailor advice to your individual circumstances.

The BMI calculator computes your Body Mass Index — a widely used screening number based on height and weight. It classifies people into four categories: underweight, normal weight, overweight and obese. It works in both metric (kg/cm) and imperial (lbs/ft/in) units.