Body Mass Index — metric or imperial
Body Mass Index is a number calculated from your height and weight that is used as a broad indicator of body fatness. The formula is simple: BMI = weight (kg) ÷ height (m)². In imperial units, the formula is weight (lb) ÷ height (in)² × 703.
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool for studying populations — not as a clinical measure for individuals. It was adopted by health organisations in the 1970s and 80s as a convenient screening tool because it requires only height and weight, both easy to measure.
The WHO and most national health agencies use four main categories: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30+). Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).
BMI is widely used but widely criticised — and for good reason. It measures the ratio of weight to height, but weight includes muscle, bone, organs, water, and fat. Two people with the same BMI can have very different body compositions and health risks.
Athletes and muscular individuals often fall into the overweight or obese BMI range despite having low body fat and excellent health. A heavily muscled rugby player may have a BMI of 30 while carrying very little excess fat.
Older adults often have a normal BMI while carrying excess fat, because muscle mass tends to decrease with age. This is sometimes called "sarcopenic obesity" — a normal weight but unhealthy body composition.
Ethnic differences also matter. Research has shown that people of Asian descent tend to have higher health risks at lower BMI values. Some health organisations use lower BMI thresholds for Asian populations — overweight starting at 23 rather than 25, for example.
Despite these limitations, BMI remains a useful first-pass screening tool at the population level and in clinical settings where more detailed measurements aren't practical. It's most meaningful when considered alongside other markers like waist circumference, blood pressure, blood glucose, and cholesterol.
The table below shows the healthy weight range for common heights in both metric and imperial units, based on a BMI of 18.5 to 24.9.
| Height | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|
| 155 cm / 5'1" | 44 – 60 kg | 97 – 132 lb |
| 160 cm / 5'3" | 47 – 64 kg | 104 – 141 lb |
| 165 cm / 5'5" | 50 – 68 kg | 110 – 149 lb |
| 170 cm / 5'7" | 54 – 72 kg | 118 – 159 lb |
| 175 cm / 5'9" | 57 – 76 kg | 125 – 168 lb |
| 180 cm / 5'11" | 60 – 81 kg | 132 – 178 lb |
| 185 cm / 6'1" | 63 – 85 kg | 140 – 188 lb |
| 190 cm / 6'3" | 67 – 90 kg | 147 – 198 lb |
This BMI calculator uses the standard WHO formula — weight (kg) divided by height (m) squared — and supports both metric and imperial input. Results include your BMI category, healthy weight range for your height, and how much to gain or lose to reach the healthy range. BMI is a screening tool only and does not constitute medical advice. Always consult a healthcare professional for personalised health guidance.