Free online tool
BMI Calculator
Calculate your Body Mass Index in metric or imperial units and see which WHO category it falls into.
What BMI measures — and what it doesn't
Body Mass Index is a simple ratio of weight to height, calculated as kilograms divided by height in metres squared. It's widely used as a population-level indicator of whether someone is under-, normal-, over- or obese-weight. It's a rough screening tool — it doesn't distinguish between muscle and fat, doesn't account for body composition, age or ethnicity, and can misclassify athletes with high muscle mass.
How to interpret your result
The WHO categories are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese. Treat your number as one data point among many. If you're concerned about your weight or health, talk to a doctor — they'll consider your full picture, not just a single ratio.
+Is BMI accurate for athletes?
Not really. A muscular athlete can have a BMI in the 'overweight' range while having low body fat. Body-fat percentage is a better indicator for very active people.
+Is my data private?
Yes. Everything runs in your browser. Nothing is sent, stored or logged.
Reading your BMI number without overreacting to it
Body Mass Index divides your weight in kilograms by the square of your height in metres, giving a single number that sorts people into underweight, normal, overweight and obese bands. It was designed in the 1830s by the Belgian statistician Adolphe Quetelet as a population-level statistic for comparing groups of people, not as a diagnostic tool meant to describe any one individual standing in front of a doctor.
Because it only needs two inputs, BMI became the fastest screening number available to clinics, insurers and researchers, and it still shows up on almost every general health checklist today. That convenience is also its biggest weakness: a formula built for tracking trends across thousands of people inevitably loses accuracy when applied to a single body with its own muscle mass, bone density and fat distribution.
Where the formula breaks down
A rugby prop at 100 kg and 178 cm and a sedentary office worker at the same weight and height get an identical BMI of about 31.6, yet their body composition could be wildly different. Muscle is denser than fat, so athletes and heavily muscled people are frequently flagged as overweight or obese when their actual health risk is low.
The formula also doesn't distinguish where fat is carried, and abdominal fat carries more cardiovascular risk than fat carried on the hips or thighs, which BMI simply cannot see. Two people with the same BMI can have very different waist-to-hip ratios and very different real-world risk profiles.
Worked example
Take someone who is 170 cm tall and weighs 68 kg. Height in metres squared is 1.7 x 1.7 = 2.89. Divide 68 by 2.89 and you get roughly 23.5, which sits inside the commonly cited normal range of 18.5 to 24.9.
Now compare a second person, 170 cm tall but 82 kg: 82 divided by 2.89 gives about 28.4, landing in the overweight band. The formula treats both people identically regardless of whether that extra 14 kg is fat, muscle from years of strength training, or water retention.
How age and sex shift the picture
Standard BMI thresholds were calibrated mostly on adult populations and don't adjust for age or sex, even though body composition naturally changes across a lifetime. Older adults tend to carry more fat at the same BMI as younger adults due to gradual muscle loss, while women on average carry a higher percentage of body fat than men at an identical BMI. Some clinicians use adjusted charts for children, teenagers and people over 65 for exactly this reason.
A quick sanity check before you worry
This tool gives you a fast estimate, not a clinical verdict — if your number surprises you, a conversation with a doctor or nurse who can look at waist circumference, blood pressure, cholesterol and family history will tell you far more than the ratio alone. Treat the result here as informational only rather than medical advice or a diagnosis, and use it as a conversation starter with a qualified professional rather than a final answer about your health.
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