BMI Explained: How It's Calculated and Where It Falls Short
How body mass index is calculated in metric and imperial units, what the adult categories mean, and why BMI is a screening number rather than a diagnosis.
By the CalClu team2 min read
Body mass index (BMI) is a quick way to compare weight with height. It's widely used because it needs only two measurements, but it's worth knowing what it can and can't tell you.
The formula
In pounds and inches, multiply by 703:
Example: someone 1.75 m tall who weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.
Standard adult categories
The World Health Organization uses these ranges for adults:
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 and above: obesity
Children and teenagers are assessed differently, using BMI-for-age percentiles, because their body composition changes as they grow.
Where BMI falls short
BMI can't tell muscle from fat, or show where fat is carried. A muscular athlete can land in the "overweight" range while being very fit, and an older adult who has lost muscle can look "healthy" on paper. Some health organizations also use different cut-offs for certain ethnic groups, because health risks can begin at different BMI levels.
Waist measurement adds useful context, since fat carried around the middle is more closely linked to heart and metabolic risks.
How to use your result
Treat BMI as a starting point for a conversation, not a verdict. If your number sits outside the healthy range, or you're worried about your weight, a doctor can look at the whole picture: blood pressure, blood tests, activity and family history.
Don't forget hydration
Everyday health numbers go beyond weight. How much water you need depends on your body size, activity level and climate, and it rises in hot weather and during exercise.
CalClu calculators and guides are for general information only and aren't medical advice.
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