The BMI formula, worked out
The formula never changes:
Take a person weighing 70 kg who is 1.75 m tall: square the height (1.75 × 1.75 = 3.0625), then divide: 70 ÷ 3.0625 = 22.9. If you think in pounds and inches instead: BMI = 703 × weight (lb) / [height (in)]².
WHO BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These cut-offs come from the World Health Organization and apply to adults aged 20 and over. For children and teens, BMI is interpreted against age-and-sex growth charts instead.
What BMI doesn't tell you
BMI is a screening tool, not a diagnosis. It can't tell muscle from fat — a muscular athlete may land in "overweight" while being very lean. It also doesn't account for fat distribution (belly fat carries more risk), age, or pregnancy. Think of it as a smoke alarm: useful for flagging that a closer look is worthwhile, not the final word on your health.
❓ BMI FAQs
What is the BMI formula?
BMI = weight in kilograms divided by height in metres squared (kg/m²). Example: 70 kg at 1.75 m → 22.9.
What is a healthy BMI?
18.5–24.9 for adults, per the WHO. Below 18.5 is underweight, 25–29.9 overweight, 30+ obese.
Is BMI accurate?
As a screening tool, yes — but it doesn't distinguish muscle from fat and is less reliable for athletes, children, the elderly and pregnant women.
How is BMI different from body fat percentage?
BMI uses only height and weight; body fat percentage measures actual fat mass. BMI is the quick proxy, body fat percentage the precise measure.
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