Quick answer: Body Mass Index estimates body fat from height and weight: BMI equals weight in kilograms divided by height in meters squared. The World Health Organization classes 18.5 to 24.9 as the healthy adult range, 25 to 29.9 as overweight, and 30 or more as obesity. BMI is a screening tool, not a diagnosis.
Last reviewed: September 2026
Key facts
- BMI = weight (kg) ÷ height (m)² — the world’s most used weight-for-height check.
- Healthy range is 18.5 to 24.9; 25–29.9 is overweight, 30+ is obesity (WHO categories).
- Used by doctors and insurers worldwide as a screening tool — not a diagnosis.
- Muscular athletes can score high while being lean, because muscle weighs more than fat.
Body Mass Index (BMI) is the world’s most widely used quick check of whether your weight is in a healthy range for your height. This free BMI calculator computes your BMI instantly from your weight and height and tells you which category you fall into.
Enter weight in kilograms and height in centimetres. The result appears at once, with the category and a reminder that BMI is a screening starting point — not a full health verdict.
How to use
- Enter your weight in kilograms.
- Enter your height in centimetres.
- Your BMI and category appear instantly.
- Compare against the category table below to see where you stand.
BMI formula
| Term | Meaning |
|---|---|
| weight | Your body weight in kilograms. |
| height | Your height in metres (divide centimetres by 100). |
Example: 70 kg at 175 cm → 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9 — comfortably in the healthy range.
Worked examples
Healthy range
70 kg, 175 cm: BMI = 70 ÷ (1.75 × 1.75) = 22.9 — Normal range (18.5–24.9).
Overweight range
85 kg, 170 cm: BMI = 85 ÷ (1.70 × 1.70) = 29.4 — Overweight (25–29.9), close to the obesity threshold of 30.
For this height, the healthy weight range is roughly 53.5–72.3 kg (18.5 × 1.7² to 24.9 × 1.7²).
WHO BMI categories
| BMI | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate undernourishment; consider medical advice. |
| 18.5 – 24.9 | Normal / healthy | Associated with the lowest health risks. |
| 25 – 29.9 | Overweight | Above the healthy range; lifestyle review sensible. |
| 30 and above | Obesity | Higher risk of heart disease, diabetes and more; see a doctor. |
For children and teens, BMI is interpreted against age-and-sex growth charts, not these adult cut-offs.
Frequently asked questions
Is BMI accurate?
It is a useful population-level screening tool, but it cannot tell muscle from fat. Very muscular people, pregnant women and the elderly should interpret it with a clinician.
How do I calculate BMI with pounds and inches?
BMI = 703 × weight (lb) ÷ height (in)². Or convert first: this calculator uses kg and cm.
What is a healthy BMI?
18.5 to 24.9 for adults, according to the World Health Organization.
Does BMI work for children?
Not with adult cut-offs. Children’s BMI is plotted on percentile charts for their age and sex — ask a paediatrician.
My BMI is 27 — should I worry?
It is above the healthy range, which statistically raises some health risks. Small, sustained changes to diet and activity move it more than crash diets do.
Why does the calculator say “screening, not diagnosis”?
Because health is more than one number: waist size, blood pressure, cholesterol, fitness and family history all matter alongside BMI.
Sources
- World Health Organization — BMI classifications and healthy ranges
- CDC — About adult BMI and how to interpret it
Key takeaways
- BMI = kg ÷ m²; 18.5–24.9 is the adult healthy range.
- It screens populations well but can misread very muscular or elderly bodies.
- Use it as a starting point for a conversation with a clinician, not a verdict.
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