What is BMI?
Body Mass Index (BMI) is a simple measure used by the World Health Organization to estimate whether an adult's weight suits their height. It's weight in kilograms divided by height in meters squared: BMI = weight ÷ height². Example: 70 kg at 1.70 m gives 70 ÷ 2.89 = 24.2 — a healthy weight.
BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 34.9 | Obesity class I |
| 35 – 39.9 | Obesity class II |
| 40+ | Obesity class III |
Healthy weight for your height
The tool calculates the weight range that puts your BMI between 18.5 and 24.9. For someone 170 cm tall, that's roughly 53.5–72 kg. It also tells you how much you'd need to lose or gain to reach that range.
Limitations of BMI
BMI doesn't distinguish fat from muscle, so a muscular athlete may be classed as “overweight” while being perfectly healthy. It's not suitable for children and teens (who are assessed with growth charts) or pregnant women, and some populations, such as people of Asian descent, may face health risks at lower BMIs. Use it alongside other measures such as waist circumference — ideally under 94 cm (37 in) for men and 80 cm (31.5 in) for women.
Next steps
- Find your daily energy needs to lose or gain weight with the Calorie Calculator.
- Work out how much water you need each day with the Water Intake Calculator.
- Aim to lose no more than 0.5–1 kg (1–2 lb) per week to protect your health.
⚕️ This tool is for general information and is not a substitute for advice from a doctor or dietitian, especially if you have a chronic condition.
Frequently Asked Questions
What is a normal BMI?
A healthy adult BMI is between 18.5 and 24.9.
Is BMI different for men and women?
The formula and categories are the same for adults, but fat distribution differs, so waist measurement is also useful.
Can I use it for children?
No. Children and teens are assessed with age- and sex-specific growth charts — ask a pediatrician.
How do I calculate BMI in pounds and feet?
Choose Imperial, enter your height in feet and inches and weight in pounds; the tool converts automatically.
Is my data stored?
No. The calculation happens in your browser and your health data goes nowhere.
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