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What is BMI (Body Mass Index)?
BMI (Body Mass Index) is a simple screening measure based on the ratio of your weight to your height, showing whether your weight sits in a healthy range. Also used by the World Health Organization (WHO), it gives a quick reference point both for populations and for tracking an individual. It doesn't measure body fat on its own, but it's a practical, widely used way to gauge the balance between height and weight.
How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres:
Example: for someone 175 cm (1.75 m) tall weighing 75 kg:
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 ≈ 24.5 → Normal range.
Method: Quetelet index (kg/m²); categories follow the World Health Organization (WHO) adult classification.
BMI categories
The WHO classification for adults:
- Underweight: BMI below 18.5
- Normal (healthy): 18.5 – 24.9
- Overweight: 25.0 – 29.9
- Obese: 30.0 and above
The limits of BMI
BMI is a fast screening tool, but it isn't perfect. It can't tell muscle from fat: a muscular athlete can score in the 'overweight' range while carrying very little body fat. Age, sex, ethnicity and where fat sits on the body are also invisible to BMI. That's why it gives a truer picture when you read it alongside measurements like waist circumference and body fat percentage.
Frequently asked questions
For adults, the range considered healthy is 18.5 to 24.9. Staying within it keeps the risk of many chronic diseases comparatively low. Our tool also shows the weight range in kilograms that matches your height automatically (18.5–24.9 × height²).
It can be. BMI doesn't separate muscle from fat, so if you carry a lot of muscle your BMI can read high without meaning you're unhealthy. In cases like this, body fat percentage and waist circumference are more telling.
The formula is the same, but the interpretation differs. In children and teenagers, BMI is assessed using percentile curves for age and sex (where a child ranks against peers of the same age); the adult categories (18.5–24.9) don't apply directly. You can use our separate Child BMI Percentile tool for this.
The weight range in kilograms that corresponds to a healthy BMI (18.5–24.9) is a good reference for your 'ideal weight' at your height. For a more personalised goal, you can also look at the Ideal Weight tool and your body fat percentage.
Yes, you can be. BMI can't see <b>where</b> fat is stored on the body — yet the real danger is visceral fat around the abdomen (the hidden fat that wraps around the internal organs). When someone has a normal BMI but a wide waist, this is called "normal-weight obesity" and heart and blood-vessel risk can be high. That's why BMI should be read not on its own, but together with your waist circumference or waist-to-height ratio — a waist less than half your height is a good sign.
Not exactly. In athletes with high muscle mass, BMI can read 'overweight' even with no excess fat — because muscle is heavy. In older adults it's the opposite: even if weight stays the same, muscle drops and fat rises, so BMI can make things look better than they are; some studies also suggest that a slightly higher BMI (23–28) over the age of 65 may be protective. For these groups, body fat percentage and waist circumference are more reliable measures.