How is BMI calculated?
BMI divides your weight in kilograms by your height in metres squared. That single division, independent of age and sex, produces a number that makes weights comparable across different heights.
BMI = weight (kg) ÷ height² (m)
If you know your height in centimetres, divide by 100 first to get metres.
For example, someone 170 cm tall weighing 70 kg: 1.7 × 1.7 = 2.89, and 70 ÷ 2.89 ≈ 24.2. That falls inside the healthy weight range.
BMI classification table
The World Health Organization defines the following ranges for adults. The boundaries are conventions — a tenth of a point either side of one does not represent a meaningful difference in your health.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obesity class 1 | 30 – 34.9 |
| Obesity class 2 | 35 – 39.9 |
| Obesity class 3 | 40 and above |
Ranges as published in the WHO adult classification.
What this number does not tell you
BMI is a population screening tool, not a measure of individual health. It takes two numbers and misses a great deal of what actually matters. If your result worries you, read this section.
It cannot tell muscle from fat
Muscle is denser than fat. A trained athlete with low body fat can land above 25 while their body composition is entirely healthy.
It does not know where the fat is
Fat carried around the abdomen behaves differently from fat distributed across the hips and thighs, but both produce the same BMI. Waist-to-height ratio — available in this page’s advanced mode — does distinguish them.
It does not apply to children or teenagers
Under 18, body composition changes with growth. Age- and sex-specific percentile charts are used instead of these ranges, and interpreting them is a clinician’s job.
It does not apply in pregnancy
Weight gain during pregnancy is normal and necessary. These ranges are not meaningful in that period; weight should be followed with a midwife or doctor.
It is less accurate in older adults
With age, muscle mass falls and fat mass rises without the scale necessarily moving. BMI holds steady while body composition has changed underneath it.
One number is not the whole picture
Blood pressure, blood sugar and lipids, sleep quality, activity level and eating patterns each say more about your health than this ratio does.
Frequently asked questions about BMI
What is a normal BMI?
For adults, 18.5 to 24.9 is classified as a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 and above falls into the obesity classes. These ranges do not apply under 18 or during pregnancy.
Is BMI different for men and women?
No. The formula and the ranges are the same for both. Average body fat percentage is naturally higher in women, though, so two people with the same BMI can have quite different body composition.
What is my ideal weight?
There is a range rather than a single number: the weight that keeps your BMI between 18.5 and 24.9. This calculator shows that range for your height. At 170 cm, for instance, it runs from roughly 53.5 kg to 72 kg.
What if I am an athlete?
If you train with weights regularly, BMI probably overstates your result, because it counts muscle the same as fat. Waist-to-height ratio or a direct body composition measurement is a better guide.
How many calories do I need per day?
That depends on your height, weight, age, sex, activity level and goal. Open the "How many calories do I need a day?" step on this page to work out a daily target and macro split using the Mifflin-St Jeor equation.
Are my measurements stored?
No. The calculation runs entirely in your browser and no numbers are sent to a server. If you choose to build a plan, your measurements are kept on that device only, to fill in the signup form for you.