How do you calculate BMI?
BMI uses a simple formula, the same for adult women and men:
BMI = weight (kg) ÷ height (m)²
Weight is in kilograms and height in metres: 170 cm becomes 1.70 m. The result is in kg/m², but people usually quote the number alone.
Worked example
- A person is 1.75 m tall and weighs 70 kg.
- Height squared: 1.75 × 1.75 = 3.0625.
- BMI: 70 ÷ 3.0625 = 22.9 (rounded to one decimal place).
- In the WHO classification, a BMI of 22.9 is in the normal weight range.
Another example: for 1.80 m and 92 kg, 92 ÷ 3.24 = 28.4, which falls in the overweight range. The calculator does this for you and rounds the result to one decimal place.
What is the WHO classification of BMI?
The World Health Organization (WHO) uses the following thresholds for adults. They are the same for women and for men.
| BMI (kg/m²) | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight (pre-obesity) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40 or above | Obesity class III |
The calculator groups the three obesity classes into a single "Obese" category from 30. In WHO terms, overweight means a BMI of 25 or more, and obesity a BMI of 30 or more.
The further BMI moves away from the 18.5 to 24.9 range, the more the risk of some health problems rises on average across the population, for example cardiovascular disease or type 2 diabetes when BMI is high. On its own, BMI does not tell you whether a given person is healthy.
What is a healthy weight for my height?
For each height, the table shows the weight that gives a BMI of 18.5 and the weight that gives a BMI of 24.9, the limits of the WHO normal weight range. The lower weight is rounded up and the upper weight rounded down to the nearest 0.1 kg, so every value stays within the normal range.
| Height | Weight at BMI 18.5 | Weight at BMI 24.9 |
|---|---|---|
| 150 cm | 41.7 kg | 56.0 kg |
| 155 cm | 44.5 kg | 59.8 kg |
| 160 cm | 47.4 kg | 63.7 kg |
| 165 cm | 50.4 kg | 67.7 kg |
| 170 cm | 53.5 kg | 71.9 kg |
| 175 cm | 56.7 kg | 76.2 kg |
| 180 cm | 60.0 kg | 80.6 kg |
| 185 cm | 63.4 kg | 85.2 kg |
| 190 cm | 66.8 kg | 89.8 kg |
For any other height, the calculation is the same: multiply 18.5 and then 24.9 by your height in metres squared. For example, at 1.68 m: 18.5 × 2.8224 = 52.21 kg, rounded up to 52.3 kg, and 24.9 × 2.8224 = 70.28 kg, rounded down to 70.2 kg. The range is wide on purpose: it covers very different body shapes.
What are the limits of BMI?
BMI is convenient because it only needs scales and a measuring tape. But it measures neither body fat nor where that fat sits, so it should be read with caution in several situations.
Muscle mass
Muscle is denser than fat. A very muscular person, a strength athlete for instance, can have a BMI in the overweight range without excess body fat.
Age
With age, muscle mass tends to decrease and fat mass to increase, even at the same BMI. For older adults, the result is best interpreted with a health professional.
Pregnancy
BMI is not used to judge body size during pregnancy, when weight gain is expected. Pre-pregnancy BMI is used instead to guide care.
Children and teenagers
Under 18, adult thresholds do not apply. BMI is compared with growth charts by age and sex, expressed as percentiles.
Ethnicity
At the same BMI, the amount of body fat and the health risk vary between populations. In some populations, notably people of Asian origin, risks can appear at lower BMI values, and some countries use different thresholds.
Fat distribution
Fat carried around the waist is associated with a higher risk than fat carried on the hips. Waist circumference is a useful complement to BMI.
A BMI outside the normal range is a good reason to talk to a doctor, not a diagnosis. Likewise, a normal BMI does not on its own guarantee good health.
How are BMI and calories linked?
BMI describes your situation at one point in time. What changes it is the balance between the energy you take in and the energy you use: eating less than you burn lowers your weight, eating more raises it.
If your BMI is above 25 and your doctor advises you to lose weight, a moderate deficit, often around 500 kcal per day, is a common starting point. If your BMI is below 18.5, talk to a health professional first to understand why.
To estimate your daily needs, use our calorie calculator: it works out your basal metabolic rate and total energy expenditure from your activity level.
Frequently asked questions
What is a normal BMI for a woman or a man?
For adults, the WHO uses the same thresholds for women and men: a BMI between 18.5 and 24.9 is in the normal weight range. At the same BMI, women have more body fat than men on average, but the thresholds stay the same.
How do you calculate BMI without an online calculator?
Square your height in metres, then divide your weight by the result. For 1.65 m and 60 kg: 1.65 × 1.65 = 2.7225, then 60 ÷ 2.7225 = 22.0.
Is a BMI of 25 bad?
A BMI of 25 is the WHO threshold for overweight. It is not a diagnosis: health risk also depends on waist size, physical activity, diet and medical history. If you have questions, it is a good opportunity to talk to your doctor.
Is BMI accurate for athletes?
Not always. BMI does not distinguish muscle from fat, so a very muscular person can be classed as overweight while having little body fat. Other measures, such as waist circumference or a body composition test, are more telling in that case.
Can I use this BMI calculator for a child?
No. Under 18, BMI must be compared with growth charts by age and sex, expressed as percentiles, because the WHO adult thresholds do not apply. Ask your doctor or check the growth charts used in your country.
Does BMI change with age?
The formula and the WHO thresholds are the same for all adults. Body composition, however, changes with age: at the same BMI, an older person often has less muscle and more fat. Interpretation is therefore done case by case with a professional.
How can I lower my BMI?
BMI goes down when weight goes down. A moderate calorie deficit, a varied diet and regular physical activity are the usual levers. Aim for gradual loss and ask a health professional for advice, especially if you live with a chronic illness.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height. Body fat percentage measures the share of fat in your body weight, which requires specific tools such as bioelectrical impedance scales. BMI is simpler to get, but less precise for an individual.
Sources
This content is for information only. It does not replace a consultation with a doctor.
Updated on 14 September 2026 · Method: Ubod team
