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Body Mass Index (BMI)
BMI (Body Mass Index) compares your weight to your height. It helps identify possible underweight or overweight, but it does not alone describe your overall health.
Body Mass Index in brief
- BMI (Body Mass Index) describes the relationship between weight and height. The normal BMI range for adults is 18.5–25.
- BMI is widely used, but it does not give a complete picture of health because it does not account for age, sex, genetics, or how muscle and fat are distributed.
- Understand your wellbeing with comprehensive body measurements and laboratory tests to track your health over time.
Table of contents
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What is BMI?
BMI is a traditional health indicator that reflects the relationship between weight and height. It helps identify whether a person is in the normal weight range or possibly underweight, overweight, or obese. It is suitable for most adults.
The normal-weight range is defined as the range where health outcomes are generally best. There is no single “ideal weight,” and the normal range is broad. Long-term weight management is best for health. Reducing excess weight can bring several health benefits:
- Lower risk of conditions linked to overweight.
- Better quality of life and energy levels.
- Potential improvements in sleep.
- In some cases, the possibility to discontinue certain disease-related medications.
How to calculate BMI
BMI is calculated by dividing weight by height squared (weight ÷ [height × height]). Use kilograms for weight and meters for height.
- Example: A person who is 1.70 m tall and weighs 85 kg: 85 ÷ (1.70 × 1.70) = 85 ÷ 2.89 = 29.4.
- In this example, a BMI of 29.4 indicates class I obesity (mild obesity).
BMI chart
A BMI chart helps interpret whether someone is underweight, normal weight, or overweight.
For adults, the normal range is BMI 18.5–25. An adult is considered overweight when BMI is over 25. For those over 65, a slightly higher BMI may be appropriate for health; for them, a normal BMI is typically 23–29.
| BMI (Body Mass Index) | Classification |
| under 17 | Severe undernutrition |
| under 18,5 | Underweight |
| 18,5–25 | Normal weight |
| 25–30 | Overweight (class I obesity) |
| 30–35 | Obesity (class II) |
| 35–40 | Severe obesity (class III) |
| over 40 | Morbid obesity |
Trefethen’s alternative BMI formula
In 2013, Professor of Numerical Analysis Nick Trefethen (University of Oxford) proposed an alternative BMI formula. His approach better accounts for height-related distortion. It can more readily flag short individuals who are overweight and, conversely, tall individuals who are actually lean. According to Trefethen, it offers an estimate closer to real human shape and size.
Trefethenin kehittämän painoindeksin laskukaava
Trefethen’s BMI calculation
The proposed formula is 1.3 × weight (kg) ÷ height^2.5 (m). The weight factor 1.3 corresponds to the square root of the average human height of 1.69 m. The exponent 2.5 is based on the idea that people are not two-dimensional, so weight should not be scaled purely by height squared.
Example: For someone 1.70 m tall and weighing 85 kg: 1.3 × 85 ÷ 1.70^2.5 = 29.7.
Limitations of BMI
While BMI is useful at the population level, there are limitations when assessing individuals, and it does not fit all situations.
- BMI can be within the normal range even if too much fat has accumulated inside the abdominal cavity (visceral fat). Measuring waist circumference can reveal this.
- Large muscle mass can increase weight even when body fat is not elevated, leading to a misleading classification.
- Fluid retention can also distort results. However, fluid must be significantly elevated to affect BMI, typically due to an underlying medical condition.
If you want to track weight management and health risks as a whole, start by measuring key baseline values. With Puhti, you can take tests easily without a referral and follow your progress in your Oma Puhti report.
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Usein kysyttyä painoindeksistä
Weight can significantly affect fertility. Both underweight and overweight can disrupt hormone balance and make it harder to conceive. Overweight may also lead to insulin resistance, which is linked to ovulation disorders. A balanced weight supports hormone health and improves the chances of conception.
Calculate BMI by dividing weight in kilograms by height in meters squared: BMI = weight ÷ height². Use kilograms and meters to get a correct result.
BMI is intended for adults over 18. For those over 65, BMI can be slightly above 25 without increased disease risk. In this age group, a normal BMI is about 23–29.
For adults, normal weight corresponds to BMI 18.5–25. For people over 65, a suitable range may be around 23–29. Overweight starts at 25 and obesity at 30.
No. BMI only reflects the ratio of weight to height and not fat distribution. A more reliable way to assess visceral fat is a body composition measurement. In such measurements, visceral fat is reported in cm² as an abdominal cross-section at navel level.
An individual is considered overweight when BMI is 25 or higher. Obesity begins when BMI exceeds 30. Overweight increases the risk of several health problems, such as cardiovascular disease and type 2 diabetes.