BMI: how to calculate body mass index, classification and limitations
What BMI is and what it is used for
Body mass index (BMI) relates weight to height to estimate whether an adult's body size is below, within or above the range associated with lower risk. The weight/height² ratio dates from the nineteenth century, but the name body mass index was proposed by Ancel Keys and colleagues in 1972, who compared it with other indices and found it the simplest and least dependent on height.
BMI is used in nutritional screening, in population studies and in weight monitoring. The World Health Organization adopted the classification ranges in Technical Report 894, on obesity, published in 2000. The World Obesity Federation points out that BMI measures size, not health: it is more useful for estimating risk in groups than for diagnosing an individual.
In the laboratory context, BMI often appears alongside metabolic tests. Insulin resistance, for example, can be estimated with the HOMA-IR index, and the lipid profile with the LDL cholesterol calculator.
How to calculate BMI: inputs, units and formula
BMI is calculated by dividing weight in kilograms by height in meters squared. This calculator asks for two inputs:
- Weight, in kilograms (kg). Example: 70.
- Height, in centimeters (cm). Example: 175.
The tool divides height by 100 to get meters and applies the formula:
BMI (kg/m²) = Weight (kg) ÷ [Height (cm) ÷ 100]²
Both fields must be greater than zero. If height is below 3, the tool warns that the value appears to be in meters; heights outside 40 to 260 cm and weights above 500 kg also produce an error message. BMI is shown with two decimal places, together with the WHO class, and a warning reminds you that the ranges apply only to non-pregnant adults.
Weight and height are also requested in other calculators on the site, such as creatinine clearance. It is worth using the same measurements in both.
Worked BMI calculation example
An adult weighing 70 kg and 175 cm tall has a BMI of 22.86 kg/m². The step-by-step calculation:
- Height in meters: 175 ÷ 100 = 1.75 m.
- Height squared: 1.75 × 1.75 = 3.0625 m².
- BMI: 70 ÷ 3.0625 = 22.857..., displayed as 22.86 kg/m².
- Class shown by the tool: Normal weight.
A second example, near the limit: 80 kg and 179.2 cm give 80 ÷ 1.792² = 24.91 kg/m². Because overweight only begins at 25.0, the calculator classifies this value as Normal weight, in line with the WHO. On the other hand, 85 kg and 175 cm give 27.76 kg/m², in the overweight range.
The examples are for teaching purposes and do not represent an assessment of any person. Interpretation of body weight takes into account body composition, age and clinical context.
How to understand the result: WHO ranges for adults
The adult BMI classification proposed by the WHO expert committee (Technical Report 894, from 2000, reproduced by the World Obesity Federation) uses the ranges below, in kg/m².
| Class | WHO range | Rule used by the tool |
|---|---|---|
| Underweight | below 18.5 | less than 18.5 |
| Normal weight | 18.5 to 24.9 | from 18.5 to less than 25 |
| Overweight (pre-obesity) | 25.0 to 29.9 | from 25 to less than 30 |
| Obesity class I | 30.0 to 34.9 | from 30 to less than 35 |
| Obesity class II | 35.0 to 39.9 | from 35 to less than 40 |
| Obesity class III | 40.0 or more | 40 or more |
The tool compares the unrounded value with the whole numbers 25, 30, 35 and 40, so that no result is left without a class: 24.95, for example, is still normal weight. The World Obesity Federation writes healthy weight as 18.5 to 24.99 for the same reason. The class is a population reference; individual assessment is up to the healthcare professional.
Common errors and BMI verification checklist
The most frequent error is entering height in meters in the field that asks for centimeters. With height entered as 1.75, the tool does not calculate and warns that the value appears to be in meters; just correct it to 175.
Check before recording:
- Height in cm: 175, not 1.75.
- Weight in kg: converted from pounds if needed, and not confused with grams.
- Decimal separator: use a point or the separator offered by the device keyboard in the numeric field.
- Measured values: measured weight and height are more reliable than values reported from memory.
- Weighing conditions: light clothing and no shoes.
- Value near a limit: a difference of a few centimeters or 1 kg can change the class; it is worth repeating the measurement before drawing conclusions.
- Population: is the person an adult and not pregnant?
For metabolic targets in the same patient, the estimated average glucose from HbA1c complements the assessment.
When BMI can mislead
BMI does not distinguish muscle from fat or show where fat is located. For that reason, it gets predictable groups wrong, according to the World Obesity Federation and the WHO report:
- Very muscular people, such as athletes, may be classified as overweight or obese without excess fat.
- Children and adolescents do not use the adult ranges: assessment is done with BMI-for-age-and-sex curves.
- Asian populations have higher cardiometabolic risk at lower BMI, and the WHO discusses specific cutoffs for them.
- Pregnant women have expected weight gain, which is not assessed with the adult ranges.
- Older adults may lose muscle mass and height, which alters the relationship between BMI and body fat.
The tool does not ask about age, sex or pregnancy: it always applies the adult ranges and shows a warning that they do not apply to children, adolescents and pregnant women. The result is a starting point for a conversation with the professional, not a diagnosis of obesity or malnutrition.
BMI, waist circumference and body composition
BMI is the simplest measure of body size, but not the only one. Other measures complement what it does not show:
| Measure | What it assesses | Main limitation |
|---|---|---|
| BMI | Weight relative to height | Does not separate muscle from fat |
| Waist circumference | Abdominal fat | Depends on measurement technique |
| Waist-to-hip ratio | Fat distribution | Two measurements, more chance of error |
| Bioimpedance | Estimate of fat mass and lean mass | Varies with hydration and equipment |
| Absorptiometry (DXA) | Body composition by region | Cost and access |
Keys and colleagues, in 1972, chose weight/height² because it correlated better with body fat than other simple indices, without claiming that it measures fat directly. In cardiovascular risk assessment, BMI is considered together with tests such as LDL and kidney function by CKD-EPI.
Frequently asked questions about BMI (body mass index)
What is the BMI formula?
BMI = weight (kg) ÷ height (m)². In this calculator, height is entered in centimeters and the tool divides by 100 before squaring.
Is a BMI of 24.95 normal weight or overweight?
Normal weight. According to the WHO, overweight begins at 25.0 kg/m². The tool compares the unrounded value with 25, so 24.90 to 24.99 remain normal weight.
Can BMI be used for children?
Not with the adult ranges. In children and adolescents, BMI is compared with curves for age and sex. This calculator applies only the adult ranges.
Does BMI measure body fat?
Not directly. It relates weight and height and correlates with fat in groups, but it does not distinguish muscle from fat. Athletes can have a high BMI without excess fat.
Can I enter height in meters?
No. The field asks for centimeters. If you enter 1.75, the tool shows a warning that height appears to be in meters and does not calculate. Enter 175.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. Geneva: WHO; 2000. (WHO Technical Report Series, 894). View
- Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329-43. Republished in: Int J Epidemiol. 2014;43(3):655-65. doi:10.1093/ije/dyu058. View
- World Obesity Federation. Obesity classification. London: World Obesity Federation; [accessed Oct 25, 2026]. View
Updated . A support tool for educational purposes; it does not replace clinical assessment.