What Is BMI?
Body Mass Index (BMI) is a simple, widely used screening measure that relates your weight to your height. It gives a single number that helps classify whether a person is underweight, at a normal weight, overweight, or obese. Because it only needs two easy-to-measure values, BMI is a quick first step in assessing weight-related health risk for most adults.
How to Use This Calculator
Enter your weight in kilograms and your height in centimetres, then read the result. The calculator converts your height to metres, squares it, and divides your weight by that value. It also shows the standard category your BMI falls into so you can interpret the number at a glance.
The Formula Explained
The formula is $$\text{BMI} = \frac{\text{Weight (kg)}}{\left(\frac{\text{Height (cm)}}{100}\right)^2}$$ Height is entered in centimetres here, so it is first divided by 100 to convert to metres. For example, 175 cm becomes 1.75 m, and \(1.75^2 = 3.0625\). Dividing your weight by this figure gives your BMI in kg/m².
Worked Example
Suppose you weigh 70 kg and are 175 cm tall. Convert height: \(175 \div 100 = 1.75\) m. Square it: \(1.75 \times 1.75 = 3.0625\). Then $$\text{BMI} = 70 \div 3.0625 \approx 22.86,$$ which falls in the "Normal weight" range (18.5–24.9).
BMI Categories and Thresholds
Body Mass Index (BMI) groups adults into weight categories based on the ratio of weight to height squared. The World Health Organization (WHO) classification for adults aged 20 and over is shown below. BMI is expressed in units of \(\text{kg/m}^2\).
| Category | BMI range (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Normal (healthy) weight | 18.5 – 24.9 |
| Overweight (pre-obese) | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | 40.0 and above |
The general formula used is:
$$\text{BMI} = \frac{\text{Weight (kg)}}{\left(\dfrac{\text{Height (cm)}}{100}\right)^2}$$For example, a person weighing 68 kg at 170 cm has a BMI of \(68 / 1.70^2 = \) 23.5, which falls in the normal range.
Interpreting Your BMI Result
BMI is a screening tool, not a diagnostic measurement. It estimates whether a person's weight may be associated with health risk at a population level, but it does not directly measure body fat, fitness, or health.
- Underweight (below 18.5): May signal insufficient energy reserves and can be associated with nutritional deficiency, reduced immune function, or an underlying condition.
- Normal (18.5–24.9): The range statistically associated with the lowest weight-related health risk for most adults.
- Overweight (25.0–29.9): Indicates higher likelihood of excess body fat and an increased risk of conditions such as type 2 diabetes and cardiovascular disease.
- Obese (30.0 and above): Associated with progressively greater risk across classes I, II and III.
Key limitations. BMI does not distinguish muscle from fat, so very muscular individuals (e.g. athletes) may register as "overweight" despite low body fat. It can also be misleading for older adults who have lost muscle mass, for pregnant people, and for children and teenagers, who require age- and sex-specific percentile charts rather than these adult cut-offs. Body-fat distribution and ethnicity also affect risk: some populations experience elevated risk at lower BMI values, and several health bodies apply lower thresholds for certain Asian populations.
Because of these factors, a single BMI value cannot diagnose any condition. A result that is notably low or elevated is best discussed with a qualified healthcare professional, who can consider waist measurement, body composition, medical history and other factors. This information is general and educational, not medical advice.
FAQ
Is BMI accurate for everyone? BMI is a useful general indicator but does not distinguish muscle from fat. Very muscular athletes may show a high BMI without excess body fat, and it is not designed for pregnant people or children.
What is a healthy BMI? For most adults a BMI between 18.5 and 24.9 is considered the healthy range.
Does age or sex change the formula? The BMI formula itself is the same for adult men and women; only interpretation may vary by individual context.
