BMI explained: your weight, height & the bigger picture
Body mass index (BMI) describes weight in relation to height. It offers a starting point for understanding weight status, but a number alone cannot describe your overall health. Here is how to read the ranges, work through the maths, and put your result in context.
What is BMI?
Adult BMI is a weight-for-height measure expressed in kg/m2. Clinicians use it for health screening alongside medical history, examination findings, and other measurements. It does not directly measure body fat percentage or diagnose a medical condition.
A BMI result is most useful as part of a conversation about health, rather than a personal score. Two people with the same BMI can have different muscle mass, fat distribution, and health needs.
How to calculate BMI: formula and examples
The BMI formula divides weight by height squared. A metric BMI calculation uses kilograms and metres; an imperial BMI calculation uses pounds and inches with a conversion factor.
| Units | Formula | Worked example |
|---|---|---|
| kg & m | kg ÷ m2 | 70 ÷ 1.752 = 22.9 |
| kg & cm | (kg ÷ cm2) × 10,000 | (70 ÷ 1752) × 10,000 = 22.9 |
| lb & in | (lb ÷ in2) × 703 | (154 ÷ 692) × 703 = 22.7 |
For height and weight measurements, keep the units consistent: 175 cm becomes 1.75 m, while 5 ft 9 in becomes 69 inches. Square the full height before dividing. The examples are rounded to one decimal place.
Adult BMI chart: understanding the categories
This BMI chart for adults follows CDC categories for people aged 20 and older. The healthy BMI range, sometimes called the normal BMI range, starts at 18.5 and ends below 25.
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity: class 1 | 30 to less than 35 |
| Obesity: class 2 | 35 to less than 40 |
| Obesity: class 3 | 40 or higher |
The underweight BMI range, overweight BMI range, and obesity classes describe screening categories. They do not establish a diagnosis on their own. Use the unrounded result when it is close to a boundary.
Weight ranges by height: a practical reference
A height and weight chart translates BMI into kilograms. The examples below use BMI 18.5 and 24.9 as reference endpoints. They illustrate the maths, rather than prescribe an ideal body weight.
| Height | Lower weightBMI 18.5 | Upper weightBMI 24.9 |
|---|---|---|
| 150 cm | 41.6 kg | 56.0 kg |
| 160 cm | 47.4 kg | 63.7 kg |
| 170 cm | 53.5 kg | 72.0 kg |
| 175 cm | 56.7 kg | 76.3 kg |
| 180 cm | 59.9 kg | 80.7 kg |
| 190 cm | 66.8 kg | 89.9 kg |
Calculated as weight = BMI × height in metres squared; rounded to 0.1 kg. The formal healthy-weight category extends to any BMI below 25, not only to 24.9.
BMI for men, women, children and teens
BMI for men and BMI for women uses the same adult calculation and standard category boundaries. Age and sex do not change that arithmetic for adults aged 20 and older, although they remain relevant to a broader health assessment.
BMI for children and teens needs a different interpretation. Between ages 2 and 19, BMI-for-age percentiles compare a young person's result with a reference population of the same age and sex. Adult cutoffs should not replace growth-chart assessment.
Beyond BMI: what the number cannot tell you
The main limitations of BMI concern body composition. Weight includes muscle, bone, and fat; BMI cannot separate them. An athlete with substantial muscle may have a high BMI without the same fat levels as someone else at that BMI.
Waist circumference adds information about abdominal fat. NIDDK notes that fat around the waist can be associated with weight-related health risks even when BMI is not high. A fuller assessment also considers blood pressure, blood tests, and personal medical history.
Put the result in perspective
A healthy weight assessment is broader than reaching one number. If your weight or result concerns you, discuss it with a qualified health professional who can interpret it in your circumstances.
Frequently asked questions about BMI
What does BMI stand for?
BMI stands for body mass index, a measure calculated from weight and height. It is expressed in kilograms per square metre.
What is a healthy BMI for an adult?
For adults aged 20 and older, the standard healthy-weight category is 18.5 to less than 25. It is a screening range, not a guarantee of health.
How do I calculate BMI using kilograms and centimetres?
Divide weight in kilograms by height in centimetres squared, then multiply by 10,000. At 70 kg and 175 cm, the result is approximately 22.9.
Is a BMI of exactly 25 considered overweight?
Yes. The adult overweight category begins at 25. The healthy-weight category ends below 25; categories are best assigned before rounding the result.
Is BMI calculated differently for men and women?
No. The adult formula and standard category boundaries are the same. Body composition and individual health factors still matter when interpreting the result.
Can muscular people have a high BMI?
Yes. BMI includes all body weight and cannot distinguish muscle from fat. A higher result in a muscular person needs additional context.
Can children use the adult BMI chart?
No. Ages 2 through 19 require BMI-for-age interpretation using sex-specific growth charts, rather than the fixed adult category boundaries.
Does BMI tell me my body fat percentage?
No. BMI is a weight-for-height measure. It does not report a percentage of body fat or show where fat is distributed.