The adult BMI formula is weight in kilograms divided by height in metres squared: BMI = kg ÷ m². If you enter height in centimetres, divide it by 100 before squaring it. The result is a screening measure that needs interpretation, especially for Indian readers, because South Asian health risks can occur at lower BMI values than the broad international categories suggest.
The World Health Organization's obesity and overweight fact sheet gives the formula and the broad adult thresholds of 25 for overweight and 30 for obesity. The calculation itself is straightforward. Understanding what it can and cannot establish is the more important next step.
| Step | Hypothetical example |
|---|---|
| Record weight | 72 kg |
| Convert height | 175 cm ÷ 100 = 1.75 m |
| Square height | 1.75 × 1.75 = 3.0625 |
| Divide weight by squared height | 72 ÷ 3.0625 = 23.5102... |
| Round the displayed result | BMI 23.5 |
How to calculate BMI in kg and cm
The example above uses invented measurements to demonstrate the arithmetic. Do not divide kilograms by centimetres squared without adjusting the units. The CDC's BMI FAQ gives an equivalent centimetre formula: weight in kg divided by height in cm twice, then multiplied by 10,000.
For the same example, 72 ÷ 175 ÷ 175 × 10,000 gives about 23.5. This is the same calculation in different units. A common input mistake is entering 175 into a field that asks for metres. Check the unit label and the measurements before interpreting an unexpectedly high or low result.
What is a normal BMI range for adults?
The CDC's adult category table, intended for people aged 20 and above, uses the ranges below. They are screening categories, not a complete assessment of a person's health. WHO's broad adult definitions also place the overweight threshold at 25 and the obesity threshold at 30.
| Broad adult screening category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 to below 25 |
| Overweight | 25 to below 30 |
| Obesity | 30 or above |
The words in the table describe the screening bands used by that source. They do not tell you someone's fitness, blood pressure, laboratory results or medical history. A BMI within the band labelled healthy weight should not be read as confirmation that every aspect of health is normal.
Why BMI interpretation is different for South Asian adults
The NHS adult overweight and obesity guidance uses lower thresholds for people from South Asian and certain other ethnic backgrounds. It lists BMI 23-27.4 as overweight and 27.5 or above as obesity for those groups, because health problems can occur at a lower BMI.
This distinction matters when an Indian reader compares calculators. The hypothetical 23.5 result sits within the broad healthy-weight band but crosses the NHS South Asian overweight threshold. That difference reflects the interpretation applied to the same number, not an error in the formula. It is a reason for an appropriate health assessment, not proof that a disease is present.
These are the named sources' screening frameworks, rather than a claim that every clinic in India uses one identical table. When discussing a result with a qualified doctor, mention your background and ask which framework is being used. Comparing the number without comparing the guidance can create needless confusion.
What can BMI not tell you?
The CDC explains that BMI cannot distinguish fat from muscle and bone, or show where body fat is located. Someone with substantial muscle may fall into a higher BMI band without having the same body composition as another person at that number. A fuller assessment can include medical history, health behaviours, examination and laboratory findings.
The NHS guidance also discusses waist-to-height ratio for adults with BMI below 35, with a general aim of keeping waist circumference below half of height. That is a different calculation from BMI and from waist-to-hip ratio. It adds information about the waist area; it does not turn either measurement into a stand-alone diagnosis.
Who should not use a standard adult BMI calculator?
The NHS adult calculator guidance excludes people under 18, pregnant people, those with a diagnosed or suspected eating disorder, and people with a condition affecting height. Its tool is for adults aged 18 and above; the CDC adult table cited here starts at 20. Follow the age range of the specific tool you use.
Children and teenagers need age-appropriate assessment, rather than the adult table applied unchanged. If you have a medical condition or concerns about your weight, consult a qualified doctor before drawing conclusions or changing your diet or activity. The NHS explicitly says its calculator should not be used to diagnose symptoms.
For other practical explainers, see our guides to reading food labels in India, adult sleep needs and beginner home exercise.
