BMI Calculator

BMI the right way: why the China cutoffs differ from WHO, and its four limits

How to compute it

BMI = weight(kg) / height(m)2

Note it is metres, not centimetres. 170 cm must be written as 1.70, and squared is 2.89. Typing 170 makes the result collapse toward zero — the most common entry mistake and the one that produces absurdly low BMI.

Weigh under consistent conditions: empty-stomach morning, after using the toilet, in light clothing. A reading after a meal, after exercise, or in the evening can differ by 1–2 kg, enough to cross a category boundary for lighter people.

The China standard differs from the WHO standard

China's overweight and obese cutoffs are stricter than WHO's (24/28 against 25/30). This is not a typo: at the same BMI, people of Asian build tend to carry more body fat, and to store more of it around the abdomen, so metabolic risk rises at lower BMI levels. Chinese authorities therefore set an independent scale rather than adopt the WHO one wholesale.

Practical consequence: someone at BMI 24.5 is "normal" by WHO but "overweight" by China. Neither scale is wrong, but you should use the one that matches the population the person belongs to, or you understate the risk. This tool uses the China standard.

Four limits you must know

One — it does not separate muscle from fat. BMI uses only height and weight, yet muscle is about 18% denser than fat. A trained lifter can read "overweight" or even "obese" with a normal body-fat percentage; conversely a sedentary, low-muscle person can read "normal" yet hide "hidden obesity", with the risk still present but invisible. High BMI is not the same as fat, and normal BMI is not the same as healthy.

Two — it ignores fat distribution. Abdominal (visceral) fat is far more metabolically harmful than subcutaneous fat, and two people with identical BMI can differ sharply — one "big limbs", one "big belly". Waist circumference is an indispensable extra:

Measure at the midpoint between the lower ribs and the top of the hip bone, at the end of a normal exhale, without sucking in.

Three — children cannot use the adult scale. Body composition changes fast with age and sex, so the same BMI means something different. Children and teens must be judged on age- and sex-matched percentile curves (P85, P95), never fixed 18.5/24/28 cutoffs. Using the adult scale on a child gives outright wrong conclusions.

Four — special groups do not fit. Pregnant women (weight gain is physiological), the elderly (muscle loss lowers BMI while body fat may rise, so a low BMI can itself signal risk), and people with edema or ascites all get distorted BMI.

What it is good for

BMI's value is fast population screening — with two easy numbers it ranks risk across large groups, which is why it is so widely used. For an individual:

• Good for: a first reference of weight status, and for tracking your own change over time (your trend matters more than a single reading).
• Not good for: a diagnosis, or for judging someone else's body and health. BMI is a screening tool, not a diagnostic standard.

For a fuller picture also look at waist or waist-to-height ratio, body-fat percentage (a scale's absolute number is unreliable, but its trend on the same device is informative), resting heart rate, blood pressure, and fasting glucose and lipids. Whether to intervene should combine these with a clinician's view, not a single BMI digit.

How the tool works

Enter height (cm) and weight (kg) for a live BMI, the China-standard category, and the normal weight range at that height (18.5 × height2 to 23.9 × height2). The range shifts with height and can serve directly as a goal band for loss or gain. All math is local; height and weight are not uploaded.