BMI Calculator
Body Mass Index using the WHO standard or the Asian cut-offs.
How it works
Body mass index is your weight divided by the square of your height. It is a population-level screening measure, not a measure of body fat: it cannot tell muscle from fat, so a heavily trained person can land in the "overweight" band while being in excellent health.
This calculator offers two sets of cut-offs. The WHO international thresholds are the familiar ones — overweight from 25, obese from 30. The Asian cut-offs, published by a WHO Expert Consultation in 2004, sit lower: overweight from 23 and obese from 25.
That difference is not arbitrary. At the same BMI, people of Asian descent carry on average a higher proportion of body fat, and their risk of type 2 diabetes and cardiovascular disease rises at a lower index than it does in European populations. Someone at BMI 24 is "normal" on the WHO scale and already overweight on the Asian one.
Formula
BMI = weight (kg) ÷ height (m)²
Height is in metres, so 170 cm is 1.70 m. Squaring height is why BMI is least reliable at the extremes of stature.
Worked example
Someone 170 cm tall weighing 68 kg:
- Height squared
- 1.70² = 2.89
- BMI
- 68 ÷ 2.89 = 23.5
- WHO classification
- Normal (18.5–25)
- Asian classification
- Overweight (23–25)
The same 23.5 produces two different conclusions depending on which scale applies — which is exactly why the choice of cut-offs matters.
Frequently asked questions
What is a healthy BMI?
On the WHO international scale, 18.5 up to 25 is classed as normal. On the Asian cut-offs the normal band is narrower: 18.5 up to 23.
Which cut-offs should I use?
If you are of Asian descent, the Asian cut-offs are the better fit. At the same BMI, Asian populations carry more body fat on average and metabolic risk rises earlier, so the thresholds are set lower.
Is BMI accurate for muscular people?
No. BMI cannot distinguish muscle from fat, and muscle is denser. Anyone carrying substantial muscle is routinely classed as overweight despite a low body-fat percentage. Body-fat percentage or waist circumference is more informative in that case.
Does BMI work for children?
Not with adult thresholds. Children and adolescents are assessed against age- and sex-specific percentile charts, because body proportions change throughout growth.
Can I use BMI during pregnancy?
No. Weight gained in pregnancy is the baby, amniotic fluid and increased blood volume, so BMI during pregnancy says nothing useful about nutrition. Clinicians track weight gain against your pre-pregnancy BMI instead.
What is my ideal weight range?
This tool derives it from whichever scale you select, multiplying the lower and upper bounds of the normal band by the square of your height.
Does a high BMI mean I am unhealthy?
A high BMI is a prompt to look further, not a conclusion. Waist circumference, blood pressure, blood glucose and lipids describe health risk far more directly than a ratio of weight to height.
Sources
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