BMI calculator
Calculate your BMI (Body Mass Index) free in seconds: clear interpretation based on WHO reference values, healthy weight range and reliable guidance.
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How BMI Is Calculated
Body Mass Index (BMI) is a simple, widely used measure of body corpulence based on a person’s weight and height. The formula is straightforward: weight in kilograms divided by height in metres, squared.
BMI = weight (kg) ÷ height (m)²
For example, a person weighing 70 kg and standing 1.75 m tall gets 70 ÷ (1.75 × 1.75), or 70 ÷ 3.0625, about 22.86. Rounded to one decimal place, this gives a BMI of 22.9, in the “Normal weight” category. The calculator above also accepts imperial units, converting them first using standard factors: 1 inch equals 2.54 cm, and 1 pound equals about 0.4536 kg.
What the WHO BMI Categories Mean
The seven reference categories
Once BMI is calculated, the key reference is the classification published by the World Health Organization, which sorts the index into several categories. Each range includes its lower bound and excludes its upper bound: a BMI of exactly 18.5 already counts as “Normal weight”, while 25 already counts as “Overweight”.
- Below 16.5: severe thinness
- 16.5 to 18.5: underweight
- 18.5 to 25: normal weight
- 25 to 30: overweight
- 30 to 35: obesity class I
- 35 to 40: obesity class II
- 40 and above: obesity class III
This classification was built for population-level comparisons, not individual diagnosis, and remains a useful but cautious reference for a single person.
What BMI Does Not Tell You
BMI is practical but has limits worth knowing before drawing conclusions from a single number. It does not distinguish fat mass from muscle mass: at the same scale reading, a highly muscular person can show a high BMI without excess body fat. It also says nothing about how fat is distributed in the body, though a waist measurement gives information BMI alone does not.
When to Talk to a Healthcare Professional
The calculator above is an informational tool: it does not replace a diagnosis or medical advice. A BMI outside the reference range, unintended weight loss or gain, or simply a question about one’s own weight are all good reasons to talk to a healthcare professional. Only a healthcare professional can interpret an individual result while taking the whole picture into account, including medical history, build, age, and lifestyle.
Frequently Asked Questions
What is BMI?
BMI, or Body Mass Index, is an indicator that relates a person’s weight to their height to place their body corpulence on a scale recognised by the World Health Organization (WHO). It is calculated by dividing weight in kilograms by height in metres squared. It is a screening reference point, not a diagnosis.
How do I calculate my BMI by hand?
Multiply your height in metres by itself, then divide your weight in kilograms by that result. For example, for 1.75 m, 1.75 × 1.75 gives 3.0625; a weight of 70 kg divided by 3.0625 gives about 22.9. The calculator above does this instantly and interprets it using the WHO categories.
What counts as a healthy BMI?
The WHO places "normal weight" between 18.5 and 25. This is not a single ideal figure but a population-level reference range used to flag values that may deserve attention. Outside this range, a result is not a verdict — it is a reference point worth discussing with a healthcare professional if needed.
Is BMI reliable for very muscular people?
Less so than for others: BMI does not distinguish fat mass from muscle mass, which weighs more than fat at the same volume. A very muscular person can therefore show a high BMI without excess body fat.
Is my BMI still valid during pregnancy?
No. During pregnancy, BMI is not interpreted using the usual categories, since the weight gain involved is physiological. The reference to keep in mind is the BMI calculated before pregnancy, best discussed with the healthcare professional following the pregnancy.
This tool is informational and does not constitute a diagnosis or medical advice. Consult a healthcare professional.
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