⚖️ BMI Calculator
Enter your height and weight to calculate your BMI, along with a category (underweight, normal, overweight, obese) and standard weight reference. Useful for checking in after a health screening or tracking your weight over time.
How to use
- Enter your height.
- Enter your weight.
- Your BMI value, category, and standard weight are displayed automatically.
How the calculation works
BMI (Body Mass Index) is your weight in kilograms divided by the square of your height in metres. Someone 170cm tall weighing 65kg has a BMI of 65 ÷ 1.70² = 65 ÷ 2.89 ≈ 22.5. The measure was devised in the 19th century by the Belgian statistician Adolphe Quetelet, originally as a way of describing the distribution of body types across a population rather than assessing individuals. It is now used as an obesity indicator by the World Health Organization and by national health bodies. Under WHO thresholds, a BMI of 30 or above is classed as obese; the Japan Society for the Study of Obesity sets that boundary at 25, so figures quoted in different countries are not always directly comparable. Dividing by height squared reflects the general tendency for body mass to scale with roughly the square of height. That is an approximation, and it is known to drift for people who are unusually tall or unusually short.
Worked example
For someone 168cm tall weighing 72kg 1. Convert height to metres: 168cm → 1.68m 2. Square it: 1.68 × 1.68 = 2.8224 3. Divide the weight: 72 ÷ 2.8224 ≈ 25.5 The BMI is about 25.5. The same 72kg at 175cm gives 72 ÷ 3.0625 ≈ 23.5, which falls in the normal range — the identical weight lands in a different band purely because of height.
Things to be aware of
- BMI uses only height and weight, so it cannot distinguish muscle from fat. Athletes with high muscle mass often show a high BMI despite low body fat.
- Adult BMI thresholds do not apply to growing children, pregnant people or the elderly. For children, age- and sex-specific growth charts are used instead.
- WHO and Japanese thresholds classify obesity at different cut-offs. When comparing figures from different sources, check which standard is being used.
- BMI is one rough indicator among many. Any decision about diagnosis or treatment belongs with a medical professional.
FAQ
How is BMI calculated?
Using the internationally standard formula: BMI = weight (kg) ÷ height (m) squared.
Is this medical advice?
No. It's a general reference only and isn't a substitute for medical diagnosis or advice. Please consult a healthcare professional for health decisions.
How is the standard weight calculated?
Using BMI 22, a widely used reference value, as: standard weight = 22 × height (m) squared.
Can I use this to evaluate a child's BMI?
The categories here are designed for adults. Evaluating a growing child's BMI requires age- and sex-adjusted references, so please rely on a pediatrician's assessment instead.
Does a high or low BMI always mean I'm unhealthy?
BMI is just one general indicator and doesn't account for muscle mass or body fat percentage. If you're concerned, it's worth discussing with a healthcare professional.