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BMI Calculator
BMI Calculator turns a weight in kilograms and a height in centimetres into a body mass index value with its standard category — underweight, normal, overweight or obese — using the World Health Organization thresholds.
How to use BMI Calculator
- Enter your weight in kilograms and your height in centimetres.
- Calculate to see the BMI figure.
- Read the category and the coloured band next to the number.
Where BMI Calculator is useful
- Getting a quick reading of where you fall on the standard range.
- Tracking a trend in your own numbers over several months.
- Converting a height and weight you already know into BMI for a form.
Formats, algorithms and limits
The formula is weight in kilograms divided by the square of height in metres. Categories follow the WHO adult cut-offs: below 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight and 30 or above obese. BMI is a population screening tool only — it does not distinguish muscle from fat, and it is not valid for children, pregnant people or athletes.
Troubleshooting and honest limits
- Use the same scale and time of day when comparing readings over time.
- Pair it with a waist measurement for a more useful personal picture.
- Treat a single reading as a rough indicator, not a diagnosis.
Frequently asked questions
Why does my BMI say overweight when I am fit?
BMI only compares weight with height, so dense muscle raises it. It cannot tell muscle from fat tissue, which is why it is a screening figure rather than an assessment.
Are these the same thresholds everywhere?
These are the WHO adult values. Some health bodies use a lower overweight threshold for people of South Asian descent, so check locally if that applies to you.
Is it suitable for children?
No. Children and teenagers are assessed against age and sex specific percentile charts, which this calculator does not implement.
Does the tool store my numbers?
No. The calculation runs in the browser and nothing is saved or transmitted.
Related tools
Page written and reviewed by the FreeConvertio engineering team on 2026-10-14. BMI Calculator runs entirely in your browser — no uploads, no account and no daily limits.