How to understand BMI
Body mass index (BMI) is the most popular way to screen weight against height — useful and quick, but with clear limits. This guide explains how it is computed, what the bands mean, and when it misleads. Its worked example sticks to the same values used by Klar’s BMI calculator.
How the index is computed
BMI = weight in kilograms ÷ height in metres squared. The calculator converts units automatically: pounds to kilograms, and metres to centimetres and back. A quick example: 75 kg and 175 cm (1.75 m) gives an index of about 24.5.
What the bands mean
The common classification: below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, and 30 and above is obese. The healthy weight range for your height is the weights that give an index between 18.5 and 24.9, and the calculator shows its two bounds directly.
A worked example
Using the same values as the calculator: weight 75 kg and height 175 cm. The index is about 24.5, and the healthy weight range for your height is about 56.7 – 76.3 kg (the low bound at 18.5 and the high bound at 24.9).
The limits of the index
The index is a screening tool, not a diagnosis: it does not distinguish muscle from fat, so it may place a muscular athlete in the overweight band without reason. Benchmarks also differ between populations. Do not use adult benchmarks for children or pregnant women, and for a better picture of body composition use tools such as body fat estimation.
Common mistakes
Among the mistakes: mixing weight or height units when entering them, entering height in centimetres where metres are expected, which miscalculates the index hugely, and using unsupported units like stone. Use the calculator with the correct units chosen and they convert automatically, and read the result as a guiding index, not a final verdict.
Key takeaways
- The index = weight (kg) ÷ height (m) squared.
- The healthy range is 18.5 to 24.9.
- The index is a guiding screen, not a medical diagnosis.
- It does not distinguish muscle from fat and can mislead for athletes.
- Choose units carefully; the calculator converts automatically.
Frequently asked questions
Is the index accurate for everyone?
No. It is a quick screening tool that does not distinguish muscle from fat, so it can misclassify athletes and some populations.
What does the healthy weight range mean?
It is the span of weights giving an index between 18.5 and 24.9 for your height — a guiding frame, not an exact target imposed on you.
When does the index not suit me?
Adult benchmarks may not fit children and pregnant women, and it can mislead people with high muscle mass or a non-typical body frame.
Related calculators
BMI calculator
Calculate your body mass index from weight and height, and see the healthy weight range for your height.
Ideal weight calculator
Find the healthy weight range for your height based on a BMI of 18.5 to 24.9.
BMR calculator
Estimate your basal metabolic rate and daily energy expenditure using the Mifflin-St Jeor equation.