Understanding BMI: What It Can and Cannot Tell You
Body mass index, or BMI, is a quick screening number that compares your weight with your height. It is widely used in health settings because it is cheap and easy, but it has real limitations that everyone should understand.
The calculation
BMI = weight in kilograms ÷ (height in metres)²
For example, a person weighing 70 kg and standing 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.
If you use pounds and inches, the formula is BMI = 703 × weight (lb) ÷ height (in)². A unit converter helps if your measurements are in mixed units.
The usual adult categories
The World Health Organization uses these ranges for adults:
- Below 18.5: underweight
- 18.5 to 24.9: normal range
- 25.0 to 29.9: overweight
- 30.0 and above: obesity
Some health bodies recommend lower thresholds for people of South Asian and some other Asian backgrounds, because health risks can rise at lower BMI values in these groups.
What BMI does well
At a population level, higher BMI is associated with greater risk of conditions such as type 2 diabetes and heart disease. As a first-step screening tool, it flags people who may want to discuss their health with a professional.
What BMI cannot tell you
- Body composition. Muscle weighs more than fat, so athletes and strength trainers may appear "overweight" while being healthy.
- Fat distribution. Fat carried around the waist is linked to higher risk than fat on the hips or thighs. Waist circumference adds useful information.
- Age and sex. Older adults can have a normal BMI but low muscle mass, and women typically carry more body fat than men at the same BMI.
- Children and teenagers. They need age- and sex-specific growth charts rather than adult cut-offs.
- Overall health. Blood pressure, blood sugar, cholesterol, fitness, sleep and mental wellbeing all matter.
How to use the number sensibly
Treat BMI as a conversation starter, not a verdict. If your result is outside the normal range, consider your waist measurement, your energy levels and your family history, and speak with a doctor or registered dietitian before making major changes.
Healthy habits matter more than the number
Regular movement, adequate sleep, a varied diet and not smoking improve health at nearly every BMI. Sudden crash diets are rarely helpful or sustainable.
Medical disclaimer
This article is general education, not medical advice. If you are concerned about your weight or health, consult a qualified healthcare professional.
Use the calculator as one data point among many, and look at the whole picture.
