BMI Explained: What It Tells You and What It Does Not
Body Mass Index is one of the most widely used health numbers and also one of the most misunderstood. This guide explains what it measures, how to read it and when to look at other indicators.
How BMI is calculated
BMI equals weight in kilograms divided by height in metres squared. Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. You can calculate yours with the BMI Calculator, in metric or imperial units.
The standard categories
For adults, the World Health Organization uses: below 18.5 underweight, 18.5 to 24.9 healthy range, 25 to 29.9 overweight and 30 or more obesity. Some countries in Asia use lower thresholds because the risk of conditions such as type 2 diabetes can rise at lower BMI in some populations.
What BMI cannot tell you
- It does not distinguish muscle from fat, so a muscular athlete can be labelled overweight.
- It does not show where fat is stored. Fat around the abdomen carries more health risk than fat elsewhere.
- It does not account for age, sex, bone structure or ethnicity.
- For children and teenagers, BMI must be compared with age- and sex-specific growth charts.
Other useful measures
Waist circumference, waist-to-height ratio, and body fat percentage give more context. The Body Fat Calculator estimates body fat from tape measurements, and the Ideal Weight Calculator shows reference ranges from several formulas. Blood pressure, blood sugar and cholesterol, measured by a professional, say more about health than any single body number.
Turning numbers into habits
If you want to change weight, small sustainable steps work better than crash diets. The Calorie Calculator and BMR and TDEE Calculator can estimate energy needs. Weigh yourself at the same time of day and watch weekly trends, since daily changes are mostly water.
These tools provide general estimates and are not medical advice. If you have concerns about your weight, or a medical condition, are pregnant, or are making significant changes, talk to a doctor or registered dietitian.