Body Mass Index (BMI) is the most common quick estimate of whether your weight is in a healthy range. It is fast and free, but it is also widely misunderstood — so here is what the number actually tells you, and what it leaves out.
How BMI is calculated
BMI is your weight in kilograms divided by your height in metres squared. A calculator does the maths for you in a second; all you enter is your height and weight.
The standard ranges
- Under 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25 to 29.9 — overweight
- 30 and above — obese
These bands come from large population studies and are a reasonable starting point for most adults.
Where BMI falls short
BMI does not distinguish fat from muscle, so a very fit person with a lot of muscle can land in the “overweight” range while carrying little body fat. It also does not show where fat sits on the body. That is why it works best as a screening signal, not a verdict.
Pair it with body-fat percentage
A second number helps fill the gap. Body-fat percentage — estimated from the simple BMI method or the U.S. Navy formula — gives a more complete view of body composition than BMI alone. Use both as rough guides, and remember any calculator gives an estimate, not medical advice.
Want to do all of this in one private place? See Weight Tracker — or try it free on Google Play.