Body Mass Index is one of the most widely used screening numbers in health, and also one of the most misunderstood. It's quick to calculate and useful for spotting trends across large groups, but a single number can never capture everything about your body. Here's how it works and how to read it sensibly.
The BMI formula
BMI compares your weight to your height using a simple ratio:
BMI = weight (kg) ÷ height (m)²
If you think in pounds and inches, convert first or use a tool that does it for you. For example, someone who weighs 70 kg and stands 1.75 m tall:
- Height squared: 1.75 × 1.75 = 3.06
- BMI: 70 ÷ 3.06 ≈ 22.9
That value sits comfortably in the healthy range.
The standard WHO categories
The World Health Organization groups adult BMI into four bands:
- Underweight: below 18.5
- Normal (healthy) weight: 18.5 – 24.9
- Overweight: 25 – 29.9
- Obese: 30 and above
These ranges apply to most adults aged 20 and over. For children and teens, BMI is read against age-and-sex percentile charts instead, so the adult bands don't apply.
Where BMI falls short
BMI only knows your height and weight, so it can't tell muscle from fat. That leads to a few well-known blind spots:
- Muscular people (athletes, weightlifters) can score as "overweight" despite low body fat.
- Older adults may lose muscle yet keep a normal BMI while carrying more fat.
- It says nothing about where fat sits — abdominal fat carries more health risk than fat on the hips.
Because of this, BMI works best as a starting flag rather than a diagnosis. Pairing it with other measures gives a fuller picture. A body-fat estimate adds the muscle-versus-fat dimension, and waist circumference adds the fat-distribution dimension.
How to use it
The fastest way to get your number is the BMI Calculator. Enter your height and weight in metric or imperial units, and it returns your BMI plus the category it falls into.
To go a step further, the Ideal Weight Calculator shows a target weight range for your height, and the Body Fat Calculator estimates the proportion of your weight that's fat tissue. Reading all three together helps you interpret a borderline BMI with more context.
A quick example to tie it together
Imagine two people who are both 1.80 m tall and weigh 88 kg. Their BMI is identical:
- 88 ÷ (1.80 × 1.80) = 88 ÷ 3.24 ≈ 27.2 (overweight band)
But if one is a regular lifter with visible muscle and the other is sedentary, their actual health risk differs a lot. Same number, different stories — which is exactly why BMI is a conversation starter, not the final word.
These figures are general estimates for healthy adults and are not medical advice. If you have specific concerns about your weight or health, talk with a qualified clinician who can consider your full history.