What Does Your BMI Really Tell You? How to Calculate It and What the Numbers Mean
BMI is one of the most widely used health metrics, but most people don't fully understand what it measures or where it falls short. Here's everything you need to know.
BMI — Body Mass Index — shows up everywhere. Doctors check it, insurance forms ask for it, fitness apps display it front and centre. But most people don't fully understand what it actually measures, how the number is calculated, or why it has some real limitations that matter.
If you've seen your BMI on a health report and weren't sure what to make of it, this guide covers everything you need to know — including what the number doesn't tell you.
What Is BMI?
Body Mass Index is a number calculated from your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a way to study weight distribution across populations — not to diagnose individual health. That origin matters, because it explains why BMI works reasonably well as a population screening tool but has clear limits when applied to a single person.
How to Calculate BMI
The formula uses only two inputs: weight and height.
Metric formula: BMI = weight (kg) ÷ height (m)²
Imperial formula: BMI = (weight in lbs ÷ height in inches²) × 703
For example, if you weigh 70 kg and stand 1.75 m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
Step-by-Step Example
Let's work through it for someone who is 5'8" (172.7 cm) and weighs 160 lbs (72.6 kg):
- Convert height to metres: 172.7 ÷ 100 = 1.727 m
- Square the height: 1.727 × 1.727 = 2.982
- Divide weight by that number: 72.6 ÷ 2.982 = 24.3
Result: a BMI of 24.3, which falls in the Normal weight category.
What the Result Means
The World Health Organisation uses these four standard categories:
| BMI Range | Category | |---|---| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 and above | Obese |
A BMI in the 18.5–24.9 range is generally associated with a lower risk of weight-related conditions like type 2 diabetes, hypertension, and cardiovascular disease. However, these thresholds aren't universal — some health organisations use different cutoffs for South Asian, East Asian, and other ethnic groups where health risks appear at lower BMI values.
What BMI Doesn't Tell You
This is the part that most BMI explainers skip.
BMI has no way to distinguish between muscle and fat. A strength athlete carrying significant muscle mass might have a BMI in the "overweight" range while having low body fat and excellent cardiovascular health. At the same time, a sedentary person with a "normal" BMI can still carry too much body fat and too little muscle — a situation sometimes called "skinny fat."
BMI also ignores where fat is stored on the body. Abdominal fat (visceral fat, around the organs) carries a significantly higher health risk than fat stored in the hips and thighs — and BMI treats both identically.
For most average adults with typical activity levels, BMI is a useful, quick screening tool. For athletes, older adults, pregnant women, and people at the extremes of muscle or bone density, it can be actively misleading.
Common Mistakes People Make
Treating BMI as a diagnosis. BMI is a screening signal, not a medical conclusion. A high BMI means it's worth looking deeper — it doesn't confirm a health problem on its own. Always pair it with other assessments.
Mismatching units without converting. Entering weight in pounds and height in centimetres without adjusting for the correct formula gives you a completely meaningless number. Always confirm which unit system you're using before calculating.
Applying adult categories to children and teenagers. BMI for under-18s uses age- and sex-specific growth charts — the four adult categories above don't apply. A paediatric BMI calculation is a different tool entirely.
When You Should Recalculate
Recalculate your BMI whenever your weight has shifted by 4–5 kg or more, after a sustained period of strength training that may have changed your muscle mass, or before any medical appointment where weight-related topics are likely to come up. A single BMI reading is less useful than watching the trend over time.
Related Calculators
- Use the BMI Calculator to get your BMI instantly in both metric and imperial units
- Use the Ideal Weight Calculator to find the weight range that puts you in a healthy BMI for your exact height
- Use the Healthy Weight Calculator to see your full healthy weight range by height and body frame size