What is BMI?
Body mass index (BMI) is a screening number that relates your weight to your height. It was designed for one job: flagging, across large populations, who is statistically more likely to be underweight or carrying excess fat. It is cheap, fast and needs no equipment — which is exactly why doctors, insurers and researchers still use it — but it says nothing about what your weight is made of. Two people with identical BMIs can look, perform and carry health risk completely differently.
The BMI formula
BMI is your weight divided by your height squared:
In imperial units the same formula uses a conversion factor:
So a 70 kg person at 175 cm works out to 70 ÷ 1.75² = 22.9 — inside the healthy range. The calculator above does this instantly and shows where you land on the scale.
BMI categories chart
The World Health Organization defines these adult categories:
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
The calculator also shows your personal healthy-weight range — the weights that would put your height between BMI 18.5 and 24.9. For many people that range is surprisingly wide, which is a useful reminder that there is no single "correct" weight.
Why BMI is often wrong for athletes
BMI's blind spot is body composition. Muscle is denser than fat, so anyone who lifts consistently drifts up the BMI scale without gaining an ounce of fat. Rugby players, sprinters and dedicated gym-goers routinely score "overweight" at 10–15% body fat. The reverse also happens: a sedentary person can sit at a "normal" BMI while carrying little muscle and a high body-fat percentage — sometimes called normal-weight obesity.
That's why coaches don't stop at BMI. They track body-fat percentage and tape measurements, which respond to what's actually changing. If you train regularly, run your numbers through our body fat calculator — it uses the U.S. Navy tape method and takes two minutes.
What to do with your result
If your BMI landed outside the healthy range and you're not carrying obvious muscle mass, treat it as a prompt, not a verdict. The practical next steps are the same ones a good coach would give you: find your daily calorie needs with the calorie calculator, pick a modest deficit or surplus depending on direction, and re-measure in four weeks. Progress you can see beats a category label.
And if you work with a personal trainer, this is exactly the kind of data worth tracking properly over time. The CoachRight client app charts your weight, body-fat and measurements automatically, so you and your coach see the trend — not the daily noise. Coaches can start a free 14-day trial. For evidence-based training and nutrition guides, see the CoachRight blog.