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Your body mass index,
and what it doesn’t tell you
You get the number right away, with the World Health Organization categories. And below it, what hardly anyone tells you: what it’s really for and when it means nothing.
| Category | BMI |
|---|---|
| Underweight | under 18.5 |
| Normal weight | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obesity class I | 30 – 35 |
| Obesity class II | 35 – 40 |
| Obesity class III | 40 or more |
For your height, the category called “normal weight” goes from 126 lb to 168 lb.
The index can’t tell muscle from fat. If you do strength training, it’s normal to get a high number without it saying anything bad about you: it measures weight and height, and that’s it. To know how you really are, your waist measurement, how you look and what a professional says are worth more.
What it measures and what it doesn’t
It’s a screening tool for populations, not a diagnosis of you.
- It can’t tell muscle from fat. This is the most important point, and it’s why a lot of people who train get “overweight” while being in better shape than average.
- It doesn’t say where that fat is. And that’s where a good part of the risk lies: fat spread out is not the same as fat around the belly. Your waist measurement says more, with a tape measure that costs a few dollars.
- It doesn’t work the same at every age. During growth other tables are used, and in older people the same number means something else.
- It doesn’t replace anyone. If the number worries you, that’s for a doctor or a registered dietitian to look at, not a web page.
So what is it for?
For a quick reference, and to see which way you’re moving over time, which is what really matters. A BMI taken on a Tuesday doesn’t say much; the same BMI logged every week for three months draws a line, and that line tells you whether what you’re doing is working.
Frequently asked questions
How is BMI calculated?
Your weight in kilograms divided by your height in meters squared (in pounds and inches, the same thing multiplied by 703). Someone who weighs 75 kg and is 1.75 m tall has a BMI of 24.5.
Which categories are the good ones?
The World Health Organization's for adults: under 18.5 is underweight, 18.5 to 25 normal weight, 25 to 30 overweight, and from 30 up the three classes of obesity.
I do strength training and I get overweight. Is that normal?
Yes, and it's the big limitation of the index: it can't tell muscle from fat. Muscle is heavy, so a lot of people who train get a high number that means nothing. Waist measurement or a body composition test measure that better.
Does it work for children, pregnant women or older people?
Not as it is. During growth, tables by age and sex are used; in pregnancy it doesn't apply; and in older people the same number means something else. If you're in one of those cases, this isn't your tool.
Do you keep my data?
No. The math is done in your browser, it isn't sent to any server and you don't need to sign up to see the result.
What actually moves the needle
It isn’t the index: it’s what you eat and how you train. The Reverfit Diet app works out your calories and macros, lets you log what you eat by scanning the package, and draws the line of your weight week by week. Free to start.
Diet
