What the BMI actually is
Body mass index is a single number derived from height and weight — in adults, weight in kilograms divided by height in metres squared. It gives a general indication of whether weight falls in a healthy range. A BMI between 18.5 and 25 is conventionally called normal, 25 to 30 overweight, and above 30 obesity.
Its virtue is simplicity: it takes two measurements anyone can obtain and produces a number that correlates, across large populations, with the risk of diabetes, cardiovascular disease, and several cancers. That is genuinely useful. But it was designed to describe populations, not to judge individuals, and that distinction gets lost constantly.
How ideal weight is calculated here
The ideal-weight figure above uses the Devine formula, long used in clinical practice: for women, 45.5 kg plus 2.3 kg for every inch over five feet; for men, 50 kg plus 2.3 kg per inch. It corresponds roughly to a BMI of 22.
Treat it as a reference point, not a target to chase. That is why the calculator also gives you a healthy range — the span of weights corresponding to a BMI of 18.5 to 25 at your height. A range is a far more honest answer than a single number, and for most people any weight inside it is fine.
What if my BMI is over 25?
For most people, a BMI over 25 does indicate excess fat. There are real exceptions. A muscular athlete may have a BMI over 25 with a low body fat percentage, because muscle is denser than fat. Older adults lose muscle and may carry more fat at the same BMI than they did at forty.
BMI does not measure body fat, and it says nothing about where fat sits — which matters. Fat around the abdomen carries more metabolic risk than fat on the hips and thighs. A waist measurement adds useful information that BMI alone misses. As a rough guide, a waist over 40 inches in men or 35 inches in women signals increased risk regardless of what BMI says.
If you want to change the number
Weight is more responsive to consistent, unglamorous habits than to any particular diet. If you want to work on it, two things on this site are more useful than another calculator: understanding how many calories your body actually burns, and reading through the weight loss series, which covers why exercise alone so often disappoints and why plateaus are normal rather than a sign of failure.
If your weight is bound up with blood sugar — prediabetes or type 2 diabetes — the remission guide I am writing addresses that specifically, in plain language.