Behind the number
What BMI actually measures
BMI relates your weight to your height so that people of different heights can be compared on one scale. It was designed for looking at populations, and it works reasonably well there. For one individual it is a rough screen: it cannot see how much of your weight is muscle, bone, or fat, or where that weight sits on your body.
How the number is calculated
The formula is weight in kilograms divided by height in metres squared. Someone who is 175 cm tall and weighs 72 kg has a BMI of 72 / (1.75 × 1.75), which works out to about 23.5. That is the entire calculation. No age, no sex, no activity level. Its simplicity is both why it is used everywhere and why it misses things.
Reading your result
For adults, the commonly used reference ranges are: below 18.5 is classed as underweight, 18.5 to 24.9 as the general reference range, 25 to 29.9 as overweight, and 30 or above as obese. These cut-offs come from population research on health risk. They were not built for children, pregnant people, very muscular people, or older adults. For those groups the ranges can genuinely mislead.
Where BMI stops being useful
Two people with the same BMI can carry very different amounts of muscle and fat. A strength athlete can read as overweight while being lean; someone in the reference range can still carry more body fat than is helpful. Treat BMI as one data point next to how you eat, move, sleep, and feel, and take questions about your health to a clinician, not a calculator.