What BMI is, and its limits
Body Mass Index is a simple ratio of weight to height: BMI = weight (kg) ÷ height (m)2. It is used as a quick screen for whether someone is a healthy weight for their height. The standard adult ranges are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, and 30 or above obese. The calculator above works out your figure and shows where it sits.
It is a rough guide, not a diagnosis. Those cutoffs are the World Health Organization's for a general adult population, and they are not universal: WHO publishes lower action points for South and East Asian populations, where the same health risks show up nearer a BMI of 23 than 25. BMI does not distinguish muscle from fat, so very muscular people (athletes, weightlifters) can read as "overweight" while carrying little fat. It also does not account for where fat is stored, and it is less reliable for older adults, pregnant women and different ethnic backgrounds, some of which face health risks at lower BMI thresholds.
Use it alongside other measures. Waist circumference and waist-to-height ratio capture belly fat, which is more closely linked to health risk than BMI alone. Treat BMI as one data point: if yours falls outside the healthy range, it is a prompt to look closer, not a verdict on its own.