To calculate BMI, divide your weight in kilograms by your height in metres, then divide again by your height in metres. As a guide, under 18.5 is classed as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight, and 30 or above as obese.
BMI is a useful population screening tool but a blunt instrument for individuals. It does not distinguish muscle from fat, so very muscular people may have a high BMI without excess fat. It also does not show where fat is stored; waist measurement adds useful information, since fat around the abdomen carries more health risk.
Healthy BMI cut-offs differ for some ethnic groups, with lower thresholds often used for people of South Asian and some East Asian backgrounds, and BMI is interpreted differently in children, pregnancy and older age. Use it as one signal alongside waist size and overall health, and discuss your numbers with a doctor.