BMI, or Body Mass Index, is the most commonly used tool to screen if someone's overweight and obese. The calculation is simple: your weight in kilograms, divided by height in meters squared. There are plenty of online calculators, like this one, that will do the work for you. A normal BMI is between 18.5 and 24.9. A BMI of 25 to 29.9 is considered overweight and above 30 is considered obese. A BMI that falls under 18.5 is considered underweight, which has its own attendant health risks, such as osteoporosis, fertility issues, and malnutrition.
Though an important first step, BMI isn't quite sufficient to fully assess your risk for weight-related health issues. The current cutoffs underestimate the risk associated with obesity in Asian and South Asian populations and overestimate the risk in black populations. Plus, because BMI doesn't differentiate between muscle mass and fat mass, it's particularly inaccurate for people who are very athletic, since the high degree of muscle mass can land them in the overweight category even if body-fat percentage is low. To get a more comprehensive picture of your risk profile, you'll also need to measure your waist circumference.