For adults, you only need to plug in weight and height to calculate Body Mass Index (BMI). For kids, you also have to take age and gender into account. A tween or teen with a BMI in the 85th to 95th percentile is considered at risk for becoming overweight; above the 95th is overweight. You can check the health of your whole family using the CDC's BMI calculators and growth charts.
Center for Disease Control and Prevention BMI Calculator