When you calculate BMR online, it offers you an idea about how much energy the body uses when you are resting. BMR measures your breathing, temperature regulation, and blood circulation. The Resting Metabolic Rate (RMR) takes additional energy consumption into account. These include digestion and minimal activities such as dressing up, brushing your teeth, lifting a spoon, and so on. In most cases, the BMR and RMR give you the nearly same result.
Conversely, the Body Mass Index (BMI) is a screening method for weight category. The result mainly depends on your age, weight and height. BMI calculators helps you determine whether you are obese, overweight, underweight or you have a healthy weight. You cannot use a BMR calculator to determine the BMI.