Find out if you are overweight by using our BMI Calculator. BMI Calculator | Check Your Body Mass Index . To be classed as medically overweight or obese your BMI (Body Mass Index) must be between 27-30 and above. (BMI of 27 – 30 must have weight related conditions (co-morbid factors). Your BMI will be determined during your consultation with one of our Doctors or Nurse Prescribers.