Simple BMI Calculator (Python 3)
It's probably best to use a function that uses SI units - you can then convert lbs to kgs and in to cm. Additionally, since the majority of the world either uses Imperial or metric, we can use a Boolean to indicate which one to use:

def BMI(height,weight,is_metric):
    if (is_metric):
        # Height in meters, weight in kg
        return weight / height**2
    else:
        # Height in inches, weight in lbs
        height = height * 0.0254
        weight = weight * 0.453
        return BMI(height,weight,True)