Simple BMI Calculator (Python 3) As my first Python program I have written a simple BMI calculator. I want to know how i can improve this program. Would functions be useful? How can I improve the way that it is presented? 

#Introduce BMI Calculator
print ('Welcome to BMI Calculator!')

#Ask if user would like to use Metric or Imperial units
input1 = input ('Would you like to input Metric units or Imperial units? ')

#If Imperial... If Metric... Calculate BMI...
if input1 == 'imperial':
    heightIN = float(input('Input your height in Inches (in): '))
    weightLB = float(input('Input your weight in Pounds (lbs):' ))
    BMI = round(((weightLB/(heightIN*heightIN))*703), 2) 


    if BMI &gt;= 19 and BMI &lt;= 24:
        print ('Your BMI is', BMI, 'so, you are healthy weight!')

    elif BMI &gt;= 25 and BMI &lt;= 29:
        print ('Your BMI is', BMI, 'so, you are overweight!')

    elif BMI &gt;= 30 and BMI &lt;= 39:
        print ('Your BMI is', BMI, 'so, you are obese!')

    elif BMI &gt; 39:
        print ('Your BMI is', BMI, 'so, you are extremely obese!')

    elif BMI &lt; 19:
        print ('Your BMI is', BMI, 'so, you are underweight!')

elif input1 == 'metric':
    heightCM = float(input('Input your height in Centimeters (cm): '))
    weightKG = float(input('Input your weight in kilograms (kg): '))
    heightM = heightCM*0.01
    BMI = round((weightKG/(heightM*heightM)), 2)

    if BMI &gt;= 19 and BMI &lt;= 24:
        print ('Your BMI is', BMI, 'so, you are healthy weight!')

    elif BMI &gt;= 25 and BMI &lt;= 29:
        print ('Your BMI is', BMI, 'so, you are overweight!')

    elif BMI &gt;= 30 and BMI &lt;= 39:
        print ('Your BMI is', BMI, 'so, you are obese!')

    elif BMI &gt; 39:
        print ('Your BMI is', BMI, 'so, you are extremely obese!')

    elif BMI &lt; 19:
        print ('Your BMI is', BMI, 'so, you are underweight!')

else:
    print ('There was an error with your input. Please restart the program!')
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)