Just change it from
i_to_c(height)
to
height = i_to_c(height)
Works the same way but actually works. You may consider working with classes.
Btw, you can rewrite the two functions like so:
def i_to_c(inches):
return inches * 2.54
def lbs_to_kg(lbs):
return lbs * 0.45359237
Or, even smaller:
i_to_c=lambda(i):i*2.54
lbs_to_kg=lambda(lbs):lbs*0.45359237