用于转换单位的HTML和JavaScript表单

I'm trying to make form, using HTML and JavaScript and using dropdown list. So I have searched over and over, I found similar problems, but coun't resolve anything from the answers. So, I want my form to be selected via Dropdown menu, so I can get like "Kilometers" and convert them to "Miles" or "Feet", depends on what I chose. I got it working for the Temparature form, but only for convertion from Celsius to Farenhait, to other way around does not work and I don't know why. Here is HTML code:I'm trying to make form, using HTML and JavaScr