Every single input field that you add to your calculator gets its own unique ID. This ID consist of a hashtag and a unique number, for example - #4. It is your formula's variables. Let's say you want to create a calculator consisting of three numeric input fields and one result's output. You want your calculator to put together values entered to those three input fields.
First of all you need to add three Numeric Input Fields. Click + icon and choose Numeric Input Fields. Name and customize this field according your needs. Then repeat it two times more to have three input fields in total. These input fields will get its unique IDs accordingly - #1, #2, #3. Then click + icon again add the Formula field this time.
Now you have the calculator consisting of three numeric input fields and one formula field. The only thing that is missing now is equation connecting all these fields. Click on your formula field and add the following line to formula editor - #1 + #2 + #3.
An you are all set. Your first calculator is ready! Don't forget to Save and Activate your calculator before you embed it to your website.