After clicking the button labeled “Add computed columns…”, a popup window appears. Initially there are no computed columns, but you can add one using the link in the first section (below "Select/add column").
Next you enter the column name, for instance “Gender ratio“. This will be the name the column appears as in the data table and in column selector drop-downs in the chart editor. You can use any character, but you cannot use a column name that already exists in the dataset.
Finally, you can enter the formula. You can refer to the values in other columns via their column name. Start by typing the first character of the column you want to address and then hit the Tab key to auto-complete. In case there are multiple columns starting with that letter you can either enter more characters or select the column from a little popup window.
While you are typing the formula, a little display in the sidebar will show you how the first five computed values do look like. To compute the gender ratio for this example, we type the formula “women / (men + women)“.
You can use access basic mathematic functions such as round and column aggregates (sum, median etc). Please see below for a complete list.
When you’re done with the formula, hit the “Close” button to get out of the popup window. You will now see the new column in the data table view, and you can use it in all visualizations just like the other columns. To edit the formula or add more computed columns, simply click the “Add computed column…” button again.
These are all the functions you can use in your formula
On top of that you can append the following suffixes to any numeric column name to get aggregated values from entire columns: