qgis - Display numeric scale in two different units, eg 1 inch = 1000 feet

How do I display a numeric scale for a map in two units, for example 1 in = 1000 ft ?

I know it's possible to display a scale bar as "Numeric," which provides only a ratio, in this example 1:12,000.

It seems like it should be possible to insert an expression into a label, only I can't figure out how to reference a map item in the expression builder. The map item has a variable "map_scale" but when I put this variable into a text item, it just displays as a string.

Answer

Use the custom python function provided in the answer to this question, then divide by 12 to get inch:feet ratio.

Step 1: Add a text label to the print composer

Step 2: Insert an expression into the label

Step 3: Create a custom function In the "Insert Expression" window, click the "Function Editor" tab. Click "New File," enter the file name get_composer_map_scale, click OK. Copy and paste the code on this page, then click "Load" to register the custom function.

Step 4: Use the custom function to display the scale as the ratio [inches on the page]:[feet in real life]

Copy/paste into your label:

1 inch = [%round(get_composer_map_scale( 'Composer 1')/12, 0)%] feet

Substituting the name of your print composer where it says Composer 1.

Note: I added the function round(value, places) to round to an integer value of feet. To display more digits, substitute the number of digits for the zero.

EDIT: This answer only works for QGIS 2. For QGIS 3, see this answer: gis.stackexchange.com/a/332759/35561. Thanks to @MrChe for the suggestion.