Can you add formula fields to a subform?

Can you add formula fields to a subform?

Is this possible? I have tried the two usual ways but they don't work:
  1. I've dragged the formula field from the field-type list on the left of the screen but it disappears when I drop it on the subform.
  2. I've clicked on "+Add Field" in the subform itself but the "Formula" field type isn't listed.
Many thanks

Steve