Go to Settings > Kitchen Settings > Units (/admin/units), click Add new, enter the Name (e.g. Ounce) and Short Name (e.g. oz) and, with the Ingredients API module, an Ingredient API mapping, then Save. The list shows ID, Name, Short Name and an inline-editable Ingredients API column. Units are used as the Base Unit of ingredients and sub-recipes, for quantities inside recipes, for stock quantities and in the shopping list. The Conversion column of the shopping list recognises common short names (oz, lb, gr, kg, floz, mls, l, tsp, tbsp, cup, pt, qt, gal, mg) and converts them according to your weight base measure setting, so use standard short names where possible.