I still remember some sound advice that was printed on the back of a packet of Co-op own-brand cornflakes, years ago:-
A litre of water's a pint and three-quarters.
Two and a quarter pounds of jam weighs about a kilogram.
You have not been charged for this information. I was, but you can...
The unit of weight that makes most sense, at least when buying food, is what the Italians call the etto. 100 grams, one hectogram. Roughly a portion, irrespective of what food it is.