What is a cup?

From Fatsecret: 1 cup of blueberries contains 21g of carb

On the other hand, 100 g of blueberries contains about about 15 g of carb

Presumably, a cup measures volume.

How big is a cup? For instance is a plastic glass of the size dispensed by a drinks machine bigger or smaller than a cup?

Somethings may be practically easier to measure by volume.

I cannot find a cup measure in Sainsbury. Perhaps it is a USA thing.

VickiT11979 said:

1 cup is 240ml. You can buy a baby's bottle to use as a measure, I have a cup measure that came in a breadmaker but don't know where else you can get them.

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Thank you, Vicki, that's what I needed. Now I just need to pour 240 ml of water into an vessel to know how big a cup is. I'll do this this evening.