Cup is a standard size for measuring liquids: 8 fl oz. It's only a problem for flour, because how much you can fit in a cup varies depending on humidity and how much you pack it.

To add to this, there are at least 3 "cup" sizes for liquid: metric (~250ml), the US version (above, ~236ml) and an imperial cup (~284ml).

Also the Japanese cup [1], which is 200 mL. I (American) have come across it twice (a rice cooker, and a dandan recipe).

[1] https://en.wikipedia.org/wiki/Cup_%28unit%29#Japanese_cup

A metric cup is exactly 250ml though, not approximately, right?

Except one cup in metric is 250 ml, not the ~236ml that a US cup is.

Thanks! Didn't know that.

> Googling for "cup size" didn't clarify anything either.

Is that because most results are in regards to bras?

If all ingredients are measured in cups, it doesn't actually matter much, since you'd still get good proportions by using any of your cups.

It's problematic if some ingredients are specified by volume, others by weight.

Specifying by volume is also a problem for compressable things like flour, chopped hazelnuts etc.

For reference, this recipe is fairly tolerant, assuming you stay reasonable (don't use some sort of machine press to get your flour into the cup hey?)

The ambiguity can be solved by specifying "heaping" units. One heaping tablespoon, or 1/2 heaping cup, etc.