One metric cup is exactly 250 millilitres. It is the only cup with a round value, and that is the entire point of it: it was created to replace the inherited cups with something that divides cleanly into a litre.
About the units
The metric cup is standard in Australia, New Zealand and South Africa, and it appears in European recipes that use cup measures at all. Four of them make exactly one litre, which is the property the older cups lack. The competition is substantial: the US customary cup is 236.588 ml, the US legal cup used on nutrition labels is 240 ml, the imperial cup is 284.131 ml, and the Japanese cup is 200 ml. The metric cup sits between the American and the imperial values, closer to the American, and the 5.7 per cent gap to the US cup is the one that causes most trouble because the two are the ones most likely to appear in the same kitchen.
The exact factor
The factor is exactly 250, so conversion is a matter of moving a decimal and multiplying by a quarter: 2 cups is 500 ml, half a cup is 125 ml, and a quarter cup is 62.5 ml. Against the US cup, 250 ÷ 236.5882365 = 1.0567, so a metric cup is 5.7 per cent larger. Across a recipe using four cups of liquid that is an extra 54 ml, roughly three and a half tablespoons — enough to change a batter's consistency visibly.
Where you meet this conversion
Australian and New Zealand recipes use it throughout, and because those recipes circulate widely online they end up in American and European kitchens where the difference goes unnoticed. The Australian measuring set compounds the problem: the cup is 250 ml, the teaspoon is 5 ml, but the tablespoon is 20 ml rather than the 15 ml everyone else uses. A recipe from Sydney therefore needs two separate corrections, not one, and the tablespoon is the larger error of the two.