Well... how much is it?

Your question is a little vague. What is it that you want to know? A cup is a cup or two 1/2 cups, three 1/3 cups, four 1/4 cups, etc. Do you want to know how may tsp, oz, grams, etc in a cup?

If you are used to the metric system it is roughly 1/4 of a liter.

It depends on what you are measuring, in liquids it is 250ml. I bought a little measuring cup set which I find really helps. It only cost a few dollars.

a cup, dry measure is 16 tablespoons, or about the size of a baseball or a tennis ball.

Thanks guys! I'm in Argentina..., so I am used to the metric system and I cannot buy an "american" measuring cup (in case there is such a thing). The "250 ml" answer was very useful. Many times, when logging in food, it says "a cup"... so now I know! I'll just take my regular measuring cup and I'll fill it up to 250 ml.

That should work!

This may help. http://www.fitday.com/fitness/forums...t-measure.html

If you don't have a measuring cup handy, you can always use your fist for comparison- assuming your fist is "average" size.

If you run into similar issues with measurements, there are websites that do conversions from one unit to another. According to this site one cup = approx. 236.6 ml.