RE: Oh, goddesses from the H-2-0 check in, a question.....
Just simply keep track one day, you know what 8oz is, so just put little tally marks on a piece of paper and count them at the end of the day. Most bottles tell you how much they hold, and if they don't, simply take something you know is 8oz and fill it up and pour it into the bottle and see how many 8oz fills it takes.
You can buy a bottle or water pretty cheap, from about anywhere. Note the size and then use it to carry with you and note how many times you fill it up. But usually if you get a bigger one that doesn't need to be filled up so often it is much easier to keep track of, then trying to count 8 to 10 8 oz cups.
Kit