Here are a couple of Metric Calculators that are easy to use.

For one of the easiest and best metric calculator I have seen go to http://www.conversion-metric.org/weight.  Simply select what you want to convert i.e. grams to ounces, put in the amount of the grams and instantly the ounces come up.

Another one is http://www.thecalculatorsite.com/conversions/liquidvolume.php.  This is also easy to use.