Here's Northern Brewers priming calculator
***BUT USE THE WEIGHT (grams or ounces) AND !!!!!NOT!!!!! THE CUPS MEASURE***
I know from experience that the cups (volume) is incorrect. (The ounces and grams are weight measures and are correct.) Volume measures are always less reliable but I measured my sugar out tablespoon by tablespoon once and found they were way off (2.5 oz simply was *not* 1/3 a cup).
[Out of curiosity, I wonder what the "volume" as in 2.7 volumes or 1.8 volumes refer to...]
Well, maybe "way off" was a bit harsh. But I had about a four thirds as much volume to my weight as they did.
+++edit of an edit+++++
Just weighed half a cup of corn sugar on two of my scales. One measured 2 2/3 oz and 75 grams. the other 2 3/4 oz and 78 grams. According to NB calculator 1/2 cup of corn sugar is 99 grams and 3.49 oz.