A little squirrel told me about this site:
It's a fun game and it is for a good cause. I just tried it this morning and reached level 43 and donated roughly 3000 grains of rice. I was curious about how much rice equaled a single serving so I did some searching on the net. This article seemed reasonable and concluded that 10,340 grains of rice was contained in 1 cup, but I found some other information that contradicted this. So, I did the only thing I could and counted some grains for myself.
I counted the number of grains of Kroger brand Long Grain Brown Rice that would fill one of my teaspoons, I then counted the number of teaspoons required to fill 1/4 cup.
~300 grains per teaspoon
9 teaspoons per 1/4 cup
So, that's just about 10,800 grains per cup. It turns out that the answers.com article is really close to what I counted, so I feel comfortable with the number. For the Kroger rice that equals roughly 640 calories. That's not too bad. You could probably donate enough rice to provide for the total calorie requirements of a single child with 25,000 grains per day.