One source I looked at gave the weight in grams for one half of a small grapefruit (3 1/2 inch diameter, 100 g) = 32 calories. Half of a large 4 1/2 inch diameter grapefruit, 166 g was 53 calories. They both give you 0.32 calories per gram of grapefruit.
I personally, would just estimate, because honestly the difference of 10 or 20 calories isnt that crucial. But if you are really concerned about accuracy, you could weigh the grapefruit and then calculate per gram or per ounce.
