srhmess says:

If you have the bag the grapes came in you could roughly estimate the mass of the grape. Divide the mass of the bag (normally in oz.) by the number of grapes it contained when it was weighed. That should give you the mass of an individual grape(in ounces, remember to convert the units), assuming the mass of the bag is small/negligible AND the grapes were not attached to a stem (a stem could seriously skew your mean grape mass).
Or you could just google the ‘ave. mass of a grape.’