Lentil,
Hesitating to disagree with KYH, doubling the area, i.e. going from 1/2 sheet (12x18) to full sheet (24x18) for the same depth, 2", is double the volume. Now, if we're talking circular pans, doubling the diameter will quadruple the volume for a given depth, in other word, a 16" diameter cake pan will hold four times the batter that an 8" cake pan would.
For rectangular pans:
V = LxWxD so for 1/2 sheet pan: L=18, W=12, and D = 2 V=18x12x2= 432 cubic inches
for a full sheet: L=24, W=18, and D=2 V=24x18x2 = 864 cubic inches.
Now, there are 14.6456 cubic inches in 1 cup so, if you use 16 cups for a half sheet, that's 234+ cubic inches, the batter is 1" deep
Remember, dimensions for sheet-pans are nominal, so they may vary slightly, also, when using extenders, the bottom measurements control, the top measurements will be greater because of the slanted sides.
Oh, one more point, in my experience, and I just went and looked, quarter, half, and full sheets are all the same depth. Mine happen to be 1" deep.
For circular pans
V = 3.14159xRxRxD where R = radius = 1/2 diameter and D=depth (it is pi R squared times depth, can't figure out exponents!)