I have always assumed that pie plate labeling was relatively straight forward, with only one dimension (the diameter) generally being given on the label. The main exception is if the plate/pan is of the deep-dish variety, in which case the depth is often given as well. However, standing in the dreaded Walmart, trying to find a pair of pie pans for the Pi-day pie crust experiment, I noticed something a bit peculiar; pie vessels that claimed to be the same size, did not appear to be the same size.

Not ashamed to be weird, I ran over to the tools section and borrowed a tape measure to check how accurate the labels were. Below are my findings. NOTE: these measurements were not done in a well controlled setting (…aisle of Walmart…) so there is likely more measurement error here than I would like.

Format:
Brand, stated diameter:

• upper: measured upper diameter, from inside edge to opposing inside edge (inches <– I was in Walmart and didn’t have a metric tape measure)
• lower: measured lower diameter of the base of the pie plate (inches)
• height: measured from where the lower crust would sit on plate, to where the top of the filling would be (below flutes if present),
• volume: (cubic inches) calculated as frustum

Anchor, 9″:

• upper: 9″
• lower: 8″
• height: 1″
• volume: 56.8 in^3 (= 3.93 cups)

Paula Deen 9″:

• upper: 9.5″
• lower: 9″
• height: 1.5″
• volume: 100.8 in^3 (= 6.98 cups)

Anchor 9.5″:

• upper 9.5″
• lower 8.25″
• height 1.125″
• volume: 69.7 in^3 (= 4.82 cups)

Pyrex 9.5″:

• upper 9.375″
• lower 8″
• height 1.5″
• volume: 89.1 in^3 (= 6.17 cups)

Conclusions:
There is a rather large difference in calculated internal volumes of pie plates, even between those that are stated to be the same size. Also, while in most cases the given diameter is the upper diameter, the version from Paula Deen gave the bottom diameter, giving that “9-inch” pie plate the largest volume of all the ones examined.

This differential volume may have large impacts on the baking time and final results of a recipe!

Future questions:
It would be nice to actually fill each plate with liquid and then measure the volume of the liquid to test accuracy of measurements.
Test baking the same recipe in plates that claim to be the same size, but actually have different volumes to see if that changes baking results.
Test to see if the observation noted at The Cookie Scoop, is due to differing volume rather than glass plate vs. metal pan. Aluminum should be a much better conductor of heat than the glass, so the pie in the aluminum pan needing to bake longer  doesn’t make much sense to me unless it is a volume issue.