Since cake baking mixes typically list instructions for 9-by-11-inch pans or 8-by- 8-inch pans, if you have an 11-by-15-inch cake pan, it's difficult to know how much cake mix to use. Also known as a sheet pan, these pans bake sheet cakes that feed over 20 people. By calculating the volume of the pan and converting that volume to the cake pan examples listed on the back of the cake mix box, you can figure out how much mix you'll need for an 11-by-15-inch pan.
To find out the volume of a cake pan, multiply the length of the pan by the width of the pan by the depth or height of the pan. For instance, an 11-by-15-inch pan that's 4 inches deep has a volume of 660 cubic inches. For the depth measurement of a cake, use the approximate height that you want the cake to end up or would expect it to end up when baked in the size of the pan in question.
Mixes for 8x8 Pans
A snack cake mix sold in bags or small boxes ideally made for an 8-by-8-inch pan with a 4-inch depth makes about 256 cubic inches of cake. If you're looking to fill an 11-by-15-inch cake pan to about the same depth, you'll need at least two cake mixes, which together make 512 cubic inches of cake. That will still leave the cake slightly more shallow than when made in the 8-by-8-inch pan, so consider buying a third cake mix, allowing a total volume of 768 cubic inches. When using three such cake mixes in an 11-by-15-inch pan, the cake will actually be a little more than half an inch higher than if using only one in an 8-by-8-inch pan.
Mixes for 9x11 Pans
When using full-size cake mixes made for 9-by-11-inch cake pans, assuming a 4-inch depth again, you'll need two cake mixes to fill the 11-by-15-inch pan. Because the two cake mixes together would make the cake almost an inch thicker than the single version made in a 9-by-11-inch pan, increase your baking time by at least a few minutes. If the mix threatens to overflow your pan upon rising in the oven, pour only part of the double mix into the pan and discard the rest. Bake the cake in a cake pan on top of a baking sheet to avoid spills.
For a thin sheet cake suitable for layering, for use in other desserts or for a bar-style cake, use only one cake mix for the large pan. Its volume will spread throughout the pan to create a thinner cake. Consider adding to the batter's volume if you don't have enough boxes of mix to get as much height to the cake as you'd prefer by beating an extra egg into the mix. In addition to adding liquid volume, the eggs leaven the batter and cause it to rise higher.