Cakes made in conventional 8- and 9-inch cake pans are perfectly sized for family use and modest occasions. But when you're hosting a larger event, a sheet cake provides the most efficient way to offer dessert. You can easily cut large rectangular cakes into uniform pieces, which is convenient for planning purposes as well as serving. With a bit of forethought and some basic math, you can avoid the twin perils of running out or over-purchasing.
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Sheet Pan Sizing
Sheet pans are made in a range of sizes, described by their relationship to the full-sized baker's sheet. Those measure 18 inches wide by 26 inches long at their flared edges, producing a cake of 16 by 24 inches after trimming. Half-sheet pans yield a cake of 12 by 16 inches, while quarter-sheet pans -- the familiar 9- by 13-inch size used by home bakers -- produce an 8- by 12-inch cake. For intermediate-sized or very large cakes, two or more can be placed together on the same cake board, and decorated as a single cake. The dimensions of your cake, and the size of slice you cut, determine how many guests you can serve.
At a backyard barbecue, a family gathering and other less-formal affairs, expect to serve a relatively generous portion of cake. A slice 3 inches long by 1 1/2 inches wide should be large enough to satisfy most adults but small enough to allow for ice cream or other treats on the side. At that portion size, a full-sized sheet cake will yield eight 3-inch rows, each containing 10 slices, for 80 portions. That number isn't exact, because some will want a smaller slice, and others will want seconds, but it's a good approximation. Half-sheet and quarter-sheet cakes provide 40 and 20 portions, respectively.
In a more formal context, such as a community event or wedding reception, slices are typically smaller. A slice 2 inches long by 1 1/2 inches is typical. In a full-sized sheet pan, that translates to a pattern of 12 slices by 10, or up to 120 guests. Half-sheet and quarter-sheet pans, cut to that size, result in 60 or 30 slices. If you prefer a larger but thinner slice, a 3-inch by 1-inch portion is also perfectly appropriate. This produces 96 slices from a full-sized sheet cake, 48 from a half-sized cake, and 24 from a quarter-sheet cake. Wedding-sized slices can be as small as 1- by 2-inch, yielding up to 192 portions. Most bakeries suggest a range, such as "90 to 100," to allow for second portions and erratic cutting.
Making the Cut
If possible, have one person designated as the cake-cutter. Your slices will be more uniform when they're all cut by the same person. Stand the knife in a large pitcher of hot water, to warm the blade, and provide your cake-cutter with a clean towel to wipe it clean and dry. Cut the cake crosswise into a 2- or 3-inch slab -- you can mark the cake board unobtrusively, if you wish -- and then clean the knife. Cut that slab into slices of the correct width, cleaning the knife after each slice and re-warming it periodically. Repeat, until all your guests have been served.