Hi Susan: As a professional baker for over 25 years, you appear to be confusing square inches with cubic inches. Cubic inches takes into consideration the depth of the pan also. At the bakery, we made many items in the half sheet and whole sheet pans. So long as the depth of the items is the same, a half sheet holds twice as much, not 1 1/2 times, as a 9x13 pan. If someone uses your formula for an item that is only 1/2 to 3/4 inches tall, they are going to be woefully short when they transfer it to a half sheet using the 1.5. Just an obsversation. If you go to http://pastrieslikeapro.com/2013/09/pan-size-conversions/#.VF98FocT2EM there is an entire article on pan size conversion. Hope this helps.

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