But... larger pans usually need LESS baking powder in the batter, because (I think) there's more surface tension to help the cake rise. I think I learned this from Rose Levy Birnbaum's Cake Bible.
What you need to know is the volume of each, and how much larger that of the 8" pan is compared to the 6" one. I use an on-line calculator for a cylinder, which a cake pan is, essentially, minus the top. I'm guessing you're using pans that are 2" deep, and I went ahead and plugged in the values for you. The volume of a 6" pan is 56.5 cubic inches; that of an 8" pan is 100.5 cubic inches. (100.5-56.5)/56.5 = .78. The 8" pan is 78% larger than the 6" pan, so use that as your increase for each ingredient in your recipe. For example, to increase 2 cups of flour using either a calculator or all your fingers and toes, whichever is easier, you'd enter 2 + 78 %, which equals 3.56 cups. Round to the nearest logical unit, which would be 3.5 cups. Works like a charm!
Using your high school math skills, use the formula
area = pi times radius squared.
Thus a 6 inch pan has an area of about 28 square inches, while an 8 inch pan has an area of about 50 square inches. If the pans are about the same depth, use a little less than twice the batter to fill the larger pan to the same depth.
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http://www.online-calculators.co.uk/volumetric/cylindervolume.php
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area = pi times radius squared.
Thus a 6 inch pan has an area of about 28 square inches, while an 8 inch pan has an area of about 50 square inches. If the pans are about the same depth, use a little less than twice the batter to fill the larger pan to the same depth.