ATriple it. You might have some extra batter but its easier to deal with that than to multiply a recipe by some fraction.
This chart should help you figure out how much batter you need per pan size: http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
This is my "calculator". It looks at the relative areas of the pans, and assumes that the depths are all the same. You can see that the area of a round 9" pan is 63.5 sq inches. The area of 6" round is 28.25 sq inches. If your recipe makes 2 6" cakes, then it is filling 56.5 sq inches. That is just a little less than the area of the 9" round. If you use the recipe for 2 6" rounds, it will fill a 9" round, but be a little less tall. If you want it taller, make 1.5 or 2 times your regular recipe ( I find it hard to measure things like eggs in smaller fractions) and make some cupcakes or 6" with any excess batter. Throw them in the freezer or give the kids a treat. Hope that helps!
PAN AREAS
SQUARE
8" 64
9" 81
10 " 100
12" 144
16” 256
RECTANGLE
9 x 12 108
11 x 15 165
12 x 18 216
ROUNDS
6" 28.25
8" 50.25
9" 63.5
10" 78.5
12" 113
14” 154
2 6" 56.5
2 8" 100.5
2 9" 127
2 10" 157
2 12" 226
HEX
6” 23.5
9” 52.5
12” 93.5
15’ 146
18” 210.5
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