i have a 6in cake recipe but want to scale up to 9in?

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