It's 3 to 1 by volume. I use the recipe in the Planet Cake book. Comes out great, especially since I got a small electronic kitchen scale so that I can do it by volume.
Measure by weight, not volume, for both the chocolate and the cream.
http://cakecentral.com/cake-decorating-ftopict-729560-.html&sid=2023a3f99b69922731ea83c84cdc313c
Here's another topic about ratios that may help you. EnglishCakeLady has posted a link to a ganache calculator which tells you how much ganache to make for each different sized cake. Click on the link and it will download an excel file to your computer.
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