Everywhere I have looked, I have read that to get a firmer ganache, you use 3 parts chocolate to 1 part heavy cream. To me, that is like comparing apples to oranges. The cream is a liquid so it would be measured in liquid ounces and the chocolate would be measured dry weight. Does three to one mean to measure the chocolate after it is melted? HELP!
An ounce is an ounce. So 3 oz chocolate for 1 oz cream. The chocolate weighs the same solid or melted.
Here is a very long, very informative thread on the subject. 
http://cakecentral.com/modules.php?name=Forums&file=viewtopic&t=633264&postdays=0&postorder=asc&&start=0
I had the same cofusion as you, zinger, when I first attempted ganache.
Sorry, Kelly, but an ounce is not an ounce. There are weight ounces and there are volume, or liquid, ounces. For example, a one cup measure holds 8 ounces by volume. If you fill it with cream you get eight liquid/volume ounces of cream. But if you weigh that cream on a scale it will weigh about 8 1/2 oz. Different liquids will weigh different amounts depending on their density.
If you fill that same 8 ounce cup with feathers or sand, each will have different weights as there is no correlation between the two different kind of ounces...weight and volume.
One 8 oz cup of chocolate actually weighs 10 oz if it's melted, less if it's chopped. So it does depend on whether it's chips or chunks or melted or blocks because each takes up a different amount of space in an 8 oz cup.
So you are correct, these recipes are all comparing apples and oranges.
The best I can figure out from recipes that give both the ratio and the specific amount is that they are measuring the cream by volume compared to the chocolate by weight. Of course, when you do it this way, the ratio is of apples to oranges.
If you did use weight measures for both as Kelly references (1 oz cream to 3oz chocolate) it doesn't match up with the references I can find that mention actual amounts used. I've even seen recipes that call for 20ml cream to 60 mg chocolate...clearly a weight to volume 3 to 1. 
artscallion is right, volume ounces and weight ounces are different. But weight ounces to weight ounces IS apples to apples.
For ganaches, there are a lot of variables. What liquid are you using? What type of chocolate? What is the purpose for use and therefore desired consistency? There are no (ok well, few) wrong recipes. My basic dark ganache is 1# dark chocolate to 1# 1oz heavy cream, both by weight.
The good news for you zinger is that cream is basically the same in either type of ounce and you can be sure the chocolate was meant to be weight.
You're right, I should have been more clear. An ounce of weight = an ounce of weight. It's like the old trick question, what weighs more, a pound of feathers or a pound of lead?
You can (almost) never go wrong in baking when you get out your scale and weigh everything.
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