I have shared that a low energy diet is endorsed by the Dietary Guidelines for Americans and is the cornerstone to Volumetrics and  Weight Watchers.  Energy density of a food or meal is determined by dividing the total calories by the weight of the food(s) in grams.
   Sometimes the serving size of a food is provided in ounces not grams.  This is usually not a problem as ounces are easily converted to grams.  If you are choosing between two products and they are both given in ounces, you can just divide calories by ounces to see which is the least caloric.  Sometimes the serving size is given in ounces AND grams.  But be careful....
   I was choosing ice cream today - ice cream always gets me.  When it is a tub of ice cream the serving size is 1/2 cup - which is not useful for two reasons.  First it is unrealistic.  We tend to eat more than one half.  And Second, it isn't calories per half of cup that we need to be aware of but calories by weight - in grams.  I have pointed out before that the 1/2 c serving sizes have unequal weights across products and flavors (the grams are always provided so you can do your own math).
   When buying ice cream bars, (I choose from Weight Watchers and Skinny Cow).  I read the grams per bar to know which one is REALLY the lower calorie option. The front of the box might say that they are 100 calorie bars, but that does not mean that you get the same amount of ice cream.  I always want more ice cream for my calorie - more bang for my buck.  Usually Skinny Cow wins the calories per gram contest.  The Weight Watchers Fudge Bar is the exception.
   But today something caught my eye in the small print on the front of the package.  This led me to check those grams again.  On all the packages (excluding the fudge), all flavors of both brands said 6- 2.65 fl ounce bars per package.  How can they all be the same fluid ounces if they are NOT the same weight in grams?    The 2.65 ounce bar in the Weight Watcher group weighs about 52 grams and the Skinny Cow ones weigh  63 or slightly more grams.  
   But wait... fluid ounces?  Ice cream is not a liquid.
Either way the ounce to gram conversion doesn't equal what the product says.  When using fluid ounces the conversion from 2.65 = 78g, when using the more appropriate ounce to gram conversion the 2.65 ounces = 75 grams.  None of the conversions were true to the label grams.  
   When in doubt, ALWAYS refer to the grams..Actually, doubt or no doubt - its calories/gram so use the grams.
Now - time for some ice cream.