November 10, 2008
This may seem obvious to those familiar with weights and measures, but
making such a statement has drawn looks that questioned my sanity!
The problem arises when people say "fluid ounce" and leave off "fluid"
Once my sister started dumping chopped walnuts from an 8 ounce bag
into a measuring cup and found that she had almost half left. She said
it should be enough since the recipe called for a cup.
I looked at the recipe and pointed out that it actually said "8 ounces",
she then said that it was the same thing. I asked her why she thought
walnuts weighed the same as water and she looked at me like I was nuts!
Of course, a "fluid ounce" is the volume of water that weighs 1 ounce. "Fluid ounce" is a meausrement of volume
equivalent to 1/8 (.125) of a cup.
So if you're speaking of actual (net) weight, the rule of "8oz = 1 cup" works only with things that have the same
density as water.
You wouldn't expect a cup of marshmallows to weigh the same as a cup of
BBs, and you shouldn't expect either to weigh 8 ounces! If you needed
8 ounces of marshmallows and you got one cup, you wouldn't have nearly
enough. On the other hand, if you needed 8 ounces of BBs and you got a
cup, you'd have way too much!