I remembered this hint this morining as I was attempting to frost some cupcakes with tub icing. (eg Pillsbury). Would have been nice had I recalled it 10 cupcakes earlier...
Put the entire tub of icing into a medium to large mixing bowl and mix on medium high speed til the volume has roughly doubled in size. Now you suddenly have enough icing :) Light, fluffy, and enough! YAY!
This not only saves $, but calories as well; the calorie thing is just a fluke, trust me.
The reason I ran short of icing to begin with:
I let the kids use tub icing and animal crackers for homemade "dunk-a-roos". It is less expensive and I can control the portion size per child. I serve them in individual applesauce containers I save and use for everything under the sun. I also use the empty icing tubs for a multitude of things. That's another story for another time..:)
Just don't decide you need the icing for something else before checking the amount in the tub. ( I am rolling my eyes , just incase you're wondering )