wrote:
It's getting toward winter and I want to pour concrete in a feedroom
on the rear of my barn...

Ignoring all the other questions people have raised about your ability to
move and trowel that much concrete before it sets, (and if your livestock
tub is strong enough), let Google to the math part for you:

http://www.google.ca/search?hl=en&q=...+in+cubic+feet

Google calculator is one of the most useful tools on the web:
http://www.google.co.in/help/calculator.html

- Rodger