we buy cement by the cubic yard in buffalo ny.
if you are buying your cement ready mixed off the truck by the cubic
yard, you already know the volume dimensions of your forms for your
project.
or this weight question may be a local question for your county weights
and measures department.
or google local for concrete suppliers near you will tell you how you
get fresh concrete delivered in your climate, plus how much it costs to
delay their trucks past the included pour time.
know your temperature and humidity and weather forcast for this pouring
and curing.
and which mix.
trucks return to the supplier for each fresh delivered load because
it's hardening by the minute.
"Concrete will probably be delivered to the site by a local ready-mix
company in a truck-mounted mixer unit. When you call for concrete, the
supplier will need to know when you want the concrete delivered, how
much you need, and what type of mix you want. Concrete can only be held
for two to three hours after mixing before it starts to harden."
http://doityourself.com/concrete/concretedelivery.htm

type of mix example: we used a "six bag mix" in buffalo ny for our new
sidewalks in the 1980's.
your state may inspect your concrete plants since they buy lots of
highways.
here's an illinois pdf on concrete plant inspections:
http://www.dot.state.il.us/materials...ncpltappvl.pdf