Gfretwell gave you the correct answer.
When ordering or specifying concrete today, it is done by
requesting a PSI strength depending on how the concrete is to be
used. 2500# for residential footings, minimum; most commercial
work is speced at 3500#; tilt up and prestress run 4500-6000#.
Each excess gallon on water per yard above the design water/cement
ratio reduces the strength by 500#, so you will often see
references to not using too much water.
The very old method of ordering, mixing, or thinking about
concrete as xxx number of sacks per yard is not used anymore, but
most concrete people (at least we old ones) do understand the
It kinda ends up knowing what you are doing with the concrete, the
stresses to which it will be exposed, your expectations for the
finish product, etc. 3 sack will set up, get hard, and will
crack. 6 sack will set up, get hard, and will crack. The six
sack can take a great deal more load before crushing.
Your call. There is an excellent explanation with facts and
figures here: http://www.wikihow.com/Make-Concrete
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