Concrete is usually sold in meter's cubed ( m3 ) but depending on what you want it for it comes in many mix ratio's etc, ( slump, aggregate size, sulpher resistant, air entrained etc etc ) the list is endless. If you simply want to cast in a slab to stand something on that's not load bearing ( dog run/kennels ) then a normal mix would suffice ( 4:2:1 mix, 25mm aggregate, 50mm slump ) but if you want it to support a load bearing structure or carry any unusual weight's then many more things come into the equasion ( reducing levels, type of hardcore, compacting, reinforcement, depth of shuttering