Answer:
Use the calculator at http://www.flexidry.com/index.php/screeding/buy_flexidry/how_to_calculate_usage. Gives the answers for 3 different mix designs 3:1, 4:1 & 5:1 depending on usage of the final floor.
add. Cement is the active component in your concrete, it is the material from which the new crystals grow that join the particles, and give your concrete strength. It is also the most expensive.
Sand has a high surface:volume ratio, and the idea is to NOT use a very fine sand - that will just consume more cement in coating it, without adding to the strength of your construction. So just enough sand to substantially fill the voids in the aggregate, and no more.
You can increase the strength of your construction by giving the concrete a good long time to set properly - keep it moist for up to 28 days if structural strength is needed - e.g. a suspended slab.
In commercial production of e.g. concrete pipes where a high strength is needed, the pipes are heat-cured which gives much quicker growth to these tiny crystals that are so important.