We now have more accurate ways to make the mixture. It was commonly believed that a "5 sac mix" was 3000 psi. Now you just order by the psi you want.
A U.S. bag (or sack) of cement contains 94 lbs. Note this is for cement not premixed concrete where you just add water. The origin of 94 lbs/sack for cement comes from 1 cubic foot of loosly bagged...
35 or 40
there is 5 shovels full in a 25kg bag of cement
10 bags can stack in one row...
A metric ton is 1000kg. 1000/40 = 25. 25 bags required.