Hi X,
Here is how I do it here, and all of this is imperial measurements, so you can adjust the technique accordingly for metric.
I multiply the area in sq ft by the average depth of the mud bed (in feet-have to keep all the units the same). This gives me cubic feet of mud needed.
If I am buying bagged mud, then I know that one 60 pound bag gives me 1/2 cubic foot, so I can just do the division and buy accordingly.
For a larger area where we need to mix our own and avoid buying bagged mud because it would be too expensive, I get the volume of the mud bed (as described above) and then buy that much sand.
We do a 4 to 1 mix for mud on floors, so I divide the amount of sand needed by 4 to determine how much portland to buy (measured in cubic feet).
Portland here is sold in 94 lb bags, which is equal to one cubic foot.
One 5 gallon drywall mud bucket or paint bucket is ~ 1/2 cubic foot, so we use those to measure out the dry sand and cement into batches for mixing.
You can use the same method to work out your volume of mud needed, using metric and the sizes of the bags or portland that you can buy.
Whoops, I didn't see that Whitebeam already answered you!