Building a wall, how much is 7 cubic meters...

Is the rule of three already too difficult? When is it taught? 4th grade? 5th grade? It used to be called word problems.

Necessary information can be found on every bag of cement or on the internet.

But you’re nice. Bulk density and all that stuff was not covered in 4th or 5th grade.

The OP has recognized that he needs to convert one unit into another. The principle of bulk density is irrelevant for solving the problem. There is a factor to apply (multiplication), and that's it.

In Step 1 he needs the density of concrete. Step 2 is then the rule of three to calculate the actual amount of the individual ingredients.

Copy and paste from an internet site:

"You can look up the density of concrete in relevant reference books or internet tables. Depending on the composition, the value for concrete is d = 2.1 - 2.6 t/m³. For the calculation, an average value of d = 2.4 t/m³ = 2400 kg/m³ is suitable, so that you are on the safe side for most applications."

No guarantee is given by me for the accuracy.

That makes 7 cubic meters then 16.8 tons of concrete. According to [Epis] mixing ratio that is 1527 kilos (liters) of water, 3054 kg of cement and 12218 kilos of gravel. A few kilos won’t get you very far. I wouldn’t want to mix these quantities myself either, just think about how many hundreds of times you would have to run a small concrete mixer. A delivery is probably more sensible....

The costs for 1cbm of concrete in a mixer are approximately 90-140 euros depending on the type and chemistry of the concrete. Just needed it last week... C25/30 for 95 euros + VAT.

I wouldn’t want to do it by hand for that price either.

Then try to get 12218 kg of gravel from the building materials dealer, because he wants to know the volume, that is cbm. For that, the bulk density is required.

The building material dealer can manage that by looking at his tables.