Concrete mix.

[conny]

Thinking of laying a concrete base for my bin area. I've looked online but different sites give different amounts/change the answers from tonnes to bags or cu metres and it's just confusing me.

My slab would be 8' x 5 ' x 4"
Can someone please tell me;

Bags of cement, (25kg?) or total weight
Bags of sharp sand, or total weight
Bags of 20mm gravel. or total weight

If I knew what the weight of each was, I could easily work out the bags.

 

[davidmiller]

I used the cemex Ready mix calculator. They said 3.2m cubed.
If you mix it yourself You'll use all in ballast usually.
If I remember rightly there's a minimum of.850 in a bulk bag. Allow 6 bags of cement per bulk bag for 6 to 1 mix. Also if I remember rightly.

 

[John D v2.0]

Bulk bags are usually between 750kg and 900kg depending where you get them from which is less than 0.5m3. I think it's to allow a weight safety margin even when saturated.
You must have massive bins if they take up 5ft by 8 ft!
That's 3.7m2 which would make .37m3 of concrete if it's 100mm deep.
So you could probably get away with 1 bulk bag and however many cement that makes. And get a mechanical concrete mixer, your back will thank you.