Planning how much concrete you need is not as simple as adding numbers.

The ratio I use is 1 part cement, 2 parts sand and 3 parts gravel. Mixing this together would give you 6 parts concrete.

Right?

Wrong! 2 parts disappear!

How can this be, you ask? It’s actually quite straightforward.

Imagine filling a bucket to the brim with gravel. Is it really “filled” with gravel? Of course not, there are air gaps between the particles. If you fill a second bucket with sand and mix everything together, you get less than two full buckets. The sand fills the pores in the gravel.

When planning how much of each component to order, you need to take this shrinking into account. In construction, they use the terms “dry volume” and “wet volume”, where the former is the volume of all components added together, and the latter is the final volume after adding water and mixing. It just so happens that the ratio of dry over wet volume is around 1.5 for concrete.

So one part cement + two parts sand + three parts gravel is not six parts concrete. In fact…

1 + 2 + 3 = 4