Is concrete mixed by volume or weight, or not too important?

[RRRRRR]

Doing a mix of 4:2:1 concrete (20mm gravel, grit sand, cement). Worked out weight I need of each from general calculators.

When mixing, I guess the cement is the heaviest, so should I mix roughly by weight or will by volume be near enough for a C20 domestic floor slab?

Thanks.

 

[hotrod]

What volume (m³) do you need in total for your floor slab? In reality, you're not going to weigh the aggregate on site are you, as you load the mixer, so you're going to have to gauge it by approximate volume (shovel).

Just watch for the sand "heaping" on the shovel whereas the cement and 20mm stone will "flatten" on the shovel and you won't get a heaped load as it were. Just something to bear in mind.

 

[RRRRRR]

Great- thanks. Clear, helpful and resolved it in my mind.