Calculate concrete orders for footings.

Hi, first time building anything, first time posting here, so please forgive me.

I'm trying to calculate what I need to order for the concrete for my footing post holes. If it's relevant, it's going to be a single story, slightly raised, 8m x 4m house.
I think I've done the calculations right, but I'd really like the confirmation (or re-direction) of someone who actually knows what they are doing!

Here's what I've come up with...

12x post holes/footings holes

Hole dimensions:
Diameter = 300mm
Depth/height = 600mm

So, approx 0.5m³ of concrete is needed.

Will be buying local 19mm concrete aggregate mix. $80/m³
Assume aggregate density = ~1500kg/m³

As recommended by manufacturer...
Cement:sand:aggregate = 1:3:3 and,
1m³ concrete = 13x 20kg bags cement (260kg cement)

So, 0.5m³ concrete = 130kg cement.

1:3:3
130:390:390 (kg)

390kg = 26% of 1500kg

This is where I confuse myself...

So, should I order:
0.3m³ of aggregate mix
7bags of cement

Or do I need to change the mixing ratio of the 1:3:3 to something like 1:6 because I don't need to add the sand?

In which case...
1:6
130:780

780kg = 52% of 1500kg

So, I should order:
0.5m³ aggregate mix
7bags cement

Please help!
TIA!

Hey Kloe Welcome to the forum
If you are hand batching order bulk
Concrete has a density of 2400kg/m3
Try batching by Volume here
General purpose concrete mix
Easy Guide to cement

Cheers
Chris

Why dont you need to add the sand, im confused?

If possible, and if youre in Sydney (dont know prices elsewhere), it would probably work out cheaper for you to order .5m3 through minicrete, and just barrow it into your holes. Would also be significantly easier, and you would get a much more reliable, and consistent mix. I would also increase the diameter of the post holes to 400mm.

I don't need to add sand cause the 19mm agg mix I'm buying already has sand blended in. All you need to do is add the cement powder and water.

Unfortunately not in Sydney. Live in a tiny rural town in the middle of nowhere, so this is definitely the cheapest way for me to get it, especially seeing as I don't need much. Was going to be over double the price to get the pre-mixed stuff.

Thank you for taking the time to reply to my post though!

Thank you for the links.
I'm still unsure what to do about the cement:sand:agg 1:3:3 ratio though, seeing as my agg is a blended mix and contains the sand already. I guess I would just add the 3:3 and make it cement:blendedmix 1:6...? Do you happen to know if this is right?

too much sand....by bulk I mean order all components separate then batch it by volume/shovels (read the cement companies instructions), mix in a cement mixer, make sure the sand, aggregate is dry, add water & check the slump, etc,etc

You are definitely correct with the math, your ratio will essentially be 1:6. Thats all.

Goodluck.