How many bags of cement does one need . . .

There are all sorts of experimenters speaking with more or less authority on how to calculate how much cement you need for a project. I have tried myself, and it is getting rather confusing, at least for me.
Is there anyone out there who could say "from their experience" approximately how many bags of cement you need for, say, a 20ft or 30 ft dome, or whatever their experience has been?
Perhaps better still, how to do the calculation?

We are building a 21 Foot Dome and up to now we have used approx 3 pallets of cement , in Australia that equates to 225 x 20kg bags, now this will get you only to lockup with no internal walls, if your unsure just google dome calculator and look at a site which will do the calculations for you. there you can have an idea of your s
urface area and calculate the materials from there. 
@dme2
Hi dme2, I’m building three connected domes in Longreach, Qld and I did calculation.
I actually wanted to know how many blocks I’d need to make and could work out the amount of concrete.
I’ll use one of my domes ok. Bedroom radius is 2250mm
The surface area of a sphere: 4xpi x radius squared. So surface area is 4 x (22/7) x (2250 x2250) or 63,643,857.
I need half a sphere (31,821,429) PLUS the part of a hemisphere enclosed by the height of the centre pole I’m using (900mm), ie 900/2250 of a hemisphere: 12,728,572.
The total surface area I have to cover is: 43,550,001sq mm.
My blocks measure 220x330mm. Total area 72,600sqmm.
So number of blocks is 44,550,001 /72,600
Thats 600 blocks.
I can make 70 blocks per pour using 6.2 bags.
That’s 9 pours And 55 bags of cement.
The numbers just look scary.
Good luck 
@TrevorMathewson no I bought up the last of the Seventh Generation liquid imported to Australia (at great cost). Apart from that I found Trucking Blue a truck wash detergent with very strong bubbles that makes a good mix.