I need to dig and fill holes for 12 concrete fence posts - slotted ones to
take fence panels.

General guidance is they should be 2' deep.
Homebase 'how to' brochure suggests 1' * 1' by 2' hole for a 4" post.
This seems generally O.K. apart from the issue that if the concrete post is
on the boundary then the concrete filling will go 4" into the neighbouring
garden. Please tell me if I am barking up the wrong tree.

That would be (allowing for the 4" post in the middle) about 8" * 8" * 2' ~
..8 cubic feet of concrete per hole.
No! The hole is 2' * 1' * 1' = 2 cu ft.
The post is .33' * .33' * 2' = .2 cu ft
The remaining hole is therefore 1.8 cu ft.
[Damn - spotted this part way through.]
Mmm.. http://www.calculator.net/concrete-calculator.html does not have a
calculator for square holes with square posts within.
Tried to frig it - 2 * 3 * 0.3 should equal 1.8.
Used 2' * 3' * 4" but it rounds up to 2 cu ft (or down to 1.5 cu ft if you
go down to 3"). Damn again.
Suck it and see gets 2' * 1' * 11" to give me 1.83 cu feet
12 off it gives me 22 cu ft or 0.623 cu m.
In built (dodgy) calculator says 12 * 1.8 = 21.6 cu ft
Web converter says this is 0.648 cu m and 22 cu ft is 0.66 cu m.
Conclusion - web concrete calculator is doing rounding all over the place
and telling porkies (on a small scale).
Also, my head hurts and I need a drink.

The calculator suggests that this volume is equal to 48.4 * 60lb bags (but
of what?).
Postfix seems to come in 20kg bags (44lb bags?).

This is all rough approximation, but this suggests that you would need more
than one bag of postfix per hole (approximately 4).
[Unless, of course, the quick setting concrete doubles its dry volume when
mixed with water.]

Assuming 48 bags at £5 per bag this would be £240 for the pre mixed fast
setting stuff.
If I have under estimated the number of bags required each extra bag adds £5
to the total.

Surely I could mix 2/3 cubic metre of concrete, including hiring a mixer,
for much less than this?

http://www.benchmarkbuildingsupplies...calculator.htm gives me, for 2/3
cu metre of 1.2.4 ratio:
211kgs cement
396kgs sand
729kgs gravel

From the Travis Perkins Trademate web site:

8 * 25kg bags cement at £6.50 a bag is £52.
balast bulk bag (at least 855kg) is £62.50
mixer £11.50 for the week (HSS special offer)

This gives £126 but I am aware that I am slightly under (sand + gravel above
= 1135kg).
In fact, if the 'one ton' bag is only 855kg then I could be looking at a bag
and a half.
[Hang on - aren't the bags supposed to be one cubic metre? In which case one
bag will be fine (unless ballast shrinks by more than 1/3 when you add
cement and water). However in that case there is something screwy about the
weights from the on line calculator!]

I do note that postfix is £7.30 a bag at TP so I may achieve better prices
elsewhere, although TP were the cheapest for the concrete posts.

All this is looking like scarily large numbers!
Do I really need a full metric ton of ballast and 8 bags of cement to secure
12 concrete posts?
I nearly finished this post once then had to go through again doubling all
the quantities when I spotted my initial sizing mistake.

Has anyone else done a similar exercise of building a fence with concrete
posts who can reassure me that I am not wildly out with my calculation?

Help!

Dave R