How many bags of concrete per fence post?

Discussion in 'Landscaping and Outdoors' started by Pete574, Dec 1, 2019.

  1. Pete574

    Pete574 New Member


    I calculated that I need 21 bags of concrete per fence post - this seems a lot to me!
    Could someone double-check my calculations?

    Hole area: 0.25 m2 (0.5 m x 0.5 m)
    Post area: 0.01 m2 (0.1 m x 0.1 m)
    Hole depth: 0.9 m

    Volume = 0.216 m3 (0.25m - 0.01m x 0.9m)

    Each concrete bag fills 0.01 m3 in volume

    Therefore I need approximately 21 bags of concrete per fence hole.

    BTW, I plan to use this concrete:
  2. Jitender

    Jitender Screwfix Select

    The calculations seem ok.

    But think the specs for the concrete is wrong.

    Holes seem a bit too big 30-40cm wide is more than adequate.

    What lenght of post will you be using? are they wooden or concrete?
  3. Jitender

    Jitender Screwfix Select

    If you have a lot of posts it will be cheaper to buy in ballast and cement, and mix your own.
  4. Wayners

    Wayners Screwfix Select

    Mix your own.

    3 sand. 3 gravel. 1 cement.

    Post hole needs to be small. Use post hole digger. Hire all the stuff so you can take back

    I mixed in wheelbarrow easy enough
    KIAB likes this.
  5. KIAB

    KIAB Super Member

    Concrete fence posts?

    How many posts?
  6. dray

    dray Screwfix Select

    Calcs are not correct giving you a cost of approx £100 per post! just for a moment imagine stacking 21 bags of concrete and how that would wrap around each of your posts!
    There was a good thread on here a little while back concerning postcrete V concrete, have a read of that and you will see that perhaps 2 bags of postcrete per post would probably do.
  7. Jitender

    Jitender Screwfix Select

    Those holes are very deep nearly 1 m deep!, unless you have very high posts thinks its a bit overkill.
  8. wiggy

    wiggy Screwfix Select

    You are way off pete.

    Your holes want to be 200/300mm square and about 600mm deep.
    You will probably need 2/3 bags of the stuff you want to use.
  9. Pete574

    Pete574 New Member

    Some essential information I missed out:
    Post length: 3.048 meters (selco link)
    Number of posts: 15

    The 0.9 m depth is due to the "general" recommendation of putting 1/3 of the length in the ground.
    It was also recommended to use C10/GEN1 concrete as I will be retaining about 0.5m of soil on my neighbour's side.

    It looks like the post holes are too big, I'll reduce the size and recalculate.
    I'll also do my sums on mixing my own concrete to see how much it will save.

    Thanks for the responses!
  10. KIAB

    KIAB Super Member

    Definitely want 900mm in the ground,400mm-500mm square,use proper concrete, get dumpy bag of ballast,will work out cheaper.
    Jitender likes this.
  11. Jitender

    Jitender Screwfix Select

    So they are essentially 10ft long posts.

    When you go above 10ft the posts get thicker dimensionally so will be 5"x5" and a lot heavier. Put in 5 of these this year and thick they costs about £70 each.

    I would have thought a 1/4 of the length of the post would be sufficient, and a lot depends on the ground condition.

    So if using 10ft post, 2.5ft would be in the ground.

    Are you planning on putting up gravel boards?
  12. Jitender

    Jitender Screwfix Select

    + for dumpy bag, the sand is likely to bet wet this time of year.

    Would rapid setting cement be a better option?
  13. Pete574

    Pete574 New Member

    Yes, two gravel boards.
    Fortunately, the soil is mostly chalk, so fairly easy to retain.
  14. koolpc

    koolpc Super Member

    21 bags per post. Lol
  15. Astramax

    Astramax Super Member

    Best to be on the safe side just in case! ;)
    koolpc likes this.

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