I am setting corner posts for my fence. I am using rail road ties and will be making the holes 3 feet deep by 10 inches in diameter. I am trying to decide how many bags I will be needing. I want to fill the holes all the way up with Concrete. I will be using 80lb bags. any help will be appreciated.
Thanks,
Clayton

A tie that is 8" by 8" (3 feet in hole) is 1.33 cubic feet
A tie that is 6" by 6" is 0.75 cubic feet

If you do a 10" hole with 8" ties you will only have 1" on either side, which is pretty minimal, so in that case I would go with a 12" hole which would be 2.36 cubic feet.

The bags should tell you how many cubic feet they make so:

Post Size - Hole Size
6" - 10" = 0.89 cubic feet per post
8" - 10" = 0.30 cubic feet per post
8" - 12" = 1.02 cubic feet per post

and if it were me, I would then add 10%

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The Following User Says Thank You to Troglodyte For This Useful Post:

Depends which set of dimensions you are using, if you are going 8" railroad tie in 10" diameter hole, and your 80lb bag represents 0.66 cubic feet then you two holes per bag. If you are going 8" - 12", then 2 bags covers three holes.

I apologize for having to clarify again, I am definately not a math wizard so I am glad you are. I am doing my posts with an 8 inch rail road tie, 3 feet deep and 12 inches in diameter. That would require less than one bag to fill the hole?