# How do I calculate the concrete needed for a fence post?

I'm trying to determine how much concrete I'll need to set fence posts for a 6ft tall privacy fence. From what I've read these are the parameters I'll be using for each post hole.

Post: 4in x 4in x 8ft

Hole Diameter: 12in

Hole depth: 30in (6in of gravel + 24in for post)

When I calculate the concrete needed, do I use the 30in depth or just the 24in for the part of the post in the ground? At the moment I plan on using 3-50lb bags of sakrete fast drying per hole. Is this too much, too little, or just enough?

• "Is this too much, too little, or just enough?" Let me ask Goldilocks. Are you sure 30" is code for the frost line in your area? Jul 29, 2019 at 21:39
• Rough calculation: Hole cross section = (6" x 6" x 3.14) - (3.5" x 3.5") = 100.79" round to 100" x 24" = 2,400 in^3. /12/12/12 = 1.38 ft^3. Various sources for "how much per bag" but using one that has it a 0.375 ft^3 per 50 lb. bag would result in 3.68 bags per hole. So seems to be in the right ballpark. Jul 29, 2019 at 22:50