# Setting 6x6 posts in concrete

We are building a pergola-swing with two 6x6 posts as the main supports. The posts are to be set in a 12" diameter sonotube footing, 36" deep. I am unsure as to how much concrete to buy being that there is only about a 3" gap around after the post is in. I have calculated the volume of the cylinder (4071.5 cubic in.) as well as that of the post(1089cubic in.) I took the volume of the cylinder and subtracted the volume of the post to get a volume of 2982.5 cubic in Than I determined that a 50lb bag of concrete is 648 cubic inches. (.375x1728=648) 2982.5cu in/648cu in=4.6--> 5 Based off my calculations shown above, I figure I would need 5 bags per post x 2 posts for a total of 10. Just looking for someone to look over my work and verify it for me. Thanks, -Nick

• What torque will the swing apply to the soil and how much shear stress/strain will that yield? You can always take back extra concrete bags. What did you assume? Sep 23, 2020 at 10:01
• A much taller swing 10’, made of 4”D steel plumbing as an inverted U set in a 6”D x 3’ deep concrete makes an excellent sturdy swing with minimal effort. Sep 23, 2020 at 13:26