The post occupies the hole
A 12 in hole with a 4 in post is not a 12 in solid pier. Construclytic subtracts the post. Cardboard tubes poured solid belong on the sonotube calculator, or set post diameter to 0 here if you mean a solid fill.
Depth is the concrete in the hole, not the frost line. Confirm embedment with the fence or deck spec. Bag yield is the sack in your hand — 0.45 ft³ is a common 60 lb planning value.
Bags versus a small truck
Ten 12 × 36 in holes around a 4 in post are about 0.78 yd³ net, 1.00 yd³ after 10% waste, or 52 of the 60 lb bags. That is still a bag job for most crews. A long fence with 40 holes is a truck conversation — do not scale 52 bags by eye.
Do it in order
- Enter hole diameter, post diameter and concrete depth. Use 0 post diameter only for a solid fill.
- Type the number of identical holes and the yield printed on the selected bag.
- Apply waste, then decide bags or a small ready-mix ticket.
Worked example — ten 12 in holes, 4 in posts, 36 in deep, 10% waste
Net concrete is 20.94 ft³ = 0.78 yd³. With 10% waste the ready-mix order is 1.00 yd³, or 52 bags at 0.45 ft³ each. A solid sonotube of the same size is more concrete — open that tool instead of zeroing the post by accident.
Planning boundary
This is a volume method. It does not set embedment, frost depth, a mix design or a guaranteed bag yield.
Does this set frost depth?
No. Embedment, bell bottoms and water in the hole sit with the responsible professional.
When do I use sonotube instead?
When the pier is poured solid in a cardboard form. This tool subtracts a post unless you enter 0.
Is 52 bags a truck?
Usually not. Past about a cubic yard, compare labour against a small ready-mix drop.