Post hole and Sonotube planning notes
Use this round concrete calculator for a post hole, Sonotube form, pier, drilled shaft, or round column. Enter the finished diameter and depth. The calculator handles the circle math, so you do not need to convert diameter into radius first.
Measure the finished diameter
Use the inside diameter of the tube form or the diameter of the actual hole. An auger bit size may not equal the final cavity size, particularly in loose, wet, or collapsing soil. Confirm the size before relying on the volume result.
Plan for irregular soil-formed holes
Post holes and drilled piers can widen below the surface or lose material from the sidewalls. The 10% default allowance is a practical starting point for a soil-formed hole. A rigid Sonotube may need less adjustment when its dimensions are consistent.
Separate enlarged bases and bell shapes
If a pier has a widened base, bell, or footing below the round shaft, calculate that added shape separately. The cylinder result on this page represents the diameter and depth entered in the tool.
Example column and post-hole projects
A common fence post hole or small pier.
A larger post hole where bag count rises quickly.
A deeper drilled shaft that should be checked against actual hole size.
Round column calculator FAQ
Can I use this calculator for several fence post holes?
Yes. Enter the number of matching holes in the quantity field. Calculate holes with different diameters or depths as separate groups.
Does the result include the post or reinforcing steel?
No. This is a concrete volume estimate only. Posts, rebar, gravel, excavation, and local delivery charges are outside the calculation.