How the Post Hole Estimate Works
Each hole is treated as a cylinder: π × radius² × depth. If you enter a post width, the calculator subtracts the part of the post occupying the hole. A round post is treated as a cylinder; a square post is treated as a square prism.
For example, a 300mm diameter × 600mm deep hole containing a 100mm square post has approximately 0.0364m³ of space around the post before an allowance. With 10% selected, the estimate becomes approximately 0.0401m³, which rounds up to 5 bags using the stated 108-bag assumption.
Measure every input
Use the excavated hole's diameter at its widest practical point and its actual depth. Enter the post dimension that sits inside the hole. Multiply by the number of identical holes, or calculate different hole sizes separately and add the results.
Dimensions are not design advice
The calculator does not choose hole diameter, depth, post embedment or concrete type. Wind loads, soil, post material, structure design and local requirements can all change those decisions. Use the project plans or advice from an appropriately qualified professional.
Read the full methodology and sources, including the bag-yield assumption.