About

Setting fence posts is a physically demanding task where running out of materials mid-job is a nightmare, but buying too much involves heavy lifting for returns. This calculator solves that precision problem for DIY enthusiasts and fencing contractors. It determines the net volume of concrete required by accounting for the displacement of the post itself within the augured hole. Whether you are installing a privacy fence, a deck footing, or a mailbox, accuracy here ensures structural stability and cost efficiency.

The tool focuses on pre-mixed bags (Quikrete, Sakrete, etc.), which are the standard for residential projects. It converts the required cubic volume into specific bag counts (50lb, 60lb, or 80lb), saving you from manual volume-to-weight conversions. It assumes standard installation practices where the post is embedded in the concrete column.

Formulas

The calculation first determines the total volume of the hole and then subtracts the volume occupied by the post (displacement) to find the net concrete volume required.

V_net = (V_hole − V_post) × N

For a circular hole:

V_hole = π × (D2)² × H

Finally, the number of bags is calculated based on yield:

Bags = V_netYield_bag

Reference Data

Bag Weight	Approx. Yield (Cu. Ft.)	Approx. Yield (Cu. Meter)	Typical Use Case
40 lb	0.30 ft³	0.008 m³	Small repairs, mailbox posts
50 lb	0.37 ft³	0.011 m³	Standard fence posts (fast-setting)
60 lb	0.45 ft³	0.013 m³	General purpose, deck footings
80 lb	0.60 ft³	0.017 m³	Heavy-duty structural posts
90 lb	0.68 ft³	0.019 m³	Commercial applications
Standard Hole	Diameter: 3x Post Width	Depth: 1/3 to 1/2 Post Length	General Rule of Thumb

Frequently Asked Questions

A general rule is to dig a hole that is 1/3 to 1/2 the total length of the post. However, in colder climates, it is critical to dig below the frost line (often 36-48 inches) to prevent 'frost heave', which can push posts out of the ground.

Yes. Adding 4-6 inches of gravel at the bottom of the hole promotes drainage and prevents the wood post from sitting in water, which significantly delays rotting. This calculator focuses on the concrete volume above that gravel base.

The diameter of the hole should generally be three times the width of the post. For a standard 4x4 post (which is actually 3.5 inches), a 10-12 inch diameter hole is recommended to provide sufficient concrete coverage.

Yes. The calculator allows you to select square or round posts while assuming a round augered hole, which is the most common installation method.