Fence Post Concrete Calculator: Holes, Bags and Cost

Formula

For a round hole, cross-sectional area = π × diameter² ÷ 4. For a square hole, area = width². Concrete depth = hole depth − gravel-base depth. Gross hole volume equals hole area × concrete depth. When selected, post displacement equals post area × the smaller of the post length inside concrete and the concrete depth. Net concrete = gross hole volume − post displacement.

Why subtracting the post matters

A general concrete calculator often treats the entire hole as concrete. A post already occupies part of that space. This calculator can remove only the portion of the post surrounded by concrete, then apply the allowance you choose. Turn the option off when you deliberately want the full-hole quantity as a conservative purchase basis.

Worked example

Ten round holes, each 10 in in diameter and 30 in deep, with a 4 in gravel base and a 4 × 4 in post embedded through the 26 in concrete zone, need about 9.41 ft³ before allowance. With 10% added and bags yielding 0.60 ft³, the purchase is 18 whole bags.

Limits

• Use actual measured hole and post dimensions. Augered holes are rarely perfect cylinders.
• The gravel-base estimate is separate; it is not included in concrete bags or cost.
• Bag yield varies by product, mix, water and consolidation. The product label must replace the example yield.
• This tool estimates quantity only. It does not select footing diameter, embedment, concrete strength, frost depth, reinforcement, drainage or structural design.
• Local code, wind, soil, fence height and gate loads may require larger or deeper footings.

Related tools: use the Fence Calculator for posts, panels and pickets, the Concrete Calculator for slabs and general shapes, or the Gravel Calculator for aggregate quantities.