Fence Post Concrete Calculator

Use this fence post concrete calculator to estimate the concrete around your posts, then compare bags and estimated costs. Enter your hole dimensions, actual post size and number of posts. Fine-tune the gravel base and concrete fill depth when your installation leaves space below or above the concrete.

How the fence post concrete calculator estimates mix

Concrete quantity comes from the space you intend to fill after subtracting the post inside it. A wider or deeper hole needs more concrete, while a larger post displaces more. The calculator measures that volume; it does not decide whether your footing is strong enough for the fence.

Use the required installation dimensions before treating the bag count as a shopping list.

Measure the hole and actual post size

Your hole measurements should describe the space you will actually fill, including any gravel or gap left at the top. For a round hole, you need its diameter; a rectangular hole needs both width and length. The post’s measured size matters too, because its nominal timber label may not match the width that displaces your concrete.

Concrete volume and bag formula

For a round hole and square post, net volume = [π × (hole diameter ÷ 2)² − post width²] × concrete fill depth. Keep every dimension in the same units. Multiply by the post count, add your chosen allowance, divide by published yield per bag and round the final job quantity up once.

Worked example: a 12-inch post hole

A 12-inch round hole with a 3.5-inch square post and 24 inches of concrete fill needs about 1.4007 cubic feet. With 10% extra, allow 1.5407 cubic feet. At approximately 0.60 cubic feet per 80 lb bag, that means 3 bags.

At the example US price of $6.47 each, those bags cost $19.41 before tax or delivery.

Compare bag sizes and estimated costs

You can compare 40, 50, 60 and 80 lb bags with separate 0%, 5% and 10% allowances. Their published reference yields are approximately 0.30, 0.375, 0.45 and 0.60 cubic feet respectively, while the cost column uses the 10% quantity. If your supplier charges a different price, you can replace the dated US example in the optional settings.

A heavier bag isn’t automatically the most practical purchase.

Check the installation before pouring

Your gravel and concrete occupy different layers, and the post’s position changes how much gravel is displaced. That’s why the selected arrangement should match what you intend to build. Irregular holes or flared bases can need more mix than this straight-sided model, so your bag count still needs checking against the excavation and the purchased mix’s placement instructions.

It doesn’t verify footing strength.

Frequently Asked Questions