How many bags of concrete do I need for post holes?

Quick answer

Find the concrete volume first, then round complete bags up

For a round hole, calculate cylindrical volume from the diameter and concrete depth. Multiply by the whole-number hole count, subtract only the post volume that actually occupies the concrete-filled depth, and then apply the project allowance. To estimate bags, divide the final order volume by the yield for the exact concrete product and round the package count upward.

The formula

Concrete volume for round post holes

Keep all dimensions in one unit system during the geometry step. BuildMeasure converts internally with exact international-foot, inch, and yard definitions and keeps full precision until display or final package rounding.

Worked example

One 12-inch diameter hole with 24 inches of concrete depth

This is a controlled calculation example, not a recommended hole size. It uses no post displacement and no extra allowance so the geometry can be checked independently.

1. Radius: 12 in diameter ÷ 2 = 6 in = 0.5 ft.
2. Concrete depth: 24 in = 2 ft.
3. Volume: π × 0.5² × 2 = 1.570796 ft³.
4. Cubic yards: 1.570796 ÷ 27 = 0.058178 yd³.
5. 80 lb example: 1.570796 ÷ 0.60 = 2.618 → 3 complete bags.

Using the same published example yields, this one-hole volume rounds to 6 × 40 lb bags, 4 × 60 lb bags, or 3 × 80 lb bags.

Reference calculation

What changes when the number of identical holes changes?

The table holds the example geometry fixed at 12 inches diameter and 24 inches of concrete depth, with no post displacement. The dimensions are examples only, not design recommendations. The last two columns use the 0.60 ft³ example yield for an 80 lb bag.

Package counts jump in whole bags because procurement rounding is applied after the full-precision order volume is calculated.

Optional geometry

Subtract a post only when the displacement assumption fits

A post occupies some of the cylindrical hole volume, but the amount depends on its shape, size, and how much of the entered concrete depth it actually occupies. BuildMeasure supports one centered round or square post per hole. When displacement is enabled, it assumes the post occupies the full entered concrete depth.

A passing fit check is geometric only. It does not certify clearance, strength, embedment, soil performance, or code compliance. If the full-depth displacement assumption does not match the project, leave displacement off rather than forcing an inaccurate subtraction.

Purchase inputs

Allowance and bag yield are separate decisions

An allowance accounts for project variation such as over-excavation, spillage, and site irregularities; it is not part of the cylinder geometry. BuildMeasure applies the selected allowance after net concrete volume is known.

The example yields come from Sakrete High-Strength Concrete Mix. Use the current yield printed for the exact product you intend to buy because other mixes can differ.

If you know your local package price, the calculator also lets you enter an optional price per selected bag and shows an approximate material cost. BuildMeasure does not fetch prices, convert currencies, or infer tax, delivery, labor, discounts, or supplier minimums.

Source: Sakrete High-Strength Concrete Mix product data.

Safety boundary

What a material calculator cannot choose for you

This guide estimates quantity from dimensions you already have. It does not determine the dimensions themselves. Before purchase or construction, separately verify:

• Hole diameter and embedment depth for the actual project.
• Soil, frost, drainage, wind, loading, and local code requirements.
• Post size, footing geometry, reinforcement, and structural details.
• The current yield and suitability of the exact concrete product.
• The allowance needed for the measured site conditions.

For unit references, BuildMeasure uses NIST conversion factors and volume-unit definitions. See the calculation methodology for how formulas, assumptions, and QA evidence are handled.