Concrete for round tubes and post holes

By Kyle Bell · Reviewed November 9, 2026

Round concrete pours use cylinder geometry: circle area multiplied by the depth actually filled with concrete. A tube's diameter, a hole's depth, and a post's dimensions are different inputs. Keep them separate so a geometric estimate does not accidentally become a rule for how a post or foundation should be designed.

Convert diameter before finding the radius

Consider a cylindrical pour 12 inches in diameter and 3 feet deep. Twelve inches is 1 foot, so the radius is 0.5 foot. The cross-sectional area is π × 0.5². Multiply by 3 feet to obtain approximately 2.356 cubic feet for one pour.

Using the full diameter as the radius would make the area four times too large. Using 12 inches directly alongside a depth measured in feet would also mix incompatible units. Label the input “diameter” explicitly and convert all dimensions to one length unit before doing the calculation.

Multiply identical components before rounding bags

Eight of these pours require 8 × 2.356, or approximately 18.850 cubic feet. A selected 10% allowance raises that to approximately 20.735 cubic feet. At an example product yield of 0.60 cubic foot per bag, the whole-bag result is 35 bags.

This yield corresponds to the 80-pound QUIKRETE Concrete Mix No.1101 example, not every 80-pound concrete product. Add the precise volumes of all identical pours before rounding to packages. Rounding each hole separately can increase the purchased total without making that difference clear.

Subtract only real displacement

If a post occupies part of the concrete volume, subtract its actual cross-sectional area multiplied by the length embedded within the pour. Suppose a post actually measures 4 inches by 4 inches and occupies the full 3-foot example depth. Its volume is (4 ÷ 12) × (4 ÷ 12) × 3 = one-third cubic foot.

The net geometric concrete volume for that one hole becomes approximately 2.356 − 0.333 = 2.023 cubic feet. This example uses actual measurements, not a nominal lumber-size assumption. A post that stops above the bottom or changes shape requires the corresponding displacement length or component calculation.

Use filled depth, not automatically excavated depth

If an approved detail includes a non-concrete layer at the bottom, that layer is not part of the concrete-filled depth. Conversely, an irregular hole can contain more space than a neat cylinder. Use measurements that represent the actual intended pour and obtain qualified guidance for design questions such as depth, width, reinforcement, and local requirements.

NIST's conversion reference supports keeping inches, feet, and metric units consistent. It does not prescribe the dimensions of a footing.

Save the diameter, filled depth, count, displacement assumptions, selected yield, and allowance together. A note saying only “35 bags for posts” loses the information needed to tell whether the estimate still applies when a hole size or post detail changes.

Sources & further reading

• QUIKRETE Concrete Mix No.1101 — Technical data sheet
• NIST Guide to SI, Appendix B.8 — Conversion factors

Source material checked November 9, 2026. This guide is an original explanation; source publishers do not endorse this website.