Choose dimensions before estimating material
This page estimates the amount of concrete for dimensions you supply. It does not size a fence foundation or deck footing. Required diameter, depth, reinforcement and concrete specification depend on the approved design, ground conditions and local requirements. Use those requirements first. A quantity result is not a structural approval.
Measure the concrete region and the post
The model uses a straight circular hole with constant diameter. Enter only the concrete fill depth, excluding gravel. For a square post use its measured side width, not a nominal lumber label; for a round post use its diameter. Enter the post length actually inside the concrete. This lets you include concrete below the post without subtracting post material that is not there. A square post must fit by its diagonal, not just by its side width. Select no post and enter two zeros for a solid cylinder.
The geometry
Hole volume in cubic inches is π × (diameter ÷ 2)² × concrete depth. Subtract square-post width² × embedded length, or π × (round-post diameter ÷ 2)² × embedded length. Divide by 1,728 for cubic feet. Multiply by the number of identical holes, then by (1 + your allowance ÷ 100). Divide cubic feet by 27 for cubic yards. The unit conversion to liters is 28.316846592 liters per cubic foot.
Worked example: six holes with square posts
For a measured 12 in round hole with 30 in of concrete, a 3.5 in square post embedded 24 in displaces 294 in³. The hole holds approximately 3,392.920 in³ before displacement. Net concrete is 3,098.920 ÷ 1,728 = 1.793356 ft³ per hole. Six identical holes need 10.760139 ft³ net; a user-selected 10% allowance gives 11.836153 ft³. At 0.60 ft³ per bag, round 19.726922 up to 20 bags for the combined batch. These dimensions demonstrate arithmetic; they are not a recommended footing design.
Bag yield and rounding
Use the yield of the mix and bag size you will actually buy. For example, the QUIKRETE Concrete Mix 1101 datasheet lists approximately 0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag and 0.375 ft³ for a 50 lb bag. These are examples from one product, not universal weight-to-volume conversions. At 0.45 ft³ per bag the same 11.836153 ft³ example needs 27 whole bags. Changing yield changes the bag count, not the concrete volume. Rounding happens once for all holes together; separate batches may leave more unused material.
Limits of the estimate
Tapered, belled, collapsed or uneven holes do not match a perfect cylinder. Different holes should be calculated separately. A single percentage cannot correct every excavation. The allowance is your explicit input, not a safety factor. Geometric fit does not establish sufficient concrete cover, footing capacity or a safe depth. This tool does not prescribe concrete mixing water, curing or installation technique; follow the product instructions and project requirements.
Before digging
In the United States, contact 811 before digging and follow the notification, waiting and marking requirements that apply at your location. This calculator does not locate underground services or determine whether digging is safe.
Sources and correction policy
N1RO maintains this quantity calculator. The mathematical model and worked examples were checked on 11 September 2026; this is not a claim of professional engineering certification. No merchant recommendations or affiliate links are used in this calculator. Report a reproducible issue with the inputs, actual result and expected result through the N1RO contact page.