Estimate the concrete volume and number of bags needed for fence posts, deck posts, mailbox posts, signs and similar post-setting projects. Enter the hole size, post size, number of holes, waste allowance and the concrete yield printed on your bag.
Choose your measurement system, enter the hole and post dimensions, and click Calculate. The calculator subtracts the volume occupied by the post from the cylindrical hole volume, then applies your waste allowance.
A post-hole concrete estimate starts with the volume of the hole. For a typical round post hole, the hole can be modeled as a cylinder. You need the hole diameter and depth, then account for the space occupied by the post if the post extends through the concrete-filled portion of the hole. This approach is useful for fence posts, deck posts, mailbox posts, sign posts and similar small post-setting jobs.
For a cylindrical hole, the basic volume formula is V = π × r² × h, where r is the hole radius and h is the hole depth. When using inches, convert the result from cubic inches to cubic feet by dividing by 1,728. When using metric measurements, keep the calculation in millimeters or meters consistently and convert the final volume to liters or cubic meters as needed.
Hole volume (ft³) = π × (diameter ÷ 2)² × depth ÷ 1,728Hole volume (m³) = π × (diameter ÷ 2)² × depth
Concrete volume = hole volume − post volume
If the post is square or rectangular, its occupied volume can be estimated as width × thickness × embedded depth. If the post is round, its occupied volume is another cylinder: π × post radius² × embedded depth. This calculator assumes the post occupies the full calculated hole depth. If the actual embedded length is shorter, the concrete volume will be slightly different and you should use the actual embedded length for the post-volume calculation.
After calculating the theoretical concrete volume, add a waste or jobsite allowance. A small amount of extra material can be useful because drilled or dug holes are rarely perfect cylinders. Soil conditions, irregular sides, loose material, spillage and over-excavation can all increase the actual amount required. Sakrete recommends adding 10% to estimates on its concrete calculator guidance, while individual jobs may justify a different allowance. The correct allowance depends on the quality of excavation and the product being used.
Once you know the required concrete volume, divide it by the yield of the selected bag. The result is rounded up because you cannot purchase a fraction of a bag. The important point is that the bag's weight alone does not tell you its concrete volume. Two products with the same bag weight can have different yields, so the manufacturer's product data should be used whenever it is available.
For example, QUIKRETE's published data for its Concrete Mix lists approximate yields of 0.30 cubic feet for a 40-pound bag, 0.375 cubic feet for a 50-pound bag, 0.45 cubic feet for a 60-pound bag, 0.60 cubic feet for an 80-pound bag and 0.675 cubic feet for a 90-pound bag. These figures are product-specific rather than universal values for every concrete mix.
Sakrete also publishes product-specific yields. Its High-Strength Concrete Mix lists 0.30 cubic feet for a 40-pound bag, 0.45 cubic feet for a 60-pound bag and 0.60 cubic feet for an 80-pound bag. This is why the calculator includes a custom-yield option rather than assuming that every brand has exactly the same coverage.
| Example bag | Illustrative yield | Approximate liters |
|---|---|---|
| 40 lb | 0.30 ft³ | 8.5 L |
| 50 lb | 0.375 ft³ | 10.6 L |
| 60 lb | 0.45 ft³ | 12.7 L |
| 80 lb | 0.60 ft³ | 17.0 L |
| 90 lb | 0.675 ft³ | 19.1 L |
The table is based on published yield figures for specific QUIKRETE concrete mix products and is provided as an estimating reference, not as a universal yield chart. Always verify the exact product you are buying.
Suppose you have 10 holes, each 12 inches in diameter and 36 inches deep. A 4×4 post is nominally 4 inches by 4 inches for this estimating example. The cylindrical hole volume is:
π × 6² × 36 ÷ 1,728 ≈ 2.356 ft³ per holeThe 4×4 post occupies approximately:
4 × 4 × 36 ÷ 1,728 = 0.333 ft³ per postThat leaves about 2.023 ft³ of concrete per hole, before waste. For 10 holes, the theoretical concrete requirement is about 20.23 ft³. With a 10% allowance, the estimate becomes about 22.25 ft³. Using a product yielding 0.375 ft³ per 50-pound bag would require roughly 60 bags after rounding up.
This example is deliberately shown as a calculation method rather than a universal fence-post recommendation. Actual hole dimensions should be based on the post, load, soil, frost conditions, wind exposure, local requirements and project design.
For a 9-inch diameter hole and 30-inch depth, the hole volume is approximately:
π × 4.5² × 30 ÷ 1,728 ≈ 1.104 ft³A 3-inch round post has a radius of 1.5 inches. Its volume through the same 30-inch embedded depth is approximately:
π × 1.5² × 30 ÷ 1,728 ≈ 0.123 ft³So the concrete volume is about 0.981 ft³ per hole before waste. With 10% waste, that becomes about 1.079 ft³. If the selected product yields 0.375 ft³ per bag, the estimate is 3 bags after rounding up.
A calculator is only as accurate as its inputs. Measure the actual hole diameter and depth rather than relying on a rough visual estimate. Augered holes may be close to cylindrical, while hand-dug holes can be irregular. If the hole is noticeably wider in places, the theoretical cylinder may understate the actual concrete required.
Do not calculate bags using weight alone. Find the yield on the product packaging or technical data sheet. QUIKRETE and Sakrete both publish product-specific yield information, and their figures demonstrate why bag yield should be treated as a product property.
A waste allowance covers ordinary estimating uncertainty, but it should not be used to hide a poorly measured hole. A 10% allowance is a useful starting point for many small concrete estimating jobs, but difficult excavation or very irregular holes may require more. Sakrete's calculator guidance specifically recommends 10% as a safety allowance.
The calculator tells you how much concrete would fill the specified geometry. It does not tell you whether the specified hole is structurally adequate. Fence and deck posts can be affected by wind, loads, soil strength, frost, drainage and local construction requirements. For structural work, use the applicable plans, specifications and local code requirements.
Post-setting methods vary. Some projects require a particular base material, drainage treatment, concrete placement method or finished grade. Follow the installation instructions for the post, fence system and concrete product rather than treating a quantity calculator as an installation manual.
The subtraction method works best when the post occupies the entire depth used in the concrete volume calculation. If the post stops above the bottom of the hole or concrete is placed around only part of the post, use the actual embedded length for the post-volume calculation.
Calculate the cylindrical hole volume using π × radius² × depth. Subtract the volume occupied by the portion of the post inside the concrete, then add a reasonable waste allowance. Finally, divide the adjusted volume by the yield of the concrete bag you plan to use and round up to a whole bag.
There is no single answer because the number depends on hole diameter, hole depth, the actual post dimensions, the concrete product and its yield. A 4×4 post in a 10-inch-diameter hole requires less concrete than the same post in a 12-inch-diameter hole. Enter your actual dimensions in the calculator above.
Depth alone is not enough to determine the quantity. You also need the hole diameter and post size. A 36-inch-deep hole can require very different amounts of concrete depending on whether it is 8, 10, 12 or 18 inches in diameter. Use the calculator with the actual dimensions.
For a volume estimate of concrete surrounding a post, subtracting the volume occupied by the embedded portion of the post is appropriate. The result should use the actual post dimensions and the length of post occupying the concrete-filled zone.
A 10% allowance is a reasonable starting point for many estimating situations, and Sakrete explicitly recommends a 10% safety allowance on its concrete calculator guidance. However, the right allowance depends on excavation accuracy, soil conditions, waste and the specific project.
You can use the volume calculation for a cylindrical concrete footing when you already know the required footing diameter and depth. However, this calculator does not design the footing. Deck footing dimensions may depend on loads, soil, frost depth, span, tributary area and local code. Use approved plans or local requirements for the required footing size.
Different calculators may use different concrete products, bag yields, post dimensions, waste allowances or assumptions about the hole. The best comparison is to use the same geometry, the same post volume, the same waste percentage and the exact product yield.
The concrete calculator for post holes estimates volume using the standard formula for the volume of a cylinder, since most post holes are dug or augered in a round shape:
For square or rectangular holes, the calculator instead uses Volume = Length × Width × Depth. In both cases, the result is converted from cubic feet to cubic yards and to the number of pre-mixed concrete bags needed, based on the manufacturer yield you select (e.g., a standard 80 lb bag yields approximately 0.6 cubic feet of mixed concrete; a 60 lb bag yields approximately 0.45 cubic feet).
Calculations and guidance on this page are checked against the following sources:
This page and its underlying formulas are reviewed periodically against the sources above and updated when manufacturer yield data or referenced standards change. The "Last updated" date at the top of this section reflects the most recent review.