From measured space to purchasable material
Fence and deck posts displace concrete inside their holes, and a drainage layer takes up part of the excavation depth. This calculator accounts for both rather than filling the entire cylinder on paper. Choose a square or round post and enter its actual size, not a nominal lumber name. Hole diameter and concrete depth must follow the applicable project specification. The result provides net concrete, bag count and material cost for a group of identical holes, making it useful for comparing bagged concrete purchases.
Measure, select yield and round the order
- Count identical holes and measure their diameter and depth.
- Enter actual post dimensions and any bottom gravel layer.
- Use the bag yield on the product label to round the total purchase up.
Net requirement and purchase calculation
A worked example
Ten 12 in holes, 30 in deep, with 6 in gravel and 3.5 in square posts use about 14.01 ft³ net, or 26 bags at 0.6 ft³ with 10% allowance.
Post Hole Concrete: what changes the result?
Compare dimensions and product yields without treating quantities as a design check. The comparisons below are calculated from this page’s default example. Each row changes only the named input; all other inputs stay at their displayed example values. These are mathematical scenarios, not measurements from a completed project.
| Changed assumption | Default input | Comparison input | Result: default → comparison |
|---|---|---|---|
| Number of holes | 10 | 11 | Concrete after post displacement: 14.0066 → 15.4072 cu ft Concrete with allowance: 15.4072 → 16.948 cu ft |
| Hole diameter | 12 in | 14.4 in | Concrete after post displacement: 14.0066 → 20.9181 cu ft Concrete with allowance: 15.4072 → 23.0099 cu ft |
| Total hole depth | 30 in | 36 in | Concrete after post displacement: 14.0066 → 17.5082 cu ft Concrete with allowance: 15.4072 → 19.259 cu ft |
| Bottom gravel depth | 6 in | 7.2 in | Concrete after post displacement: 14.0066 → 13.3062 cu ft Concrete with allowance: 15.4072 → 14.6369 cu ft |
Displayed values are rounded. Whole-pack results change in steps; continuous quantities may change before a new pack is needed. To compare your actual project, enter your dimensions above, save a scenario, then change one assumption.
Coverage, allowance and purchasing limits
- A tapered, belled or irregular excavation does not match a straight cylinder.
- Nominal 4 × 4 lumber is not necessarily 4 inches square; measure it.
- The calculator does not specify hole depth, frost protection or post anchorage.
Frequently asked questions
Does the post reduce the concrete needed?
Yes. The actual post cross-sectional area is subtracted over the concrete-filled depth.
Can I enter different hole depths?
Calculate each group of identical holes separately and add the resulting order quantities.
Is bottom gravel counted as concrete?
No. Its depth is subtracted before calculating concrete.
Read the project guides
- Concrete Bags vs Ready-Mix: Build a Quantity-Based Comparison
Work through slab volume, package yield, whole-bag rounding, ready-mix delivery charges and a complete materials comparison for a small concrete project.
- Fence Material Takeoff: Posts, Pickets, Gates and Equal Spacing
Break a fence into runs, account for gate openings, lay out posts, count pickets, and estimate finish for the surfaces actually coated.
Calculation method: JICWH methodology. Examples are planning assumptions, not product recommendations.