Concrete Pier Calculator
Size concrete for round pier footings used under decks, porches, and posts.
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Short answer
Four round pier footings 12 inches in diameter and 36 inches deep need about 0.44 cubic yards of concrete total, or roughly 20 bags of 80 lb premix. Larger diameters or deeper piers for heavier structures need proportionally more, and depth should always meet or exceed your local frost line.
Typical pier sizes by use
| Application | Diameter | Typical depth |
|---|---|---|
| Light deck post (residential) | 10–12 in | 30–36 in |
| Porch or heavier structure | 16–18 in | 42–48 in |
| Carport or small outbuilding column | 18–24 in | 36–48 in, per local frost depth |
How to size and pour pier footings
- Check your local building code for the required frost depth and minimum footing diameter in your area.
- Auger or dig round holes to the required diameter and depth, keeping the sides as clean and vertical as possible.
- Add a few inches of gravel at the bottom of each hole for drainage if required by code.
- Enter your pier size and quantity above to get the total concrete volume and bag count.
- Set post anchors or brackets in the wet concrete, checking height and alignment across all piers before it sets.
- Let piers cure at least a few days before loading them with posts and framing.
Worked example
- Pier size
- 12 in diameter × 36 in deep
- Volume per pier
- \u03c0/4 × 12² × 36 = 4,072 in³ = 2.36 ft³
- Total for 4 piers with 10% waste
- 2.36 × 4 × 1.10 = 10.38 ft³ = 0.38 yd³
Common mistakes
- Digging piers shallower than the local frost line, which can cause heaving and movement over winters.
- Not checking that every pier is the same height before the concrete sets, leading to an uneven deck or porch frame.
- Undersizing pier diameter for the load, which can lead to settling under heavier structures.
- Skipping a gravel base in areas with poor drainage, trapping water against the bottom of the pier.
Assumptions
- Standard premix concrete at roughly 150 lb per cubic foot.
- Bag yields follow manufacturer averages: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³.
- Each pier is treated as a simple cylinder; actual auger holes may vary slightly in diameter.
- Does not include post anchors, gravel base material, or forms above grade.
Frequently asked questions
How deep should pier footings be?
Depth depends on your local frost line and the load the pier supports — often 30 to 48 inches for residential deck and porch piers. Always check your local building code, since frost depth varies significantly by region.
What diameter pier do I need?
10 to 12 inches is common for light deck posts, while porches or heavier structures often use 16 to 18 inches or more. Local code may set a minimum diameter regardless of load.
Do pier footings need rebar?
Many small residential deck piers are poured without rebar, but adding a vertical bar or post anchor cage improves uplift resistance and is required by some codes, especially for taller structures.