Concrete Footing Calculator
Calculate concrete for footings, piers, and post holes — in cubic yards and bags.
Your Footing Estimate
- Choose units — Imperial (feet, inches) or Metric (meters, mm).
- Enter dimensions — length and width of the footing or diameter for round piers. Use presets for common depths.
- Select depth — 12 inches is standard for most residential footings. Check your local frost line depth.
- Set waste factor — 10% is typical. Use 15% for irregular excavations or rocky soil.
- Review estimate — for multiple footings, multiply the result by the number of piers or footing sections.
Footing Concrete Formulas
Rectangular and continuous footings use simple volume math. Round piers use the cylinder formula:
Round pier = π × (Diameter/2)² × Depth
Cubic Yards = Volume (ft³) ÷ 27
Bags (80 lb) = Volume (ft³) ÷ 0.6
With Waste = Volume × (1 + Waste %)
For Sonotube or cardboard form tubes, measure the inside diameter. The calculator uses the same cylinder formula.
How Deep Should Footings Be?
Footing depth is primarily determined by your local frost line — the depth at which soil freezes in winter. Footings poured above the frost line can heave and crack as the ground freezes and thaws. Most building codes require the bottom of the footing to sit at least 12 inches below the frost line.
In the southern US, frost lines are typically 6 to 12 inches deep. In the northern US and Canada, frost lines can reach 48 to 72 inches. Always check with your local building department before digging — the inspector will verify depth before allowing a pour.
Footing Sizes by Structure
Standard residential footing dimensions depend on what they support. Wider footings spread the load over more soil:
| Structure | Footing Width | Footing Depth | Notes |
|---|---|---|---|
| Deck post (4×4) | 12" diameter | 36–48" | Sonotube or pre-cast pier |
| Fence post | 8–10" diameter | 24–36" | Below frost line |
| Shed / small outbuilding | 12" × 12" | 12" | On compacted gravel |
| Residential wall | 16–20" wide | 8–12" | Width = 2× wall thickness |
| Garage / addition | 20–24" wide | 12" | Below frost line; rebar required |
| Retaining wall | 24–36" wide | 12–18" | Engineer-specified for height > 4 ft |
These are general guidelines. Your engineer or building code may require different dimensions based on soil bearing capacity and structural loads.
Concrete per Round Pier (Common Sonotube Sizes)
| Tube Diameter | Depth 36" | Depth 48" | Depth 60" |
|---|---|---|---|
| 8" | 0.058 yd³ (3 bags) | 0.078 yd³ (4 bags) | 0.097 yd³ (5 bags) |
| 10" | 0.091 yd³ (5 bags) | 0.121 yd³ (7 bags) | 0.152 yd³ (8 bags) |
| 12" | 0.131 yd³ (7 bags) | 0.175 yd³ (9 bags) | 0.218 yd³ (12 bags) |
| 14" | 0.178 yd³ (10 bags) | 0.238 yd³ (13 bags) | 0.297 yd³ (16 bags) |
| 16" | 0.233 yd³ (13 bags) | 0.310 yd³ (17 bags) | 0.388 yd³ (21 bags) |
Bag count based on 80 lb bags (0.6 ft³ each). Includes no waste — add 10% to your order.
Related Calculators
Footings support slabs, posts, and walls. Browse the construction calculator collection for all concrete tools.
Frequently Asked Questions
How deep do footings need to be below the frost line?
Most building codes require the bottom of the footing to sit at least 12 inches below the local frost line. In cold climates (northern US, Canada), this can mean digging 48 to 72 inches deep. Check with your local building department for the exact requirement in your area.
How wide should a footing be?
A general rule is that footing width should be at least twice the wall thickness it supports. For a standard 8-inch block wall, use a 16-inch wide footing. Local codes specify minimums based on soil type and load — always verify before pouring.
Do I need rebar in footings?
For most residential footings, rebar significantly improves tensile strength and is strongly recommended. Strip footings typically use two runs of #4 rebar. In seismic zones and frost-heavy climates, rebar in footings is usually mandatory per local code.
Do I need gravel under a concrete footing?
Many projects use a compacted base or a thin gravel layer to improve drainage and create a stable working surface, but footing details depend on soil conditions and local requirements. Use this calculator for volume planning, then confirm base requirements for your site.
How deep should a footing be?
Footing depth is driven by two requirements: frost depth and soil bearing capacity. In most of the northern US, residential footings must reach 36–48 inches below grade to get below the frost line. In the south, 12–18 inches is common. As a general starting rule, footings also need to be at least as wide as the wall they support and at least 8–10 inches thick. Always verify the required frost depth with your local building department.