Foundation Footing Formulas
\[\text{Width (ft)} = \frac{\text{Width (in)}}{12}, \quad \text{Depth (ft)} = \frac{\text{Depth (in)}}{12}\] \[\text{Net Volume (cu ft)} = \text{Length (ft)} \times \text{Width (ft)} \times \text{Depth (ft)}\] \[\text{Concrete Volume (cu yd)} = \frac{\text{Net Volume (cu ft)}}{27} \times 1.10 \quad \text{(with 10\% waste)}\] \[\text{Total Rebar (ft)} = \text{Length (ft)} \times \text{Rebar Runs} \times 1.15 \quad \text{(with 15\% lap factor)}\] \[\text{Total Footing Cost} = (\text{Concrete Vol} \times \text{Price/yd}) + (\text{Rebar Feet} \times \text{Rebar Price/ft})\]Foundation Footing Material Reference Table ($140/yd Concrete, $0.85/ft Rebar)
| Trench Length (Ft) | Footing Size (W × D) | Concrete Volume (+10%) | Rebar Runs | Total Rebar Feet (+15%) | Concrete Cost | Rebar Cost | Total Material Cost |
|---|---|---|---|---|---|---|---|
| 100 ft | 16 in × 10 in | 4.53 cu yds | 2 runs | 230 linear ft | $634.20 | $195.50 | $829.70 |
| 150 ft | 20 in × 12 in | 10.19 cu yds | 2 runs | 345 linear ft | $1,426.60 | $293.25 | $1,719.85 |
| 200 ft | 24 in × 12 in | 16.30 cu yds | 3 runs | 690 linear ft | $2,282.00 | $586.50 | $2,868.50 |
| 250 ft | 24 in × 16 in | 27.16 cu yds | 4 runs | 1,150 linear ft | $3,802.40 | $977.50 | $4,779.90 |
Step-by-Step Guide: How to Calculate Foundation Footings
- Calculate Total Perimeter: Measure the complete linear distance along foundation trench lines, including interior load-bearing footings.
- Specify Footing Dimensions: Input structural footing width and depth specified by local building code or structural blueprints.
- Select Rebar Configuration: Determine how many continuous strands of rebar (typically 2 to 4 bars) will be placed inside the footing.
- Order Ready-Mix Concrete: Use the calculated cubic yardage (which includes 10% extra for trench unevenness) when placing your ready-mix order.
Frequently Asked Questions
How much concrete do I need for a 150 ft footing?
A 150 ft long footing that is 20 inches wide and 12 inches deep requires approximately 10.19 cubic yards of concrete (including a 10% waste factor).
How is continuous footing volume calculated?
Volume (cu yd) = [Length (ft) × (Width (in) / 12) × (Depth (in) / 12) / 27] × 1.10 (waste factor).
How much rebar overlap should be added for footings?
Building codes generally require a minimum 30-diameter or 12 to 15 inch rebar lap splice. Adding 15% extra to total linear length accounts for laps, corner bends, and cutting waste.
What size rebar is standard for residential footings?
Standard residential footings use #4 rebar (1/2 inch diameter) or #5 rebar (5/8 inch diameter) laid continuously on rebar chairs.
How deep should house foundation footings be poured?
Footings must extend below your local frost depth line (typically 12 inches in warm climates up to 48+ inches in freezing northern regions).
How wide should a concrete footing be relative to the wall?
As a rule of thumb, foundation footings are twice as wide as the foundation wall they support (e.g., a 20-inch wide footing for a 10-inch stem wall).
Is my personal calculation data stored or tracked?
No. All calculation operations run privately inside your local browser.