Concrete Footing
Calculate concrete needed for strip footings and isolated pad footings. Get volume estimates, material breakdown, and cost for foundation footings.
Formula
Volume = Length x Width x Depth
Calculate footing volume in cubic feet, then divide by 27 for cubic yards. Add 10% waste factor for safety.
Worked Examples
Example 1: Strip Footing Calculation
Problem:40-foot perimeter wall needs continuous footing, 24" wide × 12" deep.
Solution:Length: 40 ft Width: 24" = 2 ft Depth: 12" = 1 ft Volume = 40 × 2 × 1 = 80 cubic feet Cubic yards = 80 ÷ 27 = 2.96 yards With 10% waste: 2.96 × 1.10 = 3.26 yards
Result:Order 3.5 cubic yards
Example 2: Isolated Footings
Problem:6 columns need 3×3 ft footings, 18" deep each.
Solution:Per footing: 3 × 3 × 1.5 = 13.5 cubic feet Total for 6: 13.5 × 6 = 81 cubic feet 81 ÷ 27 = 3.0 cubic yards With 10% waste: 3.3 yards
Result:Order 3.5 cubic yards for 6 footings
Example 3: Mixed Footing Project
Problem:Calculate concrete for 30 ft strip (2 ft wide) + 4 isolated pads (2.5×2.5 ft), all 12" deep.
Solution:Strip: 30 × 2 × 1 = 60 cu ft Pads: 2.5 × 2.5 × 1 = 6.25 cu ft each Total pads: 6.25 × 4 = 25 cu ft Total = 60 + 25 = 85 cu ft 85 ÷ 27 = 3.15 yards With waste: 3.46 yards
Result:Order 3.5-4 cubic yards
Frequently Asked Questions
What is a strip footing?
A strip footing (continuous footing) runs continuously along a wall or foundation line. It distributes the load of a wall over a larger area. Typical dimensions: 2-3 feet wide, 8-18 inches deep. Used for load-bearing walls and foundation perimeters.
What is an isolated footing?
An isolated footing (pad footing) supports a single column or post. It's typically square or rectangular. Size depends on column load and soil bearing capacity. Common sizes range from 2×2 to 6×6 feet with 12-24 inch depth.
How deep should footings be?
Minimum depth below frost line to prevent heaving (varies by region: 12" in warm climates to 48"+ in cold). Typical footing depths: 12-18 inches for single-story, 18-24 inches for two-story. Always check local building codes for frost depth requirements.
How wide should my footing be?
Width depends on wall thickness and soil bearing capacity. Rule of thumb: footing width = 2× wall thickness. For 8" wall, use 16" (1.33 ft) minimum footing. Engineer may specify wider for poor soil or heavy loads. Typical range: 16-36 inches wide.
Do I need rebar in footings?
Most building codes require rebar in footings. Typical: 2-4 continuous #4 or #5 rebar bars running lengthwise, placed in bottom third of footing. Vertical dowels connect footing to wall/column. Check local code for specific requirements.
What concrete strength for footings?
Minimum 2,500 PSI for residential footings. 3,000 PSI is common and provides good margin. Commercial or heavy-load footings may require 3,500-4,000 PSI. Check structural drawings or consult engineer for specific requirements.
How much extra concrete should I order?
Add 10-15% for waste, spillage, and slight variations in excavation depth or form dimensions. Better to have a little extra than run short mid-pour. Leftover concrete can fill post holes or small projects.
Can I pour footings and foundation walls same day?
No. Footings must cure (typically 24-48 hours minimum, 7 days better) before wall construction. This allows footing to reach sufficient strength to support wall loads. Key connection is via rebar dowels embedded in footing.
What's the difference between footing and foundation?
Footing: Bottom-most element, spreads load into soil. Foundation: Wall or structure sitting on footing. Together they transfer building loads to earth. Footing is always wider than the foundation element it supports.
Do I need an engineer for footings?
For most residential projects, standard footings follow building code prescriptive tables. Complex or heavy structures, poor soil conditions, or commercial buildings typically require engineered footings with calculations specific to loads and soil bearing capacity.
Background & Theory
Footings are the foundation elements that distribute building loads into the soil. Proper footing design prevents settlement and structural failure.
Footing Types:
Strip Footing (Continuous):
- Runs along walls
- Distributes load linearly
- Width typically 2× wall thickness
- Common: 16-36 inches wide
Isolated Footing (Pad):
- Supports single column/post
- Square or rectangular
- Size based on load and soil
- Common: 2×2 to 6×6 feet
Volume Formulas:
Strip: Volume = Length × Width × Depth
Isolated: Volume = Width × Width × Depth
Typical Dimensions:
| Application | Width | Depth |
|---|---|---|
| Single-story wall | 16-20" | 12-18" |
| Two-story wall | 20-24" | 18-24" |
| Column footing | 2-4 ft | 12-24" |
Material Breakdown:
- Cement: 15% by weight
- Sand: 30% by volume
- Gravel: 45% by volume
- Water: 10% by volume