Concrete Footing
Calculate concrete for strip and isolated footings. Enter values for instant results with step-by-step formulas.
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 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.
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.
How do I calculate the amount of concrete needed for a project?
Calculate volume in cubic feet (length x width x depth), then divide by 27 to convert to cubic yards. Add 5-10% for waste and spillage. One cubic yard of concrete covers 81 square feet at 4 inches thick.
What are the standard concrete mix ratios?
Common ratios by volume are 1:2:3 (cement:sand:gravel) for general purpose, 1:1.5:3 for structural work, and 1:2:4 for foundations. The water-to-cement ratio should be 0.45-0.55 for optimal strength. Lower water content produces stronger concrete.
What is the correct rebar spacing for concrete slabs?
Standard residential slabs use #3 or #4 rebar on 18-inch centers both ways, placed at mid-depth. Driveways and heavy-load areas use #4 rebar on 12-inch centers. Rebar should have 2-3 inches of concrete cover on the bottom. Wire mesh (6x6 W1.4xW1.4) is an alternative for light-duty slabs.