How To Calculate How Much Concrete I Need For Footings

Calculate exactly how much concrete you need for your footings with our precise calculator

Building strong footings is the foundation of any durable structure. Whether you’re constructing a house, garage, deck, or fence, proper footing calculations ensure structural integrity and prevent costly mistakes. This expert guide will walk you through everything you need to know about calculating concrete for footings, from basic formulas to advanced considerations.

Understanding Footing Basics

Footings are structural elements that transfer loads from the building to the soil. They must be:

• Wide enough to distribute the load safely
• Deep enough to reach stable soil below the frost line
• Made of proper materials (typically concrete with reinforcement)

Types of Footings

1. Spread Footings: Most common for residential construction, spreading the load over a wide area
2. Strip Footings: Continuous footings that support load-bearing walls
3. Pier Footings: Deep footings that transfer loads to deeper, more stable soil layers
4. Mat/raft Footings: Large slab footings that cover the entire building area

The Concrete Calculation Formula

The basic formula for calculating concrete volume is:

Volume = Length × Width × Depth

For footings, we typically measure:

• Length and width in feet (or meters)
• Depth in inches (or centimeters) – converted to feet/meters for calculation

Step-by-Step Calculation Process

1. Measure your footing dimensions: Get accurate measurements of length, width, and depth
2. Convert all measurements to the same unit: Typically feet or meters for consistency
3. Calculate volume for one footing: Multiply length × width × depth
4. Account for multiple footings: Multiply single footing volume by the total number
5. Add waste factor: Typically 10-15% extra to account for spillage and uneven surfaces
6. Convert to concrete ordering units: Usually cubic yards (or cubic meters)

Unit Conversions

Conversion	Formula	Example
Inches to feet	inches ÷ 12	6 inches = 0.5 feet
Cubic feet to cubic yards	ft³ ÷ 27	13.5 ft³ = 0.5 yd³
Centimeters to meters	cm ÷ 100	50 cm = 0.5 m
Cubic meters to cubic yards	m³ × 1.308	1 m³ = 1.308 yd³

Common Footing Sizes and Concrete Requirements

Building codes typically specify minimum footing sizes based on load requirements and soil conditions. Here are some common residential footing sizes:

Structure Type	Typical Footing Width	Typical Depth	Concrete per Foot (yd³)
Single-story house (bearing walls)	16-20 inches	8-12 inches	0.37-0.74
Two-story house	20-24 inches	10-16 inches	0.52-1.11
Garage	12-16 inches	8-12 inches	0.28-0.56
Deck posts	12 inches (diameter)	12-24 inches	0.09-0.18
Fence posts	8-12 inches (diameter)	12-18 inches	0.03-0.10

Factors Affecting Concrete Requirements

1. Soil Conditions

Different soil types have different bearing capacities:

• Rock: Highest bearing capacity (10,000+ psf), may require less concrete
• Gravel/Sand: Good bearing capacity (2,000-4,000 psf), standard footing sizes
• Clay: Lower bearing capacity (1,000-2,000 psf), may require wider footings
• Peat/Organic: Very low bearing capacity (<1,000 psf), may need deep footings or pilings

2. Climate Considerations

Frost depth is critical for footing depth calculations:

• Northern states: 48 inches or deeper
• Mid-Atlantic: 36-48 inches
• Southern states: 12-24 inches
• Always check local building codes for exact requirements

3. Load Requirements

Heavier structures require:

• Wider footings to distribute weight
• Deeper footings for stability
• Higher strength concrete (typically 3,000-4,000 psi for residential)

Practical Calculation Examples

Example 1: Single-Story House Footings

Scenario: 20′ × 16′ house with 16″ wide × 10″ deep continuous footings

1. Convert dimensions: 16″ = 1.33′, 10″ = 0.83′
2. Perimeter: 2(20 + 16) = 72 linear feet
3. Volume: 72 × 1.33 × 0.83 = 80 ft³
4. Convert to yards: 80 ÷ 27 = 2.96 yd³
5. Add 10% waste: 2.96 × 1.10 = 3.26 yd³
6. Order: 3.5 cubic yards

Example 2: Deck Post Footings

Scenario: 6 deck posts with 12″ diameter × 18″ deep sonotube footings

1. Radius: 6″ = 0.5′
2. Depth: 18″ = 1.5′
3. Volume per footing: π × (0.5)² × 1.5 = 1.18 ft³
4. Total volume: 1.18 × 6 = 7.08 ft³
5. Convert to yards: 7.08 ÷ 27 = 0.26 yd³
6. Add 15% waste: 0.26 × 1.15 = 0.30 yd³
7. Order: 0.5 cubic yards (minimum order at most suppliers)

Ordering Concrete: Professional Tips

When ordering concrete for your footings:

• Always round up: It’s better to have a little extra than come up short
• Check minimum orders: Many suppliers have ½ yard minimums
• Consider delivery fees: Short-load fees may apply for small orders
• Schedule carefully: Concrete has limited working time (about 90 minutes)
• Prepare your site: Ensure forms are ready and reinforcement is in place
• Have help ready: Pouring concrete is labor-intensive and time-sensitive

Concrete Strength Recommendations

For residential footings, the American Concrete Institute (ACI) recommends:

• 3,000 psi for most residential applications
• 3,500-4,000 psi for heavier loads or poor soil conditions
• Air-entrained concrete for freeze-thaw resistance in cold climates

Common Mistakes to Avoid

1. Underestimating depth: Not accounting for frost line or proper soil bearing
2. Incorrect measurements: Always double-check your dimensions
3. Forgetting waste factor: Always add 10-15% extra for spillage and over-excavation
4. Ignoring soil conditions: Unstable soil may require special footing designs
5. Poor formwork: Weak or leaky forms can lead to concrete loss and weak footings
6. Improper curing: Footings need proper curing (7+ days) to reach full strength
7. Skipping reinforcement: Most footings require rebar for structural integrity

Alternative Solutions for Special Cases

1. Poor Soil Conditions

When dealing with weak or expansive soils:

• Deep footings: Extend to more stable soil layers
• Piers/caissons: Drill deep shafts to bedrock or stable soil
• Soil improvement: Compaction, chemical stabilization, or replacement
• Mat foundations: Spread loads over a large area

2. High Water Tables

For areas with groundwater issues:

• Waterproofing: Apply membranes to footings
• Drainage systems: Install French drains or sump pumps
• Elevated footings: Raise above water table when possible
• Special concrete mixes: Use water-resistant additives

3. Seismic Zones

In earthquake-prone areas (refer to FEMA guidelines):

• Wider footings: Increase bearing area for stability
• Reinforcement: Use more rebar and proper tying techniques
• Grade beams: Connect footings for better load distribution
• Special details: Follow seismic design codes for connections

DIY vs. Professional Installation

While some footing work can be DIY, consider these factors:

Factor	DIY	Professional
Cost	Lower (labor savings)	Higher (labor costs)
Precision	Risk of errors	Accurate measurements
Equipment	May need to rent	Have proper tools
Code Compliance	Risk of violations	Know local requirements
Time	Longer learning curve	Faster completion
Warranty	None	Often included

For critical structures or complex soil conditions, professional engineering is strongly recommended. Many building departments require sealed drawings for footing designs.

Maintenance and Inspection

After installation:

• Initial inspection: Check for proper dimensions and reinforcement before pouring
• During pour: Verify concrete quality and proper consolidation
• Curing: Maintain moisture for at least 7 days (use curing compound or wet burlap)
• Final inspection: Check for cracks or defects before building on footings
• Long-term: Monitor for settling or cracking (especially first year)

Properly designed and installed footings can last 50-100+ years with minimal maintenance.

Environmental Considerations

Concrete production has environmental impacts. Consider:

• Material efficiency: Accurate calculations reduce waste
• Recycled content: Use fly ash or slag in concrete mixes
• Local suppliers: Reduce transportation emissions
• Permeable designs: Where appropriate to reduce runoff
• Proper disposal: Recycle excess concrete when possible

The EPA provides guidelines for sustainable concrete practices.

Final Checklist Before Pouring

Before ordering your concrete:

1. ✅ Verify all measurements with at least two people
2. ✅ Check local building codes for minimum requirements
3. ✅ Confirm soil bearing capacity (may require geotechnical report)
4. ✅ Calculate total volume including waste factor
5. ✅ Determine concrete strength requirements
6. ✅ Prepare forms and reinforcement
7. ✅ Schedule delivery for optimal weather conditions
8. ✅ Arrange for proper equipment (wheelbarrows, pumps if needed)
9. ✅ Have safety gear ready (gloves, boots, eye protection)
10. ✅ Plan for curing method (plastic, curing compound, etc.)

By following this comprehensive guide and using our calculator, you can ensure your footings are properly sized and your concrete order is accurate. When in doubt, consult with a structural engineer or experienced concrete contractor to verify your calculations.