Learn how to calculate concrete for foundation footings. Find the right concrete volume for deck footings, fence posts, and building foundations with our free calculator.
What You'll Learn
Standard Footing Sizes and Depths
The size of your footings depends on what you're building and local building codes. Here are common dimensions:
Deck Footings
- Diameter: 8–12 inches
- Depth: Below frost line (typically 36–48 inches)
Fence Post Footings
- Diameter: 3× the post width (e.g., 12" for a 4×4 post)
- Depth: 1/3 of above-ground post height + 6 inches
Building Foundation Footings
- Width: 2× the wall thickness (minimum 16 inches)
- Depth: 12 inches minimum for single-story structures
How to Calculate Concrete Volume for Footings
For Round Footings (Deck Posts, Fence Posts)
Use this formula:
Where:
- π = 3.14159
- r = radius in feet (diameter ÷ 2)
- depth = depth in feet
Example:
- Diameter: 10 inches (0.833 ft)
- Radius: 5 inches (0.417 ft)
- Depth: 36 inches (3 ft)
Volume = 3.14159 × (0.417)² × 3 = 1.64 cubic feet
For Square/Rectangular Footings
Use this formula:
Example:
- Length: 24 inches (2 ft)
- Width: 24 inches (2 ft)
- Depth: 12 inches (1 ft)
Volume = 2 × 2 × 1 = 4 cubic feet
To convert to cubic yards: Divide cubic feet by 27
How Many Bags of Concrete Do I Need?
For smaller footing projects, pre-mixed concrete bags are convenient. Here's the coverage:
- 40 lb bag: 0.30 cubic feet
- 60 lb bag: 0.45 cubic feet
- 80 lb bag: 0.60 cubic feet
Calculation Example:
For a 10" diameter × 36" deep round footing:
- Volume needed: 1.64 cubic feet
- Using 80 lb bags: 1.64 ÷ 0.60 = 3 bags
- Using 60 lb bags: 1.64 ÷ 0.45 = 4 bags
Pro Tip:
Always buy 10-20% extra concrete to account for spillage, uneven holes, or calculation errors.
Use Our Free Concrete Calculator
Don't want to calculate manually? Try our free Concrete Calculator:
- ✅ Select "Column/Cylinder" for round footings
- ✅ Enter your dimensions
- ✅ Instantly see cubic yards, cubic feet, and bags required
- ✅ Avoid costly mistakes
FAQs About Footing Concrete
Q: How deep should footings be?
→ Footings must extend below the frost line in your area (typically 36-48 inches in cold climates) to prevent frost heave.
Q: Can I pour footings in sections?
→ It's best to pour each footing in one continuous pour to ensure maximum strength and avoid cold joints.
Q: How long should concrete cure before building?
→ Wait at least 24-48 hours before placing weight on footings. Full strength develops in 28 days.
Q: Should I use rebar in footings?
→ For structural footings, rebar adds strength. Check local building codes for requirements.
Calculating the right amount of concrete for your footings ensures a strong foundation and keeps your project on budget. Whether you're building a deck, installing fence posts, or laying a foundation, accurate concrete estimates are essential.
Need an exact estimate? Try our free Concrete Calculator to plan your footing project with confidence!
Key Points Covered
- Standard footing sizes for different projects
- Calculate concrete for round and square footings
- Convert between cubic yards and bags
- Tips for ordering the right amount
- Common footing depth requirements
- When to use rebar reinforcement