Use this free concrete footing calculator to estimate cubic yards, cubic feet, and bag counts for isolated footings, deck posts, pergola bases, column supports, and similar foundation pads. Enter the length, width, depth, number of footings, and waste allowance to get a fast concrete estimate for your project.
Footing Calculator
Estimates only. Bag yields vary by brand and mix. Always confirm with your supplier.
Quick Answer
To estimate concrete for footings, calculate the volume of one footing by multiplying length by width by depth, all in feet, to get cubic feet. Multiply that result by the number of footings, then divide by 27 to convert to cubic yards. Add a five to ten percent waste factor for spillage, uneven excavation, and minor depth variations. This calculator is best for isolated square or rectangular footings used under posts, columns, and similar supports.
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How to Use the Footing Calculator
- Measure footing dimensions: Measure the length, width, and depth of one footing in feet. For square footings, the length and width are the same. For rectangular footings, measure both carefully.
- Confirm footing depth: Depth usually depends on frost line, soil conditions, and local code. In warm climates, footings may be shallower, while freeze zones often require much deeper excavation.
- Enter the dimensions: Input the footing length, width, and depth into the calculator. The tool handles the math and gives you the concrete volume for one footing and for the full set.
- Add the number of footings: Enter how many footings you plan to pour. This is critical because the total concrete volume scales directly with footing count.
- Choose a waste factor: A five to ten percent waste allowance is common. Increase it if your excavations are irregular, your site is rough, or you expect some overdig.
- Review the results: The calculator shows cubic feet, cubic yards, and estimated bag counts so you can compare bagged concrete with ready-mix delivery.
- Double-check before ordering: Verify your dimensions on-site before buying material. It is much easier to have a little extra than to run short during the pour.
Concrete Footing Formula
The basic footing formula is simple once all measurements are converted to feet. Start by calculating the volume of one footing as a rectangular solid:
Volume per Footing (ft³) = Length (ft) × Width (ft) × Depth (ft)
Then multiply that value by the total number of footings:
Total Volume (ft³) = Volume per Footing × Number of Footings
To convert cubic feet into cubic yards for ready-mix ordering, divide by 27:
Total Volume (yd³) = Total Volume (ft³) ÷ 27
Finally, apply your waste factor. For example, a ten percent waste allowance means multiplying the final cubic yard total by 1.10. This helps cover over-excavation, spillage, and minor jobsite variation.
Worked Examples
Square Deck Post Footings
🔢 8 footings
Dimensions per footing: Length = 1 ft, Width = 1 ft, Depth = 2 ft
Number of footings: 8
Volume per footing: 1 × 1 × 2 = 2 ft³
Total volume: 2 × 8 = 16 ft³
Volume in cubic yards: 16 ÷ 27 = 0.59 yd³
With 10% waste: 0.59 × 1.10 = 0.65 yd³
Rectangular Pergola Column Footings
🔢 4 footings
Dimensions per footing: Length = 2 ft, Width = 2 ft, Depth = 3 ft
Number of footings: 4
Volume per footing: 2 × 2 × 3 = 12 ft³
Total volume: 12 × 4 = 48 ft³
Volume in cubic yards: 48 ÷ 27 = 1.78 yd³
With 10% waste: 1.78 × 1.10 = 1.96 yd³
Common Footing Types
Most residential footing estimates fall into a few familiar categories. Understanding which type you are building helps you measure correctly and order the right amount of concrete.
Square Footings
Square footings are common for deck posts, pergola columns, and other isolated supports. They spread the load evenly in all directions and are usually the simplest shape to form, measure, and pour.
Rectangular Footings
Rectangular footings are useful when loads are not symmetrical or when space constraints require a longer shape in one direction. The volume calculation is the same, but the length and width are different.
Isolated Footings
Isolated footings support individual posts or columns rather than a continuous wall. This calculator is primarily designed for that use case, which makes it a strong fit for decks, sheds, gazebos, and similar projects.
Continuous or Wall Footings
Continuous footings run below a wall rather than below separate posts. You can still estimate them by treating the footing as a long rectangular strip with length, width, and depth, but this page is mainly intended for isolated footing layouts and repeated footing counts.
Bags vs Ready-Mix: Which Should You Use?
Bagged Concrete
Best for:
- Small footing projects under 1 to 2 cubic yards
- Hard-to-access backyards or remote pour locations
- DIY pours done at your own pace
- One or a few isolated footings
- Projects where buying bags is easier than scheduling a truck
Each 80 lb bag yields about 0.6 cubic feet, so one cubic yard usually requires around 45 bags.
Ready-Mix Delivery
Best for:
- Larger footing projects over 2 cubic yards
- Multiple footings poured in one session
- Jobs where consistency matters
- Time-sensitive work that would be exhausting with bags
- Professional or larger residential footing layouts
Ready-mix reduces labor and helps maintain consistent quality, though some suppliers charge short-load fees on smaller orders.
If you need to compare bag totals more closely, use our Bag Calculator or read our guide on how many bags of concrete you need.
Common Mistakes & Tips
Forgetting to multiply by number of footings
Many people calculate one footing correctly and then forget to multiply by the total footing count.
Measuring in mixed units
Always convert inches to feet before calculating. A 24-inch footing depth is 2 feet, not 24 feet.
Ignoring waste factor
Overdig, loose soil, and minor spillage increase the final amount of concrete needed. Always include a reasonable waste allowance.
Not checking frost depth
Footing depth is often controlled by local code. In cold climates, shallow footings can heave and fail.
Undersizing the footing
Saving a small amount of concrete is rarely worth the structural risk. Make sure the footing size matches the load and soil conditions.
Ordering too little concrete
Running short during the pour creates delays and can compromise the job. Slightly extra concrete is usually the safer outcome.
Skipping reinforcement
Many footing layouts require rebar. Check your local code and structural requirements before pouring.
Pouring on unstable soil
Footings should bear on compacted, undisturbed soil or properly prepared base material, not loose or organic fill.
Frequently Asked Questions
How deep should concrete footings be?
Footing depth depends on local frost line and soil conditions. In frost-prone areas, footings must extend below the frost line to prevent heaving, typically 24 to 48 inches or deeper. In warm climates with no frost risk, footings may be as shallow as 12 inches. Always check your local building code for minimum depth requirements before excavating.
What size footing do I need for a 4×4 post?
A common rule of thumb for light structures like decks is a 12-inch by 12-inch square footing for a 4×4 post. For heavier loads or poor soil, increase to 16 inches square or larger. Depth should meet local frost line requirements. Consult your local building inspector or a structural engineer for specific load-bearing requirements.
Can I pour footings and posts at the same time?
Yes, you can set posts directly in wet concrete as you pour footings. This method is common for fence posts and light structures. However, for decks and structural applications, most codes require pouring footings first, letting them cure, then attaching post bases with embedded anchor bolts. Check local requirements and best practices for your specific project.
How many bags of concrete do I need for one footing?
It depends on footing size. A 12-inch by 12-inch by 24-inch deep footing (2 cubic feet) requires about four 80-pound bags. A larger 24-inch by 24-inch by 36-inch deep footing (12 cubic feet) requires about 20 bags. Use this calculator to get precise estimates based on your exact dimensions.
Do I need rebar in concrete footings?
Many building codes require rebar in footings, especially for larger sizes or structural applications. Rebar adds tensile strength and prevents cracking. A common configuration is two to four rebar pieces running horizontally through the footing center. Check local code requirements and consult an inspector if you’re unsure.
How long do concrete footings take to cure?
Concrete footings reach sufficient strength for light loads in 24 to 48 hours under normal conditions. However, full curing takes 28 days. For structural applications like decks or buildings, wait at least seven days before applying significant loads. Keep footings moist during the first few days to ensure proper curing.
Can I use this calculator for round footings?
This calculator is designed for square or rectangular footings. For round or cylindrical footings (common for Sonotubes), use the cylinder volume formula: volume equals π times radius squared times height. For example, a 12-inch diameter tube that’s 36 inches deep has a volume of approximately 2.36 cubic feet.
What PSI concrete should I use for footings?
Standard 3000 PSI concrete is adequate for most residential footings. For heavy structural loads, freeze-thaw exposure, or poor soil conditions, consider 4000 PSI. Consult your local building code and ready-mix supplier for recommendations based on your specific application and climate.
Should I use gravel under footings?
Yes, a four to six inch layer of compacted gravel provides drainage, prevents frost heave, and creates a stable base. Some codes require gravel under footings. The gravel base also helps level the excavation and provides a clean surface for pouring. Compact the gravel thoroughly before placing concrete.
How do I calculate concrete for multiple different-sized footings?
Calculate each footing size separately, then add the totals together. For example, if you have four large footings and eight small footings, calculate cubic feet for one large and multiply by four, calculate one small and multiply by eight, then add both results. This calculator can handle one footing size at a time, so run the calculation twice and sum the results.
Keep Planning Your Concrete Project
Explore more tools and guides to keep your footing project on track
Disclaimer: This calculator provides planning estimates only. Always verify local code requirements, soil conditions, and supplier guidance before ordering concrete. Actual material needs can vary based on excavation accuracy, jobsite conditions, waste, and product yield. Consult a licensed contractor or structural engineer for critical structural work.