Concrete Footing Volume Calculator
Calculate the concrete volume for a rectangular footing with an inner void. Enter the outside and inside dimensions plus the thickness to get the result in cubic feet and cubic yards.
Calculate Concrete Volume for Footings
Concrete Footing Volume Formula
Where:
- \(V\) — concrete volume (ft³)
- \(\text{ow}\) — outside width of the footing (ft)
- \(\text{ol}\) — outside length of the footing (ft)
- \(\text{iw}\) — inside width of the void (ft)
- \(\text{il}\) — inside length of the void (ft)
- \(t\) — thickness of the footing (ft)
The formula subtracts the area of the inner opening from the outer rectangle, then multiplies by the thickness. If the footing is solid (no void), simply set both inside dimensions to 0.
Concrete Footing Volume – Calculation Example
Example: a rectangular footing measures 36 inches by 72 inches on the outside, with an inner void of 24 inches by 60 inches, and a thickness of 12 inches.
\[ V = (3 \times 6 - 2 \times 5) \times 1 = (18 - 10) \times 1 = 8 \text{ ft}^3 \]
\[ V_{yd^3} = \frac{8}{27} \approx 0.2963 \text{ yd}^3 \]
Result: the footing requires about 8 ft³ (0.2963 yd³) of concrete.
How to use the calculator
- Enter the Outside width of the footing and choose its unit (feet or inches).
- Enter the Outside length and select the matching unit.
- Enter the Thickness of the footing and its unit.
- Enter the Inside width of the void or opening; set it to 0 if the footing is solid.
- Enter the Inside length of the void (also 0 for a solid footing).
- Read the results: the volume is shown both in cubic feet (ft³) and cubic yards (yd³).
What is a concrete footing volume?
A footing is the widened concrete base that sits under a foundation wall, column, or pier. Its job is to spread the load from the structure above over a larger area of soil, so the ground pressure stays within safe limits. Because footings are buried and hidden once the building is complete, getting the concrete quantity right before the pour matters: too little means a cold joint or a short pour, while too much means wasted material and money.
This calculator handles rectangular footings, including those with a hollow center. A void can appear when a plumbing sleeve, utility chase, or an excavation for a future column passes through the footing. The calculator computes the area of the outer rectangle, subtracts the area of the inner opening, and multiplies the difference by the thickness. All inputs are converted to feet internally, so you can mix feet and inches without breaking the math.
The result is given in cubic feet and cubic yards because concrete in most of North America is ordered by the cubic yard — one yard equals 27 cubic feet. For small footings the cubic-foot figure is often more practical, since bagged concrete mixes are labeled by cubic feet per bag.
Assumptions, limitations and practical tips
- All dimensions must describe plain rectangles. The inner void must be fully inside the outer rectangle; otherwise the result becomes meaningless.
- The footing is assumed to have a uniform thickness. Stepped or tapered footings need to be split into sections and calculated separately.
- Order a margin of 5–10% extra concrete to cover spillage, uneven excavation, and minor over-dig. Ground that is dug slightly too deep is the most common cause of running short.
- For a solid footing with no opening, set both inside dimensions to 0 — the formula then reduces to a simple length × width × thickness calculation.
- This tool gives volume only. Whether a footing is strong enough for your soil and load conditions is a structural question; check local codes or consult an engineer.
Once you know the volume, you can plan the pour itself with the Concrete Footing/Slab Pours calculator, estimate materials with the Concrete Ingredients calculator, or check the right proportions with the Concrete Mix Ratio calculator. If you buy concrete in bags rather than by the yard, the Concrete Yardage calculator helps convert between units.
Frequently Asked Questions
How do I calculate concrete volume for a footing by hand?
Convert every dimension to feet first (divide inches by 12). Multiply outside width by outside length, subtract the inner void's width times its length, then multiply by the thickness. For example, a 4 ft × 4 ft footing with a 1 ft × 1 ft void and 1 ft thickness gives \( (16 - 1) \times 1 = 15 \) ft³. Double-check that all units match before multiplying.
How many 80 lb bags of concrete do I need for my footing?
A standard 80 lb bag of concrete mix yields about 0.6 ft³. Divide your result in cubic feet by 0.6 to estimate bags: 8 ft³ needs roughly 14 bags. An 60 lb bag yields about 0.45 ft³. Always buy a few extra bags — 5–10% extra is a common buffer for spillage and uneven excavation.
How do I convert cubic feet to cubic yards or cubic meters?
One cubic yard equals 27 cubic feet, so divide the ft³ figure by 27. For metric projects, multiply cubic feet by 0.0283168 to get cubic meters. Example: 8 ft³ is about 0.2963 yd³ or 0.2265 m³. Ready-mix suppliers usually have a minimum order of around 1 cubic yard, so small footings are often cheaper with bagged mix.
How thick should a concrete footing be?
Typical residential footings are 8–12 inches thick, but the right size depends on soil bearing capacity, the load from above, and local building codes. Many codes also require footings to project at least 2 inches beyond the wall on each side and be at least as thick as that projection. Sizing a footing is a structural decision — verify with your local code or an engineer before pouring.
Does the position of the inner void affect the concrete volume?
No. As long as the void is fully inside the outer rectangle, its position doesn't change the volume — only its width and length matter. If the opening extends past an edge, the footing is no longer a simple rectangle with a hole, and this calculator's formula will no longer be accurate. In that case, split the footing into rectangular sections and add the volumes.
Can this calculator handle round footings or piers?
No, it only handles rectangular shapes. For a round pier or column footing, use the cylinder formula \( V = \pi r^2 h \), where \( r \) is the radius and \( h \) is the depth. For example, a 2 ft diameter hole 3 ft deep needs \( \pi \times 1^2 \times 3 \approx 9.42 \) ft³. A round footing with a round void can be found by subtracting the two cylinder volumes.
What are common mistakes when estimating footing concrete?
The biggest one is mixing units — multiplying feet by inches without converting gives wildly wrong answers. Others include forgetting that the void only counts once (not on both sides), ignoring over-dig at the bottom of the excavation, and ordering exactly the calculated amount with no waste margin. Also remember that concrete is ordered in quarter-yard increments, so round up rather than down.