What It Is
The Concrete Footing Calculator estimates how much concrete you need to pour rectangular footings for foundations, posts, columns, and walls. It returns the total volume in cubic yards and cubic feet, and converts that into the number of pre-mixed bags required. Footing dimensions are typically given as width and depth in inches and length (run) in feet, which is exactly how this tool accepts them.
How to Use It
Enter the footing width and depth in inches, the length in feet, and the number of footings of that size. Set a bag yield — the volume one bag produces (an 80 lb bag yields about 0.022 yd³ / 0.6 ft³, a 60 lb bag about 0.017 yd³, a 40 lb bag about 0.011 yd³; check your product). Add a waste allowance (10% is common) to cover spillage, over-excavation, and uneven subgrade.
The Formula
Width and depth are divided by 12 to convert inches to feet. Multiplying width × depth × length gives the volume of one footing in cubic feet. Multiply by the quantity, then divide by 27 (cubic feet per cubic yard) to get cubic yards. A waste factor (1 + waste/100) is applied, and the bag count is the total volume divided by the bag yield, rounded up.
$$\begin{gathered} V = \frac{\dfrac{\text{Width (in)}}{12} \cdot \dfrac{\text{Depth (in)}}{12} \cdot \text{Length (ft)} \cdot \text{Qty}}{27} \cdot \left(1 + \frac{\text{Waste \%}}{100}\right) \\[1.5em] \text{Bags} = \left\lceil \frac{V}{\text{Bag yield}} \right\rceil \end{gathered}$$
Worked Example
A footing 16 in wide × 8 in deep × 20 ft long: \(16/12 = 1.3333\) ft, \(8/12 = 0.6667\) ft. Volume = \(1.3333 \times 0.6667 \times 20 = 17.78\) ft³. As cubic yards: $$17.78 \div 27 = 0.6584 \text{ yd}^3$$ With 10% waste that's 0.724 yd³ and 19.56 ft³. At 0.011 yd³ per 40 lb bag you'd need $$\left\lceil 0.724 / 0.011 \right\rceil = 66 \text{ bags}$$
FAQ
Should I order bags or ready-mix? Bags are practical for small jobs (under ~1 yd³). Larger pours are usually cheaper and faster with ready-mix delivery, ordered by the cubic yard.
How much waste should I add? 5–10% is typical; use the higher end for hand-dug trenches with uneven walls.
What if my footing isn't rectangular? This tool assumes a rectangular cross-section. For stepped or tapered footings, split them into rectangular sections and add the volumes.
