1. Start with the actual footing geometry
A rectangular footing is a rectangular prism, so its material volume is length × width × concrete depth. All three dimensions must describe the same physical footing and must be converted to compatible units before multiplication.
V = length × width × depthThe formula answers a material question only. It does not tell you what the width or depth should be. Use project drawings, engineering information, and applicable requirements for those dimensions.
2. Combine identical footings before rounding
If several footings have the same dimensions, multiply the unrounded volume of one footing by the quantity. This preserves the actual total instead of accumulating package-rounding error at every footing.
Vnet = V × quantityIf footing sizes differ, calculate each size separately and combine the unrounded material volumes rather than averaging the dimensions.
3. Add a visible project allowance
Field conditions can make the purchased quantity higher than the ideal formed volume. BuildNumbers keeps the extra allowance explicit instead of hiding a universal percentage inside the formula.
Vorder = Vnet × (1 + allowance ÷ 100)4. Convert to ready-mix volume or complete bags
Imperial results can be viewed in cubic yards and cubic feet; metric results use cubic meters and liters. For bagged concrete, BuildNumbers divides the final allowance-adjusted volume by the selected published package yield and rounds upward once to a complete bag.
A controlled example of three 10 ft × 2 ft × 8 in rectangular footings equals 40 ft³ before allowance. At a published 0.60 ft³ yield, 40 ÷ 0.60 = 66.667, so the purchase quantity is 67 complete 80 lb bags. Those dimensions are an arithmetic example, not a footing recommendation.
5. Check the estimate before ordering
Recheck the actual formed dimensions, project requirements, product yield, supplier order increments, and whether the project uses bagged or ready-mix concrete. Excavation volume is not automatically the same as concrete volume, and this calculator does not estimate forms, reinforcement, labor, delivery, or structural requirements.
Run the measured dimensions
Calculate the concrete quantity.
Footing concrete FAQ
What measurements do I need for footing concrete?
For a rectangular footing, measure or obtain the project length, width, and concrete depth. If several footings are identical, also record the quantity. Do not use this guide to choose structural dimensions.
Should I round concrete bags for each footing separately?
No. Combine the unrounded concrete volume for identical footings first, apply the chosen allowance to the total, then round the final bag quantity upward once.
Does the calculator design the footing?
No. Footing dimensions, reinforcement, bearing capacity, frost depth, concrete strength, and other structural requirements are outside the calculator scope.