Footing Concrete Calculator Guide
Footings can be simple to calculate, but they are also more sensitive than decorative slabs because they may support posts, walls, decks, additions, or other loads. The calculator can estimate volume for strip footings, square pads, and round piers, but footing size, depth, reinforcement, and inspection requirements should be confirmed with local code guidance or a qualified professional.
Strip footing formula
A continuous strip footing is estimated like a long rectangular shape. Measure length, width, and depth in feet, multiply them together, then divide by 27 to convert cubic feet to cubic yards.
Example: a 40 foot footing that is 16 inches wide and 8 inches deep is 40 x 1.333 x 0.667 = about 35.6 cubic feet. Divide by 27 for about 1.32 cubic yards before waste allowance.
Square or rectangular pad footings
For a pad footing, multiply length by width by thickness or depth. If several identical footings are required, calculate one footing and multiply by the quantity. If sizes vary, calculate each size separately. A small mistake in depth can matter when there are many footings, so measure the excavated holes after digging rather than relying only on the planned dimensions.
Round piers and post holes
Round footings use the circular area times depth. The area is pi times radius squared. Make sure to use radius, not diameter. A 12 inch diameter hole has a 6 inch radius, or 0.5 feet. A 12 inch diameter hole that is 36 inches deep is 3.1416 x 0.5 x 0.5 x 3 = about 2.36 cubic feet. Multiply by the number of holes, then convert to cubic yards or bags.
Depth, frost, and structural assumptions
Footing dimensions are not only a math choice. Frost depth, soil bearing capacity, load, seismic or wind requirements, and local code can affect size and reinforcement. The calculator does not determine safe footing design. It estimates concrete volume for dimensions you provide. For load-bearing work, confirm footing requirements before ordering concrete.
Reinforcement and inspection planning
Rebar, dowels, anchor bolts, post bases, and chairs can affect job sequencing. Some jurisdictions require inspection before concrete is placed. If an inspector needs to see excavation depth or reinforcement, do not schedule concrete delivery before the inspection window is clear. Delays can create ready-mix waiting fees or cancellation charges.
Bagged concrete versus ready-mix
Small post holes are often handled with bags because each hole can be mixed as needed. Larger continuous footings may be easier with ready-mix because placement is faster and consistency is better across the pour. Compare total bag count, labor, mixer capacity, access, delivery fees, and whether the concrete must be placed continuously.
Waste allowance
Footing excavations are rarely perfect. Soil can cave in, sides can widen, and bottoms can be uneven. A 10 percent allowance is often a practical planning starting point, but rough or hand-dug holes may need more. When using bags, round up because partial bags cannot always be measured conveniently in the field.
Information to gather before ordering
List each footing type separately before using the calculator. For example, separate continuous wall footings from porch post pads and round deck piers. Record length, width, depth, diameter, quantity, and any bell-shaped or enlarged base. If holes are inspected before the pour, measure after the inspection-ready excavation because loose soil and cleanup can change the final depth.
Ask the supplier or contractor what mix strength, slump, aggregate size, and placement method are appropriate. A footing order may also need anchor bolts, templates, rebar, chairs, and enough workers to place concrete before it stiffens.
Use this guide to estimate material quantity, then verify footing dimensions, reinforcement, inspection requirements, and mix strength before ordering. Structural and code decisions should not be made from a calculator estimate alone.