Combine rectangular concrete pours and cylindrical piers
Concrete volumes for projects with more than one shape should be calculated by section before being combined into a single order. A footing can be treated as a rectangular prism when its length, width and depth are constant, while a drilled pier can be represented as a cylinder when its actual diameter and depth are known. Measuring each geometry avoids applying a flat-slab area formula to elements of different thicknesses. The quantities in this calculator assume every rectangular section shares the entered dimensions and every pier shares the entered diameter and depth.
Concrete is commonly supplied in cubic yards, but plans can specify lengths in feet and pier diameters or depths in inches. Converting those units before adding the volumes is essential. A purchasing allowance accounts for reasonable spills, uneven excavation and other project-specific tolerances; it should not be used to conceal unknown element sizes or substitute for a structural design. Enter measurements taken from the plans or a qualified contractor's material schedule.
Combined concrete volume formulas
Pier volume (cu ft) = pier count × π × (diameter (in) ÷ 24)² × pier depth (in) ÷ 12
Ordered volume (cu yd) = (rectangular volume + pier volume) × (1 + allowance % ÷ 100) ÷ 27
Bag count = round up(ordered cubic feet ÷ stated cubic feet per bag)
Worked example: two footings and four drilled piers
Two rectangular pours measuring 10 feet long, 2 feet wide and 12 inches deep require 40 cubic feet in total. Four cylindrical piers measuring 12 inches in diameter and 36 inches deep require another approximately 9.42 cubic feet. The combined placed volume is therefore about 49.42 cubic feet, or 1.83 cubic yards before extra ordering allowance.
With an 8% allowance, the combined order becomes approximately 53.38 cubic feet, or 1.98 cubic yards. An illustrative bagged mix yielding 0.6 cubic feet per bag would require 89 whole bags, but the bag label—not the bag's stated weight—is the correct authority for yield.
Using the concrete volume to place an order
Ready-mix suppliers may have minimum orders, increments, delivery or short-load charges and separate fees for pumping or waiting time. The cubic-yard price field gives an itemized planning total when a real supplier quote is available. The bag count is an alternative quantity view, not another cost to add automatically on top of ready-mix concrete. For projects with irregular bottoms, bell-shaped footings, sloped forms or additional slabs, calculate each distinct volume separately and combine the totals.
Depth is especially important: doubling the depth doubles the concrete required at a fixed footprint. The quantities do not determine whether footing dimensions, reinforcement, frost depth, soil bearing or curing method are adequate. Those decisions belong to the responsible designer and local construction requirements.
Concrete volume questions
How are round concrete piers calculated?
Round piers are approximated as cylinders using π times the square of the radius times the depth. Because the diameter is entered in inches, it is divided by 24 to obtain the radius in feet. Drilled holes that widen toward the bottom or have irregular sides need different geometry or a measured excavation volume.
Is the bag count the same as the ready-mix order?
No. The bag count is derived from the cubic-foot yield entered for a particular packaged product, while the ready-mix quantity is expressed in cubic yards. Use whichever supply method is actually intended. Supplier batching and bag yield specifications can produce a different purchase quantity from the theoretical volume.
Does the calculator include rebar, gravel or forms?
Those materials are not calculated from concrete volume alone. Reinforcement schedules, subbase thickness, form length, and any excavation or labor charges require their own measured quantities. The optional delivery and additional-material field can incorporate an itemized quote without implying that the tool has designed those components.