🧱 Concrete Calculator
Estimate how much concrete you need for a slab, footing, column, or stair project in cubic feet, cubic yards, and 80lb bags.
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Concrete is typically ordered by the cubic yard from a ready-mix supplier for larger jobs. For smaller jobs, pre-mixed 80lb bags (about 0.6 cubic feet each) are a practical alternative.
How the Concrete Calculator Works
Concrete volume is calculated from the shape of the form you're filling. The calculator converts your measurements to cubic feet, then to cubic yards (the standard unit for ready-mix orders) and estimates the number of pre-mixed bags as an alternative for small jobs.
Formulas
Column Volume (ft³) = π × (Diameter ÷ 2)² × Height
Cubic Yards = Cubic Feet ÷ 27
80lb Bags = Cubic Feet ÷ 0.6
Why Add a Waste Factor?
A waste factor accounts for spillage, uneven subgrade, over-excavation, and minor measurement error. A 10% waste factor is a common default for most residential pours.
Frequently Asked Questions
Why is concrete ordered in cubic yards?
Ready-mix concrete trucks measure and deliver concrete by the cubic yard, so converting your project's volume to cubic yards makes it easy to place an accurate order.
How many bags of concrete do I need?
A standard 80lb bag of concrete mix yields about 0.6 cubic feet once mixed. Divide your total cubic feet by 0.6 to estimate the number of bags for smaller projects.
Should I always add a waste factor?
Yes — most contractors add 5-10% extra to account for spillage, uneven ground, and form irregularities. Larger or more complex pours may need up to 15%.