Concrete Calculator (Volume & Bags Needed)

Volume is worked out in cubic feet first: a rectangular slab or footing is length x width x thickness, while a cylindrical column or post uses the standard cylinder formula, pi x radius^2 x height (radius being half the diameter you enter, height reusing the 'thickness' field). Whichever unit system you're using, the dimensions are converted to feet before that formula runs, so the same physical size always gives the same volume.

That cubic-foot volume is then shown three ways: cubic yards (dividing by the exact 27 cubic feet per cubic yard — the unit ready-mix concrete and delivery trucks are usually quoted in), cubic feet directly, and cubic meters (the exact 0.0283168 m³ per ft³ conversion), so it's useful whether you're ordering ready-mix or working in metric.

Bags needed is the total cubic feet divided by however many cubic feet one bag of pre-mix yields once mixed, rounded up to a whole bag. The default, 0.6 cubic feet, is the commonly quoted yield for an 80 lb bag — a 60 lb bag yields about 0.45 cubic feet and a 40 lb bag about 0.3, so change the yield field to match whatever bag size you're actually buying rather than trusting the default blindly.