Free construction tool
Concrete calculator: slabs, footings, and columns
Enter dimensions, get cubic yards with waste included, plus bag counts for small pours. The formulas are printed below the results so you can check the math.
Formula: volume in cubic feet divided by 27 = cubic yards. Bag yields: 60 lb makes about 0.45 cf, 80 lb about 0.60 cf. Ready-mix is sold by the cubic yard; confirm minimum-load terms with your supplier.
How the concrete calculator works
Concrete is ordered in cubic yards, drawings are dimensioned in feet and inches, and the calculator bridges the two. Every shape reduces to the same chain: convert dimensions to feet, multiply to cubic feet, divide by 27 for cubic yards, then add a waste factor before ordering.
Slab: length (ft) x width (ft) x thickness (ft). A 6 in slab is 0.5 ft thick. A 30 x 50 slab at 6 in is 30 x 50 x 0.5 = 750 cf, and 750 / 27 = 27.8 cy.
Continuous footing: total run (lf) x width (ft) x depth (ft). A 120 lf footing at 16 in wide and 12 in deep is 120 x 1.333 x 1.0 = 160 cf = 5.9 cy.
Round column: pi x radius (ft) squared x height (ft) x count. Four 18 in diameter columns at 10 ft are 3.1416 x 0.75 x 0.75 x 10 x 4 = 70.7 cf = 2.6 cy.
Why the waste factor matters
Field conditions consume concrete that the geometry does not show: uneven subgrade, form deflection, spillage, and pump line loss. Most estimators carry 5 to 10 percent on slabs and footings, more on irregular pours. Running short mid-pour forces a second truck dispatch and risks a cold joint, so the waste factor is cheap insurance. The calculator applies your percentage to the ordered volume, and bag counts round up to whole bags.
Truck loads and bag math
Ready-mix trucks carry about 10 cy, so a 30 cy order is three trucks, and suppliers commonly charge short-load fees below their minimum. For small pours, premixed bags substitute: a 60 lb bag yields about 0.45 cf and an 80 lb bag about 0.60 cf. A 10 cf repair therefore needs 10 / 0.60 = 17 bags of 80 lb mix, rounded up.
From volume to a priced line
Volume is only the quantity half of the estimate. The priced line multiplies the cy by an all-in unit cost covering the mix design, placement labor, pump or crane time, and finishing. That is exactly the step Ruh automates: it reads the slab and foundation dimensions off your drawings, computes the volumes with your waste factors, and prices them against your own price book, with your estimator reviewing the result. See the takeoff guide for the full quantity workflow, or the cy glossary page for the unit itself.
Concrete calculator FAQs
How many cubic yards of concrete do I need for a 24x24 slab at 4 inches?+
24 x 24 = 576 sf, times 0.333 ft thickness = 192 cf, divided by 27 = 7.1 cy. With 8 percent waste that rounds to about 7.7 cy, so order 8 cy.
How many 80 lb bags equal one cubic yard?+
One cy is 27 cf and an 80 lb bag yields about 0.60 cf, so roughly 45 bags make a cubic yard. That volume is why anything beyond small repairs is ordered as ready-mix rather than bagged.
What waste factor should I use for concrete?+
A common range is 5 to 10 percent for slabs and footings on well-prepared subgrade, trending higher for irregular shapes, thin toppings, or rough grade. Your own pour history is the best calibration.
Ruh does this for the whole drawing set.
AI takeoff reads your plans, computes every volume, and prices it on your price book.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.