Estimating & quantities
Volume math is the one part of a concrete job you can get exactly right before anyone shows up. Formulas, worked examples, bag counts and the waste factor to add.
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How much concrete do I need?
Multiply length by width by thickness in feet, divide by 27, and add about 10% for waste. One cubic yard covers 81 square feet at 4 inches thick.
2How do you figure concrete in yards?
Multiply length by width by thickness in feet, then divide by 27. Worked examples, a coverage table, round and irregular shapes, and how much waste to add.
3How many bags of concrete do I need?
One 80 lb bag yields about 0.60 ft3, so a cubic yard takes roughly 45 bags. Get your volume in cubic feet, divide by the yield on the bag, then add waste.
Questions in this topic
8Calculators
Concrete slab calculator
Work out cubic yards, cubic feet and bag counts for a concrete slab. Enter length, width and thickness; includes a waste factor and a metric conversion.
CalculatorsBag count calculator
How many bags of concrete you need for a given volume, for 40, 50, 60 and 80 lb bags. Yields are manufacturer figures and vary by formulation.