Concrete Volume: How Much You Need and How to Order
Ordering concrete is a one-shot decision: too little and the pour fails, too much and you pay for waste. The math is simple volume arithmetic, but the details, waste, thickness variation and bag yields, separate smooth pours from expensive mistakes.
This guide shows the calculation, the conversions suppliers use, and the ordering rules contractors follow.
The core calculation
Volume = length x width x depth, all in the same unit. A 10 ft by 10 ft slab at 4 inches thick: 10 x 10 x (4/12) = 33.3 cubic feet.
Suppliers sell by the cubic yard: divide cubic feet by 27. That slab needs 33.3 / 27 = 1.23 cubic yards.
Watch the depth conversion: 4 inches is 0.333 feet, 6 inches is 0.5 feet. Mixing inches and feet is the classic error.
Bags vs ready-mix
An 80 lb bag of concrete mix yields about 0.6 cubic feet. Our 33.3 cubic foot slab needs 33.3 / 0.6 = 56 bags. A 60 lb bag yields about 0.45 cubic feet.
Rule of thumb: under about 1 cubic yard, bags are practical. Above that, ready-mix delivery is usually cheaper and far less labor.
Ready-mix has minimum loads and short-load fees, so tiny pours can cost more per yard. Ask the supplier about minimums.
Waste and thickness reality
Always add 5 to 10% for waste, uneven subgrade and spillage. Subgrades are never perfectly level, so real thickness varies.
For the example slab, order 1.23 x 1.1 = 1.35 cubic yards, or about 61 bags. Running short mid-pour creates a cold joint, a weak seam.
It is cheaper to have half a yard extra than to reorder a short load. Leftover concrete can fill a walkway form or a sonotube.
Footings, columns and odd shapes
Footings are just long boxes: length x width x depth. A 40 ft footing 16 inches wide and 8 inches deep: 40 x 1.33 x 0.67 = 35.6 cubic feet, or 1.32 yards plus waste.
Columns and sonotubes are cylinders: pi x radius squared x height. A 12-inch diameter, 3 ft deep hole: 3.14 x 0.5^2 x 3 = 2.36 cubic feet, about 4 bags.
For L-shaped slabs, split into rectangles, compute each, and add. Never try to eyeball irregular areas.
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Try the Concrete Slab calculatorFrequently asked questions
How many bags of concrete do I need?
Divide cubic feet by 0.6 for 80 lb bags, then add 10% waste.
How many cubic yards in a 10x10 slab?
At 4 inches thick, about 1.23 cubic yards before waste.
Should I order extra concrete?
Yes, 5 to 10% extra. Short loads cost more than slight overages.
Bags or ready-mix?
Bags under roughly 1 cubic yard; ready-mix above it.
How thick should a driveway slab be?
Typically 4 inches for cars, 5 to 6 for heavy vehicles, on compacted base.
What is a cold joint?
A weak seam where fresh concrete meets concrete that already started setting, caused by delays.