Concrete Slab Calculator
Enter your slab dimensions to find cubic yards of concrete needed, plus how many 60lb or 80lb bags to buy.
*Estimates only, based on illustrative pricing. Get quotes from local suppliers for accurate costs.
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The math behind slab volume and how to translate it into bags or a truck order.
How it Works
Enter your slab's length, width, and depth. The calculator converts depth from inches to feet, multiplies all three dimensions for volume, and converts to cubic yards — the unit concrete is sold in.
The Formula
Cubic Yards = (Length × Width × Depth in feet) ÷ 27, plus your waste factor. A 60lb bag yields ~0.017 yd³; an 80lb bag yields ~0.022 yd³.
Pro Tip
For anything over roughly 1 cubic yard, ordering ready-mix delivered by truck is almost always cheaper per yard and far less physically demanding than mixing dozens of bags by hand.
Frequently Asked Questions
How do I calculate how much concrete I need for a slab? +
Multiply the slab's length by width by depth, all in feet, to get cubic feet. Then divide by 27 to get cubic yards. For a 10ft x 10ft slab at 4 inches (0.33ft) deep: 10 x 10 x 0.33 = 33 cubic feet, or 33/27 = 1.22 cubic yards.
How many bags of concrete do I need instead of ready-mix? +
A 60lb bag yields about 0.017 cubic yards, and an 80lb bag yields about 0.022 cubic yards. For small jobs like fence posts or a small patio, bagged concrete is often more practical than ordering a ready-mix truck, which usually has a 1 cubic yard minimum.
Why should I add a waste factor to my concrete order? +
Uneven subgrade, spillage, and slightly-off measurements typically mean you need more concrete than the exact geometric calculation suggests. A 5-10% waste factor is standard practice, and it's cheaper to over-order slightly than to run short mid-pour.
How thick should a concrete slab be? +
A standard residential walkway or patio slab is usually 4 inches thick. A driveway that supports vehicles typically needs 5-6 inches. Always check local building codes before pouring.
Is it cheaper to buy bagged concrete or order ready-mix? +
For small jobs under roughly 1 cubic yard, bagged concrete is usually cheaper and more practical. For anything over about 1-2 cubic yards, ready-mix delivered by truck is typically more cost-effective per yard and far less labor-intensive.