How Much Concrete Do I Need?
Updated
To find how much concrete you need, multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. Add 5–10% for waste. A 10×10 ft slab at 4 inches needs about 1.23 cubic yards, or roughly 56 80-lb bags.
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The basic formula
Concrete is ordered by volume. Measure your slab in feet, convert the thickness from inches to feet (divide by 12), then multiply all three dimensions for cubic feet. Divide by 27 to get cubic yards — the unit ready-mix is sold in.
Don't forget waste
Always add 5–10%. Subgrades are rarely perfectly level, and running short mid-pour creates a weak cold joint. A little extra is far cheaper than a second delivery.
Bags vs ready-mix
- Under ~1 cubic yard: bagged concrete is convenient.
- 1 cubic yard or more: ready-mix delivery is usually cheaper and faster.
- A cubic yard ≈ 45 80-lb bags or 60 60-lb bags.