Estimate how much concrete you need for a slab, footing, or
round column — in cubic yards and bags — plus a cost estimate. Enter your
dimensions and get instant, accurate results for your project.
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Concrete needed
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Enter slab dimensions
Concrete needed
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Enter footing dimensions
Concrete needed
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Enter column dimensions
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How to calculate concrete for a slab
Concrete is ordered by volume in cubic yards. To find the volume of a slab,
multiply length by width by thickness — but all three must be in feet first.
Since thickness is usually given in inches, divide it by 12 to convert.
Cubic Yards = (Length ft × Width ft × Thickness ft) ÷ 27
There are 27 cubic feet in one cubic yard, which is why we divide by 27 at
the end.
Example: A 20 ft × 12 ft slab poured 4 inches thick:
4 in ÷ 12 = 0.333 ft. Volume = 20 × 12 × 0.333 = 80 cu ft.
80 ÷ 27 = 2.96 cubic yards. Order about 3.3 cubic yards to allow for
waste.
How many bags of concrete do I need?
For small jobs you can mix bagged concrete instead of ordering a truck.
Each bag size yields a known volume:
Bag size
Yield per bag
Bags per cubic yard
40 lb
0.30 cu ft
~90 bags
60 lb
0.45 cu ft
~60 bags
80 lb
0.60 cu ft
~45 bags
The calculator above shows the bag count for all three sizes automatically.
As a rule of thumb, anything over about 1 cubic yard is cheaper and easier to
order as ready-mix from a truck.
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Tips for ordering concrete
Always add 5–10% extra. Spillage, uneven subgrade, and over-dig
mean you'll use more than the exact calculation.
Standard slab thickness is 4 inches for patios and walkways,
5–6 inches for driveways that hold vehicles.
Ready-mix trucks usually carry up to 10 cubic yards and may
charge a short-load fee for small orders.
Compact and level the base before pouring — a sloppy subgrade
wastes concrete and weakens the slab.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab at 4 inches thick needs about 1.23
cubic yards of concrete (10 × 10 × 0.333 ÷ 27). Order roughly 1.4 cubic
yards to allow for waste.
How many 80 lb bags of concrete in a cubic yard?
An 80 lb bag yields about 0.6 cubic feet, and there are
27 cubic feet in a cubic yard, so you need about 45 bags per cubic yard.
How do I calculate cubic yards of concrete?
Multiply length, width, and thickness in feet, then divide
by 27. Convert any inch measurements to feet first by dividing by 12.
How much does a yard of concrete cost?
Ready-mix concrete typically costs between $120 and $180
per cubic yard, varying by region and mix. Enter your local price in the
calculator for an accurate estimate.
Should I order extra concrete?
Yes. Add 5–10% to your calculated amount to account for
spillage, uneven ground, and waste. Running short mid-pour causes a cold
joint and a weak slab.
