Work out how much concrete you need for slabs, footings and columns — in cubic yards, plus 40, 60 and 80 lb bag counts.
How concrete is worked out
Enter the dimensions of your slab or column and this concrete calculator returns the volume in cubic yards and cubic feet, then converts it into bags of pre-mix concrete in the three common sizes and adds a waste allowance.
For anything larger than a few bags, ready-mix delivered by the cubic yard is usually cheaper and easier, so the tool shows both.
Try it with these numbers
Worked through, a length of 10, width of 10, diameter of 1 produces a concrete needed of 1.3. Underneath, Cubic yards comes out at 1.3 and Cubic feet at 35. Change any field and every figure updates as you type.
Using the calculator
- Pick Slab for flat pours or Column for round pours, and enter the dimensions.
- Set the thickness and a waste allowance (5% is a good default).
- Read the cubic yards for ready-mix, or the 80/60/40 lb bag counts for bagged concrete.
How the number is calculated
volume (ft³) = area × thickness (slab)
80 lb bag ≈ 0.60 ft³ · 60 lb ≈ 0.45 ft³ · 40 lb ≈ 0.30 ft³
bags = ft³ ÷ bag yield (round up)
Frequently asked questions
How many 80 lb bags of concrete in a yard?
A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.6 cubic feet, so roughly 45 bags per cubic yard.
When should I order ready-mix instead of bags?
Bags are fine up to about 20–30; beyond that, ready-mix delivered by the yard is usually cheaper and far less work.
How much extra concrete should I order?
Add about 5–10% for spillage, uneven subgrade and slight over-excavation so you don’t run short mid-pour.