Concrete Calculator
Calculate the volume of concrete needed for slabs, footings, columns, and steps.
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Concrete required
Bag yields are typical manufacturer figures and vary slightly by brand.
How to use the Concrete calculator
Choose the shape you are pouring and your units, then enter the dimensions. Thickness and diameter are in inches (or centimetres) because that is how they are usually measured on site.
Add how many identical pieces you need (e.g. 6 fence-post holes) and a waste allowance — 5–10% is typical. You’ll get the volume in cubic yards, cubic feet and cubic metres plus how many 40, 60 or 80 lb bags of pre-mix it takes.
For anything over about 1 cubic yard, ordering ready-mix from a supplier is usually cheaper and easier than bags.
Formula
Slab / footing: Length × Width × Thickness. Column: π × (Diameter ÷ 2)² × Height.
1 yd³ = 27 ft³ = 0.7646 m³. Bags = volume in ft³ ÷ bag yield (40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³), rounded up.
Frequently asked questions
Should I order extra concrete?
Yes — allow 5–10% extra for spillage, uneven sub-base and over-excavation. Running short mid-pour leaves a weak cold joint, so it is better to have a little left over.
How many 80 lb bags of concrete are in a cubic yard?
About 45. An 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags. You need about 60 bags of 60 lb or 90 bags of 40 lb for the same volume.
How thick should a concrete slab be?
Around 4 inches (10 cm) is standard for patios, walkways and shed bases. Driveways usually need 4–6 inches (10–15 cm), and heavier loads may need more plus reinforcement — check local building codes.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. For example, a 10 ft × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, which is 1.23 yd³.