Concrete Calculator
Work out how much concrete you need for slabs, footings and columns - in cubic yards, cubic meters and bags.
Use the concrete calculator to figure out exactly how much concrete you need for a slab, footing, column or post hole. Enter the dimensions and the calculator returns cubic yards (the unit ready-mix is sold in), cubic feet, cubic meters and the number of 40lb, 60lb or 80lb bags. The bag count includes a 10% waste factor so you do not run short halfway through a pour. Common questions like "how much concrete do I need for a 10x10 slab" or "how many bags of concrete for a fence post" can be answered in seconds. For dedicated post-hole math (including post displacement), use the post hole concrete calculator instead.
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Frequently asked questions
A 10 ft x 10 ft slab at 4 inches deep is 33.3 cubic feet, or 1.23 cubic yards. You would need about 56 bags of 60lb concrete or 42 bags of 80lb concrete including 10% waste. For pours over 1 cubic yard, ready-mix delivered is usually cheaper than bagged.
One cubic yard equals 27 cubic feet. An 80lb bag yields about 0.60 cubic feet of cured concrete, so you need 45 bags per cubic yard. A 60lb bag yields 0.45 cubic feet, requiring 60 bags per yard. A 40lb bag yields 0.30 cubic feet, requiring 90 bags per yard.
A standard 4x4 fence post hole (10 in diameter, 24 in deep) needs about 1.1 cubic feet. That is roughly 2 bags of 60lb concrete or 1.5 bags of 80lb concrete per post. Always round up - it is cheaper to throw away a partial bag than stop a job for one more.
Concrete pours rarely come out exact. Sub-grade is never perfectly level, forms flex, and some material is always wasted in the wheelbarrow or mixer. Most concrete suppliers and bag manufacturers recommend ordering 10% more than the calculated volume to avoid running short at the end of a pour.
For pours over 1 cubic yard, ready-mix delivered by truck is almost always cheaper per cubic yard. Bagged concrete makes sense for small jobs (under 0.5 cubic yards) where you do not want a truck minimum or short-load fee. Check local prices - typical break-even is around 1 cubic yard.