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Concrete Calculator
Work out the cubic yards and bags of concrete for a slab or footing.
How to use this calculator
Enter the length and width of your slab in feet and the thickness in inches, then press Calculate concrete. The tool returns the volume in cubic yards — how ready-mix concrete is ordered and delivered — plus the equivalent number of 80-pound bags if you are mixing it yourself. For footings or several pours, run each section separately and add the totals. It is sensible to order around 10% extra to cover spillage, uneven sub-base and the small amount lost when you screed the surface.
The formula explained
Concrete volume is the slab's three dimensions multiplied, converted to cubic yards:
cubic yards = (length × width × thickness_inches ÷ 12) ÷ 27
Length times width gives the area in square feet; dividing the thickness in inches by 12 turns it into feet, so the product is cubic feet. Because ready-mix is sold by the cubic yard and one cubic yard is 27 cubic feet, dividing by 27 gives yards. For bag counts, each standard 80-pound bag of mix yields about 0.6 cubic feet, so dividing the cubic feet by 0.6 gives the number of bags.
A worked example
A 10 ft × 10 ft slab poured 4 inches thick has an area of 100 sq ft and a depth of 0.33 ft, giving about 33.3 cubic feet. That is 1.23 cubic yards, or roughly 56 80-pound bags. With a 10% allowance you would order about 1.35 cubic yards, or buy a few extra bags.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length, width and thickness in feet, then divide by 27 for cubic yards. This tool handles the conversions.
How many bags in a cubic yard?
About 45 80-pound bags, since each yields roughly 0.6 cubic feet.
Should I order extra?
Yes — around 10% extra covers spillage, uneven ground and waste.