Concrete Calculator
Work out how much concrete a job needs in cubic yards, cubic feet and bags. Add slabs, footings, round columns or post holes, steps, and curbs and gutters, include waste, and compare the cost of ready-mix against 80-lb bags.
Concrete calculator
Add each part of the job; the calculator totals them. Enter feet and inches as labeled.
Concrete yard calculator: the formula
Volume in cubic feet = length × width × thickness, all in feet (divide inches by 12). Divide by 27 to get cubic yards. A 10 × 10 ft patio 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cubic yards, about 1.36 yards with 10% extra.
- Bags: Quikrete Concrete Mix yields about 0.60 cu ft per 80-lb bag, 0.45 per 60-lb bag and 0.30 per 40-lb bag, so one cubic yard takes 45 bags of 80 lb, 60 of 60 lb or 90 of 40 lb.
- Bags or a truck? Bags make sense for post holes and small pads; past roughly a cubic yard most people order ready-mix. Many suppliers charge a short-load fee below a minimum order, so ask.
- Thickness: 4 inches is typical for sidewalks and patios; driveways are commonly 4–6 inches. Follow your local code or engineer for structural work.
For gravel, topsoil or other bulk materials, use the cubic yard calculator.
Common questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet and divide by 27 for cubic yards. Add about 10% for waste.
How many bags of concrete are in a yard?
About 45 of the 80-lb bags, 60 of the 60-lb bags or 90 of the 40-lb bags, based on Quikrete's yields.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, 1.23 cubic yards: about 56 bags of 80 lb, or 62 with 10% extra.
How much concrete for a fence post?
Use the round column section with the hole diameter and depth. A 10-inch hole 2 feet deep holds about 1.09 cu ft, roughly two 80-lb bags, minus the post's volume.
How thick should a concrete slab be?
4 inches for most patios and walkways, 5–6 inches where vehicles or heavy loads go.
Sources
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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.