How to calculate concrete
Concrete is ordered by volume — cubic feet for bags, cubic yards for ready-mix trucks. For a slab or footing, multiply length by width by thickness (converting the thickness from inches to feet). For a round column, the volume is pi times the radius squared times the height. This calculator handles both shapes, adds a small waste allowance for spillage and uneven subgrade, and converts the result into bag counts for whichever bag size you choose.
How many bags make a cubic yard?
A cubic yard is 27 cubic feet. Since an 80 lb bag yields about 0.6 cubic feet, it takes roughly 45 of them to fill a yard; 60 lb bags take about 60, and 40 lb bags about 90. That's why bags only make sense for smaller pours.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | ~45 |
| 60 lb | 0.45 cu ft | ~60 |
| 50 lb | 0.375 cu ft | ~72 |
| 40 lb | 0.30 cu ft | ~90 |
Why add a waste allowance?
Subgrade is never perfectly flat, forms flex, and some concrete is always lost to spillage and the mixing process. A 5–10% cushion keeps you from running out mid-pour, which is the worst time to make a supply run — concrete sets on a clock.
Frequently asked questions
How thick should a slab be?
Four inches is standard for patios and walkways; driveways and areas carrying heavier loads are often poured at five to six inches over a compacted base. Always check local code for structural pours.
Should I order ready-mix?
Past about a cubic yard, yes — hand-mixing dozens of bags is exhausting and it's hard to keep a consistent mix. Ready-mix trucks have minimum loads, so confirm quantities with the supplier.
Does this include rebar or base?
No — this estimates the concrete only. Gravel base, rebar, and forms are separate. For structural work, follow an engineer's or local code's specifications.
Concrete calculation example
A 10-by-10-foot slab poured 4 inches thick contains about 33.3 cubic feet, or 1.23 cubic yards, before waste. With a 5% allowance, plan for about 1.30 cubic yards. That is roughly 59 bags at 80 pounds each, so ready-mix delivery may be more practical.
Related project calculators
Estimate supporting materials with the gravel calculator, plan a patio with the paver calculator, or calculate posts and concrete with the fence calculator.