Concrete calculator
Figure out how much concrete you need for a slab, footing or column — in cubic yards, cubic feet and bags.
Calculator
Detailed breakdown
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How to use this calculator
Measure
Measure length and width in feet, and thickness in inches. For a column, enter the height as length and the diameter in inches.
Calculate
Once you hit calculate, results update as you type. Change any number to compare options instantly.
Order
Print the estimate and take it to your supplier. Quantities are rounded up so you don't come up short.
How to calculate concrete the way suppliers price it
Concrete in the United States is sold two ways: by the cubic yard from a ready-mix truck, or by the bag from a home center. This calculator gives you both, because the right answer depends entirely on the size of your pour.
The math is simple volume. Multiply length by width by thickness — with thickness converted from inches to feet — to get cubic feet, then divide by 27 to get cubic yards. A 10 by 10 foot patio at 4 inches thick comes to 33.3 cubic feet, or about 1.23 cubic yards before waste. The calculator above adds 10 percent by default, which is why it shows 1.36 — that is the number you order.
In bags, that same patio is 75 sixty-pound bags bare, or 82 with the waste allowance. Which is exactly why nobody pours a patio out of bags.
The practical dividing line sits around one cubic yard. Below it, bagged mix is cheaper and you avoid delivery minimums. Above it, ready-mix wins on price — and on your back. Once you pass about 10 cubic yards, you're into full-truck territory, where the price per yard drops again.
Always add a waste factor. Subgrade is never perfectly level, forms bow slightly, and some concrete stays in the truck or the wheelbarrow. Ten percent is standard for slabs; use fifteen percent for footings in soft or uneven soil. Coming up half a yard short on a slab means a cold joint you'll see forever, so the extra is cheap insurance.
One thing this calculator deliberately doesn't do is tell you the mix strength, rebar schedule or thickness your project requires. Those are structural decisions that depend on soil, load and local code. Get them from an engineer or your building department — this tool answers how much, not what kind.
Frequently asked questions
How many cubic yards is a 10x10 slab?
At 4 inches thick, a 10 by 10 foot slab is 33.3 cubic feet, or about 1.23 cubic yards before waste — 1.36 with the standard 10 percent allowance. At 6 inches thick it rises to about 1.85 cubic yards bare.
How many 80 lb bags in a cubic yard?
An 80 lb bag yields about 0.60 cubic feet, so a cubic yard (27 cubic feet) takes 45 bags. A 60 lb bag yields 0.45 cubic feet, so the same yard takes 60 bags.
Should I use bags or ready-mix?
Under about one cubic yard, bags are usually cheaper and simpler. Above that, ready-mix delivery costs less per yard and saves hours of mixing. Check your supplier's minimum delivery charge.
How much waste should I add?
Ten percent for slabs on prepared subgrade, fifteen percent for footings or uneven ground. Running short mid-pour creates a cold joint, which is a structural and cosmetic problem.
How thick should a concrete slab be?
Four inches is typical for patios and walkways; five to six inches for driveways and anything carrying vehicles. Your local code and soil conditions have the final say — confirm before pouring.
How do I calculate a round column or pier?
Pick "Round column", enter the height in feet and the diameter in inches. A standard 10 inch Sonotube is 10 — don't convert it to feet, the calculator does that.
Does this include rebar or mesh?
No. This calculates concrete volume only. Reinforcement depends on span, load and code requirements, and should come from an engineer or your building department.
Sources
Formulas and material assumptions on this page follow published industry guidance: