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Concrete Calculator
Use this free Concrete Calculator to work out exactly how much concrete your job needs before you order. Enter the size of your slab, footing, wall, or column and it gives you the volume in cubic feet, cubic yards, and cubic meters, the number of 40, 60, and 80 lb bags, and an estimated cost — so you can compare bagged mix against a ready-mix truck delivery.
10% is a safe allowance for most pours.
Typical ready-mix: about $125–$175 per yard.
| Volume (cubic feet, with waste) | — |
| Volume (cubic yards) | — |
| Volume (cubic meters) | — |
| 80 lb bags (0.6 ft³ each) | — |
| 60 lb bags (0.45 ft³ each) | — |
| 40 lb bags (0.3 ft³ each) | — |
| Estimated cost (ready-mix) | — |
How it works
Concrete is sold by volume, so the calculator first works out the volume of the shape you are pouring, then converts it into the units suppliers actually use. For a slab, footing, or wall, the volume in cubic feet is length times width (or height) times thickness, with the thickness converted from inches to feet by dividing by 12. A round column is different: its volume is pi times the radius squared times the height, with the diameter converted from inches to feet first. Once the calculator has the raw volume, it adds your waste factor, because a real pour always uses a little more than the neat math says — the base is never perfectly level, some concrete stays in the wheelbarrow, and edges spill.
The waste factor is a percentage added on top of the raw volume. A 10 percent factor on an 80 cubic foot slab adds 8 cubic feet, giving 88 cubic feet to order. The bag counts come from the standard yield of each bag size: an 80 lb bag of mix makes about 0.6 cubic feet of concrete, a 60 lb bag about 0.45 cubic feet, and a 40 lb bag about 0.3 cubic feet. The calculator divides the volume (with waste) by each yield and rounds up to whole bags, because you cannot buy part of a bag and running short mid-pour is worse than having one spare. The estimated cost multiplies the cubic yards by the price per cubic yard you entered, which should be the delivered ready-mix price your supplier quoted — not the price of bagged mix, which is far higher per yard.
Choosing the project type changes which measurements matter. A slab needs length, width, and thickness; a footing is the same shape but usually deeper and narrower, so it uses the same three measurements. A wall swaps width for height, since you measure along the wall and up it. A round column only needs its diameter and its height, so the third box disappears. Measure twice and enter the finished dimensions — the size of the concrete itself, not the size of the hole you dug, which is usually a little bigger.
Worked example
A patio slab measures 20 feet long and 12 feet wide, and it will be 4 inches thick. The waste factor is 10 percent and ready-mix costs $150 per cubic yard.
Step 1: Convert the thickness to feet: 4 inches ÷ 12 = 0.333 feet.
Step 2: Raw volume: 20 × 12 × 0.333 = 80 cubic feet.
Step 3: Add 10 percent waste: 80 × 1.10 = 88 cubic feet.
Step 4: Convert to cubic yards: 88 ÷ 27 = 3.26 cubic yards. In cubic meters: 88 × 0.0283168 = 2.49 cubic meters.
Step 5: Bags: 88 ÷ 0.6 = 146.7, so 147 bags of 80 lb mix; 88 ÷ 0.45 = 195.6, so 196 bags of 60 lb mix; 88 ÷ 0.3 = 293.3, so 294 bags of 40 lb mix.
Step 6: Cost: 3.259 × $150 = $488.89.
Enter those same numbers in the calculator — slab, 20 ft by 12 ft, 4 in thick, 10% waste, $150 per yard — and it gives exactly the same result: 3.26 cubic yards, 88 cubic feet, 2.49 cubic meters, 147 eighty-pound bags, and an estimated cost of $488.89. At that size, ready-mix delivery is far cheaper and faster than mixing nearly 150 bags by hand.
FAQ
How many cubic yards of concrete do I need for a slab?
Multiply the slab length by its width in feet, then multiply by the thickness in feet (thickness in inches divided by 12) to get cubic feet. Divide the cubic feet by 27 to get cubic yards. For example, a 20 by 12 foot slab 4 inches thick is 80 cubic feet, or about 2.96 cubic yards before any waste allowance. Add 5 to 10 percent for waste and uneven ground, and round up when ordering.
How many 80 lb bags of concrete make a cubic yard?
It takes 45 bags of 80 lb concrete mix to make one cubic yard, because each 80 lb bag yields about 0.6 cubic feet and a cubic yard is 27 cubic feet. A 60 lb bag yields about 0.45 cubic feet, so 60 bags make a yard, and a 40 lb bag yields about 0.3 cubic feet, so 90 bags make a yard. Bagged mix is practical for small jobs; for anything over about a cubic yard, ready-mix delivered by truck is usually cheaper.
What waste factor should I use for concrete?
Use 5 percent for a straightforward pour on a well-prepared, level base, and 10 percent for a first-time pour, an uneven base, or a job with lots of corners and edges where spillage is likely. A waste allowance covers spillage, over-excavation, and a base that is not perfectly level. It is far better to have a little extra than to run short mid-pour, because a cold joint where fresh concrete meets partly set concrete is a weak spot.
How much does a cubic yard of concrete cost?
Ready-mix concrete in the US commonly costs about $125 to $175 per cubic yard, depending on where you live, the mix strength, and delivery distance. Small loads often carry a short-load fee, and extras such as fiber reinforcement, a faster set, or a weekend delivery add to the price. Bagged mix costs more per yard — often $200 or more — but needs no minimum order and no truck access. Enter your supplier's quoted price per cubic yard in the calculator for an accurate estimate.
How thick should a concrete slab be?
A 4 inch slab is standard for a patio, walkway, or shed floor, and it suits light foot traffic and garden furniture. Use 5 to 6 inches for a driveway or any slab that will carry cars, and 6 inches or more for heavy vehicles. Thickness is measured after the concrete is screeded level, so set your forms and base to the finished depth, not the depth of the hole. Check local building codes before pouring a structural slab, footing, or foundation wall.