How to use this calculator
- Choose rectangle for most slabs, circle for round pads, or enter a total area if you already know it.
- Enter the length and width in feet and the thickness in inches. Four inches is common for patios and walkways.
- Pick the bag size you plan to buy. The calculator uses the typical yield printed on premix bags.
- Keep the waste allowance at 10%, which covers uneven ground, spillage and a slightly thicker pour.
- Add a price per cubic yard if you want a rough ready-mix cost to compare with buying bags.
How the numbers are calculated
Volume is area multiplied by thickness. The calculator turns the thickness from inches into feet by dividing by 12, multiplies by the area to get cubic feet, adds your waste allowance, and divides by 27 to get cubic yards.
For bags it divides the cubic feet by the yield of one bag and rounds up. Typical yields are about 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. That works out to roughly 90, 60 and 45 bags per cubic yard. Brands differ slightly, so check the label on the bag you buy.
Worked example
A slab 10 ft long and 10 ft wide has an area of 100 sq ft. At 4 in thick the volume is 100 × 0.333 = 33.3 cubic feet. With 10% extra you need 36.7 cubic feet, which is 1.36 cubic yards, or 62 bags of 80 lb mix.
Tips before you buy
- Order a little more than you calculate. Running short in the middle of a pour leaves a cold joint that can crack.
- For anything beyond a few dozen bags, ready-mix delivery is usually less work than mixing by hand. Ask about minimum loads and short-load fees.
- Prepare a compacted gravel base and use forms that are level or sloped slightly for drainage before the concrete arrives.
- Driveways and areas that carry vehicles are usually poured thicker than a patio, often five to six inches, and may need reinforcement.
- Keep new concrete damp and out of strong sun for several days so it cures slowly and evenly.
Frequently asked questions
How much area does one cubic yard of concrete cover?
One cubic yard covers about 108 square feet at 3 inches thick, 81 square feet at 4 inches, and 54 square feet at 6 inches.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix, based on typical label yields.
How thick should a concrete slab be?
Four inches is common for walkways, patios and shed bases. Driveways and heavily loaded areas are usually thicker. Check local requirements for structural work.
Should I add extra for waste?
Yes. A 5% to 10% allowance is normal because forms are rarely perfect and the ground is rarely perfectly level.
Quick answers
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These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.
Last updated October 11, 2026.