About this tool
This concrete calculator covers the five pours a homeowner or small contractor is most likely to price: a rectangular slab or pad, a continuous footing, round columns or post piers, a set of solid stairs, and a curb with gutter. Each tab asks only for the dimensions that shape needs and returns the volume in cubic feet, cubic yards, and cubic meters, the number of 40, 60, or 80 lb premixed bags, the approximate weight of the finished concrete, and the ready-mix order rounded up to the next quarter yard. A waste allowance (10 percent by default) covers uneven subgrade, spillage, and forms that bow. Enter a price per yard or per bag for a material cost estimate. Use it before calling the ready-mix plant or loading bags at the store.
How it works
Every shape is reduced to cubic feet with inches converted to feet (divide by 12). Slab and footing: length x width x thickness. Column: pi x (diameter / 2)^2 x height x count. Stairs: the solid staircase volume is width x rise x run x n(n + 1) / 2, because step k sits on a block k treads deep, plus an optional landing (width x depth x total rise). Curb and gutter: (curb height x curb width + gutter width x gutter thickness) x length. The result is multiplied by 1 + waste%, divided by 27 for cubic yards and by 35.3147 for cubic meters, and rounded up to the nearest 1/4 yard for ordering. Bag counts use the standard yields printed on premixed concrete: a 40 lb bag makes 0.30 cu ft, a 60 lb bag 0.45 cu ft, and an 80 lb bag 0.60 cu ft (all 0.0075 cu ft per pound). Weight assumes normal-weight concrete at 150 lb per cubic foot (2,400 kg/m3).
Frequently asked questions
How many bags of concrete do I need for a 10 x 10 slab?
A 10 x 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Without waste that is 112 bags of 40 lb, 75 bags of 60 lb, or 56 bags of 80 lb mix. Add 10 percent waste and it becomes 123, 82, or 62 bags. At that size a short load of ready-mix is often cheaper and far less work than mixing 60-plus bags by hand, so compare both prices.
How thick should a concrete slab be?
Patios, sidewalks, and shed floors are usually 4 inches over compacted gravel. Driveways are 4 inches for cars and 5 to 6 inches where trucks or RVs park. Garage floors are 4 to 6 inches. Footings depend on soil and local code but are commonly 8 to 12 inches deep and twice the wall width; frost-line depth applies to the bottom of the footing, not the footing itself. Always check the local building code for structural pours.
Why round up to a quarter yard, and what is a short-load fee?
Ready-mix trucks meter concrete in quarter-yard increments, and running short mid-pour is far worse than having a wheelbarrow left over, because a second truck means a cold joint and a second delivery fee. Most plants also charge a short-load fee for orders under 4 to 6 yards, so a 2.5-yard pour may cost nearly as much as 4 yards. The calculator shows the rounded quarter-yard figure to order and the exact volume so you can judge the margin.
How much does concrete weigh?
Normal-weight concrete weighs about 150 lb per cubic foot, or roughly 4,050 lb (2 tons) per cubic yard, which is 2,400 kg per cubic meter. That matters for what your trailer or wheelbarrow can carry and for the load on decks or floors under planters and countertops. Lightweight structural mixes run 90 to 115 lb per cubic foot; the calculator uses the standard 150.