How to use the concrete calculator
- Choose a Shape. Use Slab, footing or wall for anything flat or boxy, Round column or pier for tubes and post holes, and Steps for solid concrete stairs.
- For a slab, footing or wall, enter Length (ft), Width or height (ft) and Thickness (inches). For a column, enter Diameter (inches) and Height (ft). For steps, enter Rise (inches), Run (inches), Width (ft) and Number of steps.
- Set Number of identical pieces for repeats such as six deck footings.
- Leave Extra for spillage and uneven ground (%) at 10, or change it.
- Read the cubic yards to order, then the bag counts if you are mixing it yourself.
How the volume is worked out
A slab is length x width x thickness in feet, so inches are divided by 12. A column is pi x (diameter / 24) squared x height, since diameter divided by 24 is the radius in feet. Solid steps stack up, so step 1 is one rise tall, step 2 is two rises tall, and so on, which makes the volume rise x run x width x (1 + 2 + ... + steps). Cubic feet are divided by 27 for cubic yards. This is the same formula Concrete Network uses, and it says never to order the exact amount. Bag counts use QUIKRETE's published yields of 0.45 cu ft for a 60 lb bag and 0.60 cu ft for an 80 lb bag, from its Concrete Mix data sheet.
Worked example with the default inputs
A 10 x 10 ft slab, 4 inches thick. The volume is 10 x 10 x (4 / 12) = 33.33 cu ft, which is 33.33 / 27 = 1.235 cu yd. With 10% extra that is 1.36 cu yd, and rounding up to the next quarter yard gives 1.5 cubic yards to order. In bags, 33.33 x 1.1 divided by 0.45 is 82 bags of 60 lb, or divided by 0.60 it is 62 bags of 80 lb.
Limits and edge cases
- Steps are figured as solid concrete from the ground up with no landing. Add a landing or a footing as a separate slab, and subtract the space taken by any hollow form.
- Ground is rarely flat, which is why the extra percentage exists. Concrete Network suggests budgeting an extra quarter inch of thickness for uneven ground. Rebar and mesh take up almost no volume.
- Yields differ between brands and mixes, so check the bag.
- Above roughly 2 cubic yards, hand-mixing bags gets impractical and the tool suggests ready-mix. Suppliers may charge more for small loads, so ask for their minimum. Building forms? Try the lumber calculator. For units, see the volume converter and the cylinder volume calculator.
Frequently asked questions
How many bags of concrete do I need for a 10 x 10 slab?
For a 4 inch slab with 10% extra, you need 62 bags of 80 lb mix or 82 bags of 60 lb mix. Without the extra it is 56 and 75 bags. Bag counts are rounded up to whole bags.
How do I calculate cubic yards of concrete?
Multiply length by width by thickness, all in feet, and divide by 27. A 10 x 10 ft slab 4 inches thick is 10 x 10 x 0.333 = 33.33 cubic feet, or 1.235 cubic yards. As a shortcut, 81 square feet at 4 inches thick makes one cubic yard.
How much extra concrete should I order?
Order about 10% more than the calculated volume, because forms move, the ground is uneven and some concrete is lost in the pour. Running a little over is cheaper than running short.
How many 80 lb bags make a cubic yard?
An 80 lb bag makes about 0.60 cubic feet, so 45 bags make one cubic yard. A 60 lb bag makes about 0.45 cubic feet, so it takes 60 bags.
Sources: QUIKRETE Concrete Mix No. 1101 data sheet; Concrete Network, Concrete Calculator.