A concrete calculator helps answer a crucial question before the pour begins: how many cubic yards should you order? An incorrect estimate could result in the concrete truck leaving before the slab is finished. Conversely, a miscalculation could leave you paying for concrete that ends up hardening in the wheelbarrow.
Enter your dimensions below. The tool accounts for slabs, footings, and round columns, adds a margin for waste, and converts the result into cubic yards, cubic feet, and the number of bags required (for a DIY mix).
How the concrete calculator works
Concrete is sold by volume; therefore, any calculation simply involves applying the formula: length x width x depth. The only challenge lies in the fact that the slab thickness is expressed in inches, while the other dimensions are in feet; this is precisely where errors often occur during manual calculations.
For a 20-by-20-foot concrete slab with a thickness of 4 inches, the calculation is as follows: 20 x 20 x 0.333, resulting in 133 cubic feet. Dividing this figure by 27 yields 4.94 cubic yards. Adding the recommended 10% to account for waste results in an order quantity of 5.5 cubic yards.
It is worth keeping the number 27 in mind: one cubic yard is equal to 27 cubic feet, since it measures three feet in each dimension.
Calculating for a concrete slab
For a concrete slab, patio, or walkway, you need three measurements: length, width, and thickness. Thickness is often underestimated, even though it is the factor that most influences the amount of material to order.
- 4 inches (approx. 10 cm) is the standard thickness for patios, walkways, and garden shed foundations.
- 5 to 6 inches (approx. 13 to 15 cm) is typical for driveways intended for passenger cars.
- 6 inches (approx. 15 cm) or more is required if heavy vehicles or machinery will be parked there.
If you increase the thickness of a 20 x 20 foot slab from 4 to 6 inches, the amount of material needed rises from 5.5 to 8.25 cubic yards (a 50% increase), because the thickness applies to the entire surface area. Minimum thicknesses for residential work are set out in the standards published by the American Concrete Institute, and your local building code may be stricter still. If the slab has an irregular shape, first calculate the surface area (for example, using our square footage calculator) and use that total value as the basis for the area to be concreted.
How much does a yard of concrete cost?
This is the question underlying most searches regarding concrete. The honest answer is: it depends largely on your location. Ready-mix concrete is billed by the cubic yard; the price varies based on the cost of cement, fuel prices, and local competition. Therefore, a national average is not very useful for estimating the cost of a specific construction project.
However, when planning, you can take into account the billing structure, which is consistent everywhere:
- The concrete itself (calculated by the cubic yard)
- Delivery (sometimes included in the price for orders above a certain minimum, or billed separately if below that threshold)
- Small-load surcharge (common for orders under approximately 1 cubic yard; this cost can sometimes nearly equal the cost of the concrete itself)
- Additional services, such as weekend delivery, extended wait times at the job site, or the use of a pump if the truck cannot directly access the pour site
Due to the small-load surcharge, purchasing very small volumes of concrete is proportionally very expensive. For quantities of less than about half a cubic yard, it is usually more economical to buy bagged concrete. Request a price quote per cubic yard from a local supplier and enter that figure into the calculator above; this will give you a total cost reflecting actual market conditions, rather than relying on a theoretical average.
Bagged concrete or ready-mixed concrete?
The critical threshold is generally around half a cubic yard (approximately 0.4 cubic metres). Below this volume, bagged concrete is more economical and easier to handle; furthermore, it avoids the need to wait for a ready-mix truck delivery at your driveway. Above that amount, mixing by hand becomes an extremely arduous task.
The calculated quantities are based on standard yield values listed for most mixes. An 80-pound bag (about 36 kg) yields approximately 0.60 cubic feet (about 17 liters); a 60-pound bag (about 27 kg), close to 0.45 cubic feet; and a 40-pound bag (about 18 kg), around 0.30 cubic feet. Based on these figures, 45 bags of 80 pounds are needed to obtain one cubic yard (about 0.76 cubic metres); this amounts to well over a ton of material that must be transported, opened, and mixed.
For a person working with a concrete mixer, the practical limit is between 20 and 25 bags per work session; otherwise, the first batch begins to set before the last one is ready. For larger volumes, it is preferable to order ready-mixed concrete, as there is a risk that the concrete will harden in visible layers.
Footings, post holes, and columns
Footing calculations are performed in the same way as for slabs, with the difference that they are narrower and deeper. Measure the total length of the trench, as well as its width and depth, to determine the volume.
In contrast, for post holes and round columns, the formula for a circle must be used; the calculation tool applies this automatically as soon as the shape is changed. Measure the diameter of the hole, not the post, and specify the number of holes to be filled, as the volume per hole adds up quickly when considering the entire fence.
An important note regarding post holes: the walls are rarely as clean as one might expect when using an auger, and soil tends to crumble into the cavity. In practice, the actual amount required often exceeds the theoretical value; therefore, the 10% allowance for waste addresses a genuine need rather than serving as a mere, unnecessary safety margin.
Why it is important to allow for a margin for material loss
Concrete is unforgiving when material runs short. Once pouring begins, the mix already in place starts to harden; if a second delivery arrives twenty minutes later, it will not bond with the previous batch to form a homogeneous, monolithic slab. This creates what is known as a cold joint: a visible line that also acts as a point of structural weakness.
A 10% margin is generally recommended. This accounts for irregularities in the base surface, the most common cause of excess consumption, since a slab poured over uneven ground (with depressions of around half an inch in places) requires more concrete than a simple surface-area calculation would suggest. This margin also covers losses from spillage, residue left in the truck or wheelbarrow, and slight formwork deformation caused by the pressure of the concrete.
Ordering a slightly larger quantity is a cost-effective preventive measure. Plan a use for the surplus in advance, such as a hole for an extra post or a short section of pathway, to ensure the material does not go completely to waste.
For the other materials in a build, see all our construction calculators.