Concrete Calculator

The short answer

A 10 by 10 foot slab that is 4 inches thick holds about 1.37 cubic yards of concrete, which works out to roughly 2,060 pounds and needs ordering with about ten percent waste on top. Most suppliers sell by the yard and delivered loads are partial, so a project this size is normally one load rather than two. The same slab at 6 inches takes about 2.05 yards.

What it depends on

Bag count is the practical question for small jobs. Sixty-pound bags yield about half a cubic foot each, so the 10 by 10 slab needs about 27 bags plus waste. Eighty-pound bags yield about two thirds of a cubic foot and need about 20. Strength mix and the required PSI change the mix design but not the volume, and mixing conditions matter more than most buyers expect.

Where this stops being useful

Volume is straightforward, but a slab this size involves formwork, base preparation, reinforcement, curing time and often a permit — all of which can cost more than the concrete itself. Cold weather changes how long cure takes and can require additives. Figures here are for planning quantities; a qualified contractor can assess the structural and site requirements, which vary by location.

Volume is straightforward, but a slab this size involves formwork, base preparation, reinforcement, curing time and often a permit — all of which can cost more than the concrete itself. Cold weather changes how long cure takes and can require additives. Figures here are for planning quantities; a qualified contractor can assess the structural and site requirements, which vary by location.

oncrete is bought by volume, and the weight needed depends on the mix. A standard mix is roughly one hundred and fifty pounds per cubic foot, so a delivery order needs both the volume and the strength grade. Many suppliers deliver by the quarter, half or full cubic yard, so a small job may attract a minimum delivery charge that dwarfs the material cost — buying bags from a builders merchant is cheaper below about one cubic yard. If the volume falls awkwardly between delivery increments, rounding up is cheaper than a second delivery.

Concrete is one of the few materials where the delivery method can cost more than the material. Knowing the volume first makes it easy to ask the supplier for a quote on both options, and to see whether bags make sense for the amount you actually need.

Concrete strength is chosen for the job, not for the job you wish you had. A footing needs more compressive strength than a shed base, which needs more than a patio slab, because the loads are different. Check the bag or delivery ticket for the PSI or megapascal rating and match it to what is being supported. Also plan the pour: fresh concrete has a limited working time once the truck arrives, so the forms, the base and any reinforcement need to be ready before it is on site.

Concrete is ordered by volume, priced by the cubic yard or cubic metre, and once it is on site there is no going back — you cannot add more halfway through a pour without creating a cold joint. That is why a concrete estimate is worth doing twice, and why every experienced builder adds a margin rather than ordering exactly what the geometry suggests. The geometry itself is simple for a slab: length times width times thickness, with the thickness converted from inches to feet so the units agree. A four-inch slab over ten by twelve feet is forty cubic feet, which is about one and a half cubic yards. Footings, post holes and steps are the same calculation applied to a different shape; break a complicated pour into simple boxes, work out each one and add them together. The margin matters more than the maths. Concrete is lost to uneven subgrade, formwork that bows under the weight, spillage and the small amount left in the chute. Five to ten per cent extra is typical, and it is far cheaper than a short load, which may mean a second delivery charge or a weak, patched edge. Running slightly over is normal and the surplus can be used for a small pad. This calculator gives the volume in cubic feet, cubic yards and cubic metres, and estimates the number of eighty-pound bags if you are mixing by hand rather than ordering ready-mix. Mixing by hand is only sensible for small jobs — a cubic yard is well over a tonne and more than sixty bags. Confirm the subgrade depth and any reinforcement with your supplier before ordering.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.