Concrete Calculator

Concrete is a material made from coarse aggregates such as sand, gravel, crushed stone and slag, bonded together with cement. Cement is the binder: it adheres to those materials and hardens over time. Many types exist, but Portland cement is by far the most common, and it is an ingredient in concrete, mortar and plaster alike.

The distinction is worth getting right, because the words get used interchangeably and mean different things. Cement is the powder. Concrete is the finished material once cement, aggregate and water are combined. Nobody builds a driveway out of cement.

Concrete can be bought in several forms, including 60 or 80 pound bags, or delivered in bulk by a mixer truck. Proper mixing matters: strong, uniform concrete needs water, aggregate, cement and any additives combined thoroughly. Production is time-sensitive, since the mix is normally placed as a viscous fluid and has to be in position before it hardens. Some mixes are designed to set rapidly for applications that need it. In factory settings, concrete is mixed dry and pressed into precast products such as wall panels.

How much concrete do I need?

Every shape below reduces to the same question: what volume does the pour occupy? Work that out and the weight and bag count follow from the density of the mix.

A slab, square footing or wall is a rectangular box, so the volume is:

A 5 by 2.5 metre slab poured 5 cm thick works out at 0.625 cubic metres, which is 22.07 cubic feet or 0.82 cubic yards.

A hole, column or round footing is a cylinder, so the area of the circular face is multiplied by the depth:

Note that the input is the diameter, not the radius, which is the more common source of a doubled or quartered answer than any other mistake on this page. Halve it before squaring.

A circular slab or tube is a cylinder with a cylinder removed from the middle, so subtract the inner circle from the outer one before multiplying by the height:

A curb and gutter barrier is an L-shaped cross-section run along a length. The upright part of the L is the curb and the flat part is the gutter flag:

Stairs are the awkward one, because the concrete underneath forms a staircase of slabs rather than a simple wedge. Each step sits one rise higher than the one below it, so the volume is the platform plus a series of rectangles that grow by one rise each time:

The n(n − 1)/2 term is just the sum 1 + 2 + … up to the number of risers, which is where the staircase shape enters the arithmetic.

Weight, bags and ordering

Pre-mixed concrete has a density of roughly 2,130 kg/m³, or 133 lbs/ft³. Multiply the volume by that figure and you have the weight to move, which matters as much as the volume when you are carrying bags rather than taking a delivery.

The bag counts follow directly. A 60-pound bag holds about 0.45 cubic feet of finished concrete and an 80-pound bag about 0.60, which is exactly consistent with 133 pounds to the cubic foot. A single cubic yard is 27 cubic feet, weighs about 3,591 pounds, and would take roughly 45 of the 80-pound bags or 60 of the 60-pound ones. That figure is the usual tipping point: much past a cubic yard and mixing by hand stops being sensible, so a ready-mix delivery is both cheaper and far less work.

Order slightly more than the calculator says. Sub-grade is never perfectly level, forms bulge slightly under the weight of wet concrete, and some is always lost to spillage and to what stays in the barrow. Five to ten percent extra is the normal allowance. Running short mid-pour is the outcome worth avoiding: concrete placed against a partly set surface forms a cold joint, a plane of weakness through the slab that no amount of finishing fixes.

Ready-mix suppliers usually charge a short-load fee below about a cubic yard, so on small pours the economics can favour ordering a full yard even if you need less.

Curing and strength

The process of concrete hardening once it has been placed is called curing, and it is slow. Concrete typically takes around four weeks to reach over 90% of its final strength, and it can carry on strengthening for up to three years. Curing is a chemical reaction called hydration rather than simple drying, which is why keeping the concrete damp increases its strength in the early stages. That is achieved by spraying slabs with a compound that forms a film to retain water, or by ponding, where the surface is submerged and wrapped in plastic.

Letting a slab dry out early is the most common way to weaken it. Concrete that loses its water to the sun in the first days stops hydrating and finishes permanently weaker than the same mix kept damp.

The single biggest control over final strength is the water-to-cement ratio. Duff Abrams established in 1918 that strength falls as the ratio rises, and the relationship still carries his name. Extra water makes concrete easier to place and produces a weaker result once cured, which is why adding water on site to loosen a stiff mix is a bad trade. Workability is measured by the slump test, where a cone of fresh concrete is inverted and the sag measured in inches.

Residential concrete is usually specified between 2,500 and 4,000 psi in compression. What the number hides is that concrete is roughly ten times weaker in tension than in compression, which is why it is reinforced. Steel rebar carries the tensile load that the concrete cannot, and the two work together because steel and concrete expand at almost the same rate with temperature. In cold climates, air entrainment is added: microscopic bubbles give freezing water somewhere to expand into, which prevents the surface spalling apart over successive winters.

Weather matters at both ends. Below about 5°C hydration slows sharply and fresh concrete can be ruined by freezing. Above about 30°C it sets faster than it can be finished, which is why large summer pours often start at dawn.

Some background worth knowing

Concrete is the most used building material on Earth and the second most used substance of any kind after water. That scale carries a cost: cement production accounts for roughly 8% of global carbon dioxide emissions, partly from the fuel needed to heat kilns to about 1,450°C and partly from the chemistry itself, since converting limestone to clinker releases CO₂ directly.

The Romans built with concrete two thousand years ago, using volcanic ash called pozzolana, and structures such as the Pantheon dome still stand while modern concrete often degrades within decades. Research published by MIT in 2023 suggested part of the reason is hot mixing with quicklime, which leaves reactive lime clasts through the material that can dissolve and re-seal cracks when water gets in. Roman concrete may be self-healing in a way modern mixes are not.

Using this calculator

Pick the shape that matches your pour, enter the dimensions with each measurement in whatever unit suits it, and set a quantity if you are pouring several of the same thing. Every result is given in cubic feet, cubic yards and cubic metres at once, since suppliers in different countries quote different ones, along with the weight in pounds and kilograms and the equivalent number of 60 and 80 pound bags.

The density used is that of standard pre-mixed concrete. Different mixes vary considerably: lightweight concrete can run under 1,900 kg/m³ and heavyweight mixes used for radiation shielding go well past 3,000, so treat the weight as a solid estimate rather than a figure to size a vehicle by.