Concrete Calculator

Choose the shape of your pour, enter its dimensions, and the calculator returns the concrete to order in cubic yards, the volume in cubic meters, and a table of how many 40, 60 or 80 lb bags of dry mix it would take. Slabs and footings are treated as rectangular prisms and columns as cylinders; a quantity field covers several identical pours, and a waste allowance pads the total against spillage and uneven ground.

How the volume is worked out

Each shape reduces to a cross-section times a length, in consistent units. A slab is length times width times thickness; the only trap is that thickness is quoted in inches while the other dimensions come in feet, so it is divided by 12 first.

V = length × width × (thickness ÷ 12)

A footing works the same way with two of its dimensions, width and depth, entered in inches and each divided by 12. A column or post hole is a cylinder, and because the diameter arrives in inches, dividing it by 24 gives the radius in feet directly:

V = π × (d ÷ 24)² × h

The shape volume is then scaled for the number of identical pours and the waste allowance:

total = V × quantity × (1 + waste ÷ 100)

Cubic yards are the total divided by 27, a conversion with no rounding in it: a yard is three feet, so a cubic yard is exactly 3 × 3 × 3 = 27 cubic feet, and the foot behind both is defined as exactly 0.3048 meters. The metric figure multiplies cubic feet by 0.02831685, the factor NIST lists in Special Publication 811 alongside that foot definition, and bag counts divide the total by each bag's yield and round up, because nobody sells a fraction of a bag.

A worked example

Take a 10 by 12 ft patio slab poured 4 inches thick. The thickness is 4 ÷ 12 = 0.333 ft, so the bare volume is 10 × 12 × 0.333 = 40 cubic feet. Divided by 27 that is 1.48 cubic yards, and multiplied by 0.02831685 it is 1.13 cubic meters. An 80 lb bag yields approximately 0.60 cubic feet, so the pour needs 40 ÷ 0.60 = 66.7, which rounds up to 67 bags. Add the default 10 percent waste allowance and the total grows to 44 cubic feet, which is 1.63 cubic yards or 74 bags. The calculator's own defaults, a 10 by 10 ft slab at 4 inches, run the same way: 33.3 cubic feet bare, 36.7 cubic feet with waste, 1.36 cubic yards, and 62 bags of 80 lb mix.

Roman concrete and the Pantheon

Roman builders discovered that a volcanic ash dug near Baiae and around Mount Vesuvius, mixed with lime and rubble, made a concrete that hardens even under water. Vitruvius, writing in the first century BC, gives it a chapter of De architectura (Book II, chapter 6): "There is also a kind of powder which from natural causes produces astonishing results," he begins in Morris Hicky Morgan's 1914 translation, adding that "even when piers of it are constructed in the sea, they set hard under water." Pliny the Elder, in the translation quoted by Berkeley Lab's news service, wrote that such concrete, once submerged, "becomes a single stone mass, impregnable to the waves and every day stronger." The ash itself came to be called pozzolana after Pozzuoli on the Bay of Naples.

The Pantheon in Rome was completed under the emperor Hadrian and probably dedicated around AD 126, though its facade still carries Marcus Agrippa's name from a predecessor that burned in AD 80 and again in 110, and some scholars argue the present building was begun under Trajan in 114. Its dome remains the largest unreinforced concrete dome in the world at about 43 meters, or 142 feet, across, careful sources splitting between 43.2 and 43.3, a record held for some 1,900 years. The builders graded the aggregate as the dome rose, from travertine at the base through broken tile to porous tufa and pumice at the crown, and thinned the shell as they went; an MIT course page on the building puts it at only about 1.2 meters thick around the oculus, itself reported at about 8 to 9 meters across depending on the source. In 2017 a team led by Marie Jackson at the University of Utah showed in American Mineralogist that Roman marine concrete grows interlocking phillipsite and aluminous tobermorite crystals as seawater percolates through it, adding cohesion over the centuries, and in January 2023 Admir Masic, Linda Seymour and colleagues at MIT reported in Science Advances that hot mixing with quicklime left reactive lime clasts that let the material heal its own cracks when water gets in. Neither finding claims Roman concrete was stronger than the modern material; both explain why it has lasted.

From Aspdin's patent to the ready-mix truck

The modern chapter opens with a name. On 21 October 1824 Joseph Aspdin, a Leeds bricklayer, was granted British Patent 5022, enrolled that December, for what the specification, as transcribed by the Cement Kilns archive, titles "An Improvement in the Modes of Producing an Artificial Stone"; later accounts often render it as mode, singular, but the enrolled text has modes. The patent describes "a cement or artificial stone" for stuccoing buildings and waterworks, "and which I call Portland cement", a name the standard account says he chose because the hardened material resembled Portland stone, the prestige limestone quarried on the Isle of Portland in Dorset. What he patented was the name and a lower-fired process rather than the exact material sold as Portland cement today, which emerged from later, harder-fired refinements, his son William's among them.

Delivery caught up with chemistry in the twentieth century, though every date here rests on trade retrospectives such as Agg-Net's history of the industry rather than on primary records. Those retrospectives usually credit the idea to Germany, where the architect Jurgen Heinrich Magens patented ready-mixed concrete in 1903, and place the first American delivery of centrally mixed concrete in Baltimore in 1913. Stephen Stepanian of Columbus, Ohio filed a patent application for a motorized transit mixer in 1916; the patent office rejected it, and the first load of truck-mixed concrete waited until about 1926, with the National Ready Mixed Concrete Association following in 1930. The bagged mix this calculator counts is the small-job descendant of all that: dry materials batched and packaged under ASTM C387 and rated by both Quikrete and Sakrete at 4,000 psi of compressive strength at 28 days.

Bags, yields and when to call a truck

The bag table uses the yields the two major vendors publish in their technical data sheets: approximately 0.30 cubic feet from a 40 lb bag, 0.45 from a 60 lb bag and 0.60 from an 80 lb bag. Quikrete's document attaches the word approximately to every figure and covers sizes the table omits, 0.375 cubic feet for a 50 lb bag and 0.675 for a 90 lb bag it flags as regional; Sakrete's data sheet puts the 90 lb bag at 0.66, so even the vendors disagree at the margins. Dividing 27 by the yields gives the numbers worth remembering: one cubic yard takes 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix. Metric 25 kg bags have no equally settled figure; published yields cluster around 12 liters but vary by manufacturer, so check the number printed on the bag. Finished normal-weight concrete weighs approximately 140 pounds per cubic foot by the Quikrete sheet's ASTM C138 figure, so a placed yard comes to about 3,780 pounds. Once a pour reaches a full yard, mixing 45 bags by hand rarely beats one continuous discharge from a truck; that threshold is practicality, not a published rule.

Assumptions and caveats

The 10 percent default waste allowance is editorial, not an industry standard. No vendor mandates a figure; Quikrete's own calculator simply warns that all yields are approximate and make no allowance for uneven substrate or waste. Five to ten percent is a common rule of thumb for pours on well-prepared ground, and the field accepts 0 to 25 percent for rougher excavations. The yields are vendor figures for the standard Quikrete and Sakrete concrete mixes; fast-setting and other specialty blends yield differently, so check the figure printed on the bag. Nothing here is structural advice. Quikrete's mix is designed for pours 2 inches thick or more and 4 inches is typical for walkways and patios, but slab thicknesses, footing dimensions and post-hole depths must follow local building codes, a point the Quikrete data sheet itself makes for load-bearing applications. Quikrete and Sakrete are trademarks, cited here as the source of the yield figures rather than as an endorsement.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

A cubic yard is 27 cubic feet and an 80 lb bag yields approximately 0.60 cubic feet, so 27 divided by 0.60 gives 45 bags. On the same basis a 60 lb bag works out to 60 bags per yard and a 40 lb bag to 90. The yield figures come from the Quikrete and Sakrete technical data sheets, and Quikrete labels every yield approximate.

How many bags of concrete do I need for a 10x10 slab at 4 inches?

Multiply 10 by 10 by 4 and divide by 12 to get 33.3 cubic feet. At approximately 0.60 cubic feet per 80 lb bag that is 56 bags before any allowance, or 62 bags once 10 percent waste is added. At that scale it is worth pricing a ready-mix delivery against mixing five dozen bags by hand.

How much water does a bag of concrete mix take?

Quikrete's data sheet starts a 40 lb bag at 3 pints of water, a 60 lb bag at 4 pints and an 80 lb bag at 6 pints, with maximums of 4.5, 7 and 9 pints respectively. Final water content for an 80 lb bag is approximately 6 to 9 pints, which is 2.8 to 4.3 liters. Too much water weakens the mix, so start low and add gradually.

How strong is bagged concrete mix?

The standard Quikrete and Sakrete concrete mixes are both rated at 4000 psi of compressive strength at 28 days, and both data sheets cite ASTM C387, the specification for packaged dry combined materials for concrete; Quikrete names ASTM C39 as the compressive test method. Strength is only part of the picture, though: load-bearing work still has to follow local building codes for footing dimensions.

Should I order ready-mix concrete instead of bags?

At the published yields a single cubic yard takes about 45 bags of 80 lb mix, each mixed by hand or in a small drum mixer. The threshold is practicality rather than a published rule: pours of a yard or more usually favor a ready-mix truck, while post holes, small pads and repairs stay bag territory.

Why should I buy more concrete than the calculation shows?

Vendor yields are approximate, and Quikrete's own calculator warns that its figures make no allowance for uneven substrate or waste. Ground that is not perfectly level, spillage and mixing losses all eat into the margin, so a 5 to 10 percent allowance is a common rule of thumb rather than an official standard. This calculator defaults to 10 percent and accepts 0 to 25.