Concrete Calculator

A concrete calculator has one job: turn the size of a pour into an order you can place, whether that is a number of bags at the store or a number of cubic yards on the phone to a ready mix plant. This one starts from the job rather than the shape, so a slab, a strip footing, a poured wall, a round column, a row of post holes and a flight of steps each ask for the measurements a builder would actually take. The answer comes back in cubic yards and cubic metres, with 80, 60 and 40 lb bags and 25 and 20 kg bags counted side by side and the waste allowance already included.

If what you need is the volume of a shape that is not a pour, such as a tank, a cone of gravel or a room, the volume calculator covers generic solids and the area calculator handles the footprint. This page stays with concrete.

The only conversion you have to remember

Concrete in the US is sold by the cubic yard, and a cubic yard is 3 ft by 3 ft by 3 ft, which is 27 cubic feet. So every job comes down to the same two steps: find the volume in cubic feet, then divide by 27. Thickness is the trap, because it is measured in inches and has to become feet first. A 4 inch slab is 4 / 12 = 0.333 ft thick, and that is where the old trade shortcut comes from: at 4 inches, one cubic yard covers 81 square feet, so square footage divided by 81 gives cubic yards. At 6 inches the same yard covers only 54 square feet.

In metric countries the unit is the cubic metre, and the arithmetic is kinder because thickness in centimetres divides by 100. One cubic yard is exactly 0.764555 cubic metres, so a metric order is about three quarters of the yard figure. The tool shows both so a US spec and a UK or Australian supplier can be compared without a second calculation.

Slabs, patios and driveways

A slab is length times width times thickness. Concrete Network puts residential patios, walks and driveways at 4 inches and garage floors and heavy duty driveways at 5 to 6 inches; CalculatorSoup's guide adds 6 inches for a hot tub pad. The thickness choice moves the order more than anything else on a slab: a 10 by 10 ft slab is 1.24 cubic yards at 4 inches and 1.85 at 6, which is 56 against 84 bags of 80 lb mix before any waste. Worked through the tool, a 20 by 10 ft patio at 4 inches is 66.7 cubic feet; with 10% waste that is 73.3 cubic feet, 2.72 cubic yards or 2.08 cubic metres, and 123 bags of 80 lb mix if you insisted on mixing it by hand.

Strip footings and poured walls

A strip footing is a long, shallow slab under a wall, so enter the total run length in feet and the width and depth in inches. Under the International Residential Code, section R403.1.1, a concrete footing is never less than 12 inches wide or 6 inches thick, and Table R403.1 widens it for more stories or weaker soil; on 2,000 psf soil a two story light frame house needs 15 inches. Check your local code before you dig, because your building department sets the real number. A poured wall is the same box turned on its side: length times height times thickness, with the height in feet and the thickness in inches.

Round columns and piers

A column or pier poured in a cardboard tube form is a cylinder, so the volume is π times the radius squared times the height. Enter the tube's diameter in inches and the height in feet, and set the count to the number of piers. The count matters more than it looks: four 12 inch piers 8 feet tall are just over 25 cubic feet, which passes a full cubic yard once 10% waste is added.

Post holes, with the post taken out

This is where almost every concrete calculator overestimates. A post hole is not a solid cylinder of concrete, because the post stands in it. Quikrete's data sheet sizes the hole at about three times the post width and one third of the post's height deep, with 6 inches of concrete placed under the post first. The tool follows that: it works out the hole, then subtracts the post from the level of that bottom layer up to the surface. Enter the post's actual size, not its name. A nominal 4x4 is 3 1/2 by 3 1/2 inches and a 6x6 is 5 1/2 inches. For a 4x4 in a 10.5 inch hole 32 inches deep, the concrete is 1.42 cubic feet rather than 1.60, so the post fills about 11% of the hole, and a calculator that ignores it orders 13% too much. Over a fence line of twenty posts that is several bags you do not have to buy.

Steps

Solid steps poured against the ground are a staircase of boxes. The bottom step is one rise tall, the next is two rises tall, and so on, so the volume is width times tread times rise times n(n + 1) / 2 for n steps, plus any landing at the top. The code limits under IRC R311.7.5 are a riser of no more than 7 3/4 inches and a tread of at least 10 inches, with no more than 3/8 inch variation between steps in one flight. Four steps at a 7 inch rise and 11 inch tread, 4 feet wide, come to about 21.4 cubic feet. If you fill the core of a large set of steps with compacted gravel, measure the concrete as the shell you are actually pouring.

Why the waste allowance is 10 percent, and when it should be more

Nearly every calculator tells you to add 10 percent. The reason is in the National Ready Mixed Concrete Association's note on yield: forms bulge, subgrade is never perfectly flat, and thickness errors are proportionally huge. A slab dug just 1/8 inch deeper than planned on a 4 inch pour is 3 percent short, which on a 32 yard order is a whole yard. Hardened concrete in place can also measure about 2 percent less than the fresh volume that was delivered. NRMCA's advice is to order at least 4 to 10 percent over the estimate, and more for irregular work such as footings, walls and fill around pipes. Cemex suggests 10 to 15 percent. The tool defaults to 10 and offers 15 for dug holes and footings, where the ground itself is the form.

What a bag actually makes

Bag counts only make sense against the maker's stated yield. Quikrete's Concrete Mix data sheet gives 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag, and Sakrete's data sheet gives the same 0.60 for its 80 lb bag. That is 90, 60 and 45 bags to the cubic yard. Metric bags are less consistent. Cockburn Cement in Western Australia states 0.009 cubic metres, or 110 bags per cubic metre, for its 20 kg concrete pack, while Blue Circle's 20 kg ready to use concrete in the UK is sold as making around 0.01 cubic metres. The tool counts 20 kg bags at the lower yield so you do not run one bag short, and counts 25 kg bags at about 0.012 cubic metres, the figure UK suppliers quote. Always check your bag's printed yield before a large order, because it changes with the brand.

The number that surprises people is the weight. A cubic yard of 80 lb bags is 45 bags and 3,600 lb of dry mix to lift, tip and stir. Quikrete also asks for about 6 pints of water per 80 lb bag and at least 1 cubic foot of mixer drum for every 80 lb bag mixed at once. That is why the tile row shows the weight to lift under each bag count.

Bags or ready mix

Bags win on small jobs: fence posts, repairs, a pad for a shed. Ready mix wins on anything big: Concrete Network suggests bags for walkways, small patios and footings and ready mix for driveways and larger patios. The switch point is a matter of volume and effort. Cemex will not deliver less than one cubic yard, and UK guidance puts the point where ready mix becomes cheaper somewhere between 0.5 and 1.0 cubic metres. The tool reads the order and says so: under half a yard it says bags, from half a yard to a yard it says price both, and from a yard up it says ready mix. A full truck carries about 8 yards on average and up to 10, so anything well under that attracts a short load fee.

What concrete costs in 2026

Prices vary by region and by how far the plant is from you, so treat these as a sense check rather than a quote. LawnStarter's guide, updated in December 2025, puts ready mix at $120 to $150 per cubic yard, with a short load charge of about $43 per yard below a truckload and delivery of $18 to $20 per yard. SlabCalc's 2026 guide, updated in September 2026, puts delivered ready mix at $125 to $175 per cubic yard with a $50 to $100 short load fee. In the UK, 2026 guidance puts delivered ready mix at £110 to £185 per cubic metre before VAT, with 25 kg bags at roughly £5.50 to £9 each. Ask your plant for its minimum load and its short load fee before you compare against bags, because on a one yard pour the fee can be a large part of the bill.

How much it weighs

Normal ready mix weighs about 150 lb per cubic foot, so a cubic yard is roughly 4,050 lb; Concrete Network and Cemex round that to about 4,000 lb, or two tons. In metric terms that is about 2,300 to 2,400 kg per cubic metre. Bagged mix is a little lighter; Quikrete lists its Concrete Mix at about 140 lb per cubic foot. The weight tile uses 150 lb per cubic foot, which is the figure to plan a trailer, a wheelbarrow route or an elevated form around.

After the pour

Quikrete asks for moist curing for 5 days in weather of 70 °F or warmer and 7 days between 50 and 70 °F, and for protection from freezing for the first 48 hours. CalculatorSoup's rule of thumb is foot traffic after 24 to 48 hours, cars after about 7 days, and near full strength at about 28 days, which matches Quikrete's 4,000 psi rating at 28 days against 2,500 psi at 7. Cut control joints about 1 inch deep every 6 to 8 feet on a slab so the cracks go where you choose.