Concrete Calculator

Enter your dimensions above and the calculator returns the answer in the units suppliers actually sell in — cubic yards, cubic feet, tons and bags.

Typical concrete depth by application

Concrete is ordered by the cubic yard from a ready-mix plant, or mixed from 60 lb and 80 lb bags for small pours.

What the numbers assume

The result uses a bulk density of 150 lb per cubic foot and a default waste allowance of 5%. Both are typical values, not measurements of your material — bagged products and different aggregate sources vary by 10% or more. Change the waste field if your subgrade is uneven or you are pouring against forms with complex edges.

Ready-mix plants almost always have a minimum charge — often 1 cubic yard — plus a short-load fee below about 3 yards, so a 0.5-yard pour can cost nearly as much as a 3-yard one.

Costs shown are material only, based on a national average price you can edit. They exclude delivery, short-form fees, equipment rental and labour.

Ready-mix is ordered by the yard, and priced by the load

A ready-mix plant is not a hardware store with a concrete aisle. You order a truck, and that truck costs the plant about the same whether the drum is full or nearly empty — the driver, the batch time, the washout and the round trip do not shrink with the load. That is why nearly every plant has a minimum charge, often one cubic yard, and a short-load fee on anything under roughly three yards. The per-yard number is the marginal price. The minimum is what you pay.

So the first questions for your plant are not about concrete: what is the minimum charge, what is the short-load fee below three yards, and what delivery fee and window will you get? The $165 per cubic yard default here is a US national average for material only, excluding delivery, short-load and minimum-order fees — the methodology page says the same in more detail. Type your own plant's number into the price field before you believe any total; if a figure looks wrong for your market, the contact page is how it gets corrected.

Load size is capped by weight rather than by the drum. Normal-weight concrete is 150 lb per cubic foot, which makes a cubic yard 4,050 lb. Ten yards is more than 20 tons of concrete, which already puts a mixer close to the 80,000 lb highway weight limit once the truck itself is counted. A 25-yard pour arrives as three trucks nose to tail, and none of them will wait while you finish setting forms.

Where bagged concrete actually wins

Bagged mix is sold by the bag, so you pay for packaging and for the water you add yourself. An 80 lb bag yields about 0.6 cubic feet, which is 45 bags to the cubic yard. At $6.85 a bag that is $308 per yard, or $11.42 per cubic foot; ready-mix at $165 per yard is $6.11 per cubic foot. Bagged is nearly twice the material cost before you lift a bag. A 60 lb bag yields about 0.45 cubic feet, so it takes 60 of them to make the same yard.

The arithmetic flips for small pours, because the plant's minimum applies to the truck and not to your pour:

• A 3 × 3 ft stoop, 6 inches thick, is 9 × 0.5 = 4.5 cubic feet, or 0.17 cubic yards. That is 8 bags of 80 lb mix, about $54.80. Ready-mix would bill a full cubic yard plus a short-load fee — $165 and up before the truck moves.
• A half-yard pour is 13.5 cubic feet, or 23 bags, about $157.55. Still cheaper than a one-yard minimum plus a short-load fee in most markets.
• A full yard is 45 bags, about $308, and 3,600 lb of material to move and mix. At that point the one-yard minimum is the better deal — if the plant will send a truck for one yard at all. In a busy season, plenty will not.

The crossover sits between half a yard and a yard, depending on the fee schedule you were quoted. Above two yards there is nothing to decide: two yards is 90 bags, 7,200 lb, and a full day even with a mixer and a helper. If you are blending your own mix from cement, stone and sand, the sand calculator covers the aggregate side. If you are weighing a concrete pad against a stone one, the material calculator runs every material on the same formula.

The clock is why you order extra, not exactly enough

Standard ready-mix gives you about 90 minutes from batching to the point where it stops being workable. On a hot day, or with a fast-setting mix, the useful window can be closer to 45 minutes. Inside that window the concrete has to come off the truck, get placed, screeded, floated and finished. Outside it, you are not pouring concrete any more, you are moving rock.

A slab is monolithic: one continuous placement. If the truck runs out with six feet left to fill, you cannot pause and resume. You either order a second truck and accept the cold joint where the batches meet — a weak line, and the first place a slab cracks — or you break out what you placed and start again. Both cost several times what the leftover concrete would have cost.

This is the whole argument for the waste allowance: 5% on flatwork over well-compacted, formwork-controlled ground, and 10% on footings or anything with an irregular edge, where concrete disappears into the ground faster than the geometry suggests. The waste field is editable and is the right place to absorb your own uncertainty: on a three-yard pour, moving from 5% to 10% costs about $25, while running short costs a second short-load fee, a cold joint, or a tear-out.

Everything else has to be ready before the truck arrives: forms braced and oiled, reinforcement tied and chaired, a clear path for the chute, and two people minimum with a bull float within reach. The 90-minute clock starts at the plant, not at the end of your driveway.

Thickness comes from the load, not the area

Two 100 sq ft pours need the same volume per inch of depth and completely different depths. Four inches is right for a shed slab, patio or walkway, and it is the default the calculator starts from. A driveway taking cars and light trucks needs 5 to 6 inches with reinforcement. Anything heavier — an RV, a loaded truck — wants 6 to 8 inches reinforced. Steps and stoops start at 6 inches because their edges take a beating. Footings start at 12 inches and go deeper wherever frost reaches further down, and that number is set by local code, not by any national figure.

Concrete is strong in compression and weak in bending, so thickness and steel are what stop a driveway sagging into the soil beneath it — which means the ground under the slab does as much work as the slab. Four inches of concrete over four inches of compacted crushed stone will outperform six inches poured over soft clay every time. The gravel calculator sizes that base layer, and if you want the same structure under pavers, the paver base calculator explains how the compaction is achieved.

Steel holds the slab together, it does not thicken it

Reinforcement changes the concrete volume by less than a rounding error, which is why no calculator here tries to model it — but it is the reason a driveway can be 5 inches thick instead of 8. For a slab on grade, 6 × 6 in welded wire mesh; for a driveway or anything structural, #4 rebar at 12 to 18 inches on center, tied where the bars cross. Neither does much good lying on the gravel. Mesh and bar belong in the middle third of the slab, on chairs or pulled up with a hook as the concrete goes in, with 2 to 3 inches of cover so they cannot rust and spall the surface later. A mat on the bottom is a mat doing nothing.

Control joints are the other half of the same job. Space them at two to three times the slab thickness in inches, so a 4-inch slab gets joints every 8 to 12 feet, and in a slab wider than it is long, break it into panels close to square. Saw-cut them within about 6 to 12 hours, while the concrete is still green enough to cut cleanly. Concrete is going to crack — the only question is whether it cracks along the lines you chose or diagonally across the middle of your new patio.

Reinforcement is cheap insurance and the easiest line item to skip. A 10 × 12 ft slab takes roughly 120 linear feet of #4 bar, under $60 at any lumber yard — less than the short-load fee on a second truck. Tie it, chair it, cut the joints on schedule. The steel holds the concrete together after it cracks, because it will.