Work out how much concrete a slab, footing or column needs — in cubic yards, cubic feet and bags.
Concrete Calculator
Inputs
4 in suits patios and walkways. Use 5–6 in for driveways.
Covers spillage, uneven subgrade and over-excavation. 5–10% is typical.
Results
Concrete needed
7.11 yd³
V = length × width × thickness, then ÷ 27 for cubic yards
Calculated with BuildCalculators.com
Scale model
Proportions follow your inputs — thickness is enlarged to stay visible. The numbers are the real ones.
As 80 lb bags
352 bags — 304 more than shown
Every concrete quantity is the same three steps: work out the volume of the form, convert it to cubic yards, then add an allowance for what you will lose. The maths is simple. What makes it error-prone is that the three measurements arrive in different units — length and width in feet, thickness in inches, and the order in cubic yards.
That is two unit conversions hidden inside one calculation, and missing either one throws the answer out by a factor of 12 or 27. Both are large enough that the mistake is usually discovered when the truck arrives.
V = length(ft) × width(ft) × (thickness(in) ÷ 12)V = length(ft) × (width(in) ÷ 12) × (depth(in) ÷ 12)V = π × (diameter(in) ÷ 24)² × height(ft) × quantityyd³ = ft³ ÷ 27order = yd³ × (1 + waste ÷ 100)A cubic yard is a cube three feet on every side — 3 × 3 × 3 = 27 cubic feet. Concrete is measured in cubic feet by geometry but sold in cubic yards, so that division happens on every single order. It is also the step people skip when working quickly, which produces an answer 27 times too large and is at least obvious when it happens.
Because that is how slabs are specified. Nobody says a 0.333 ft slab; they say a 4 inch slab. The calculator takes the unit you actually use and converts it, rather than asking you to do the conversion first and risking an error before the tool even sees your numbers.
A round pier is π × radius² × height. The diameter is entered in inches and the height in feet, so the radius in feet is diameter ÷ 24 — dividing by 2 for the radius and by 12 for the units in one step. Multiply by the number of piers, since deck footings are almost never poured one at a time.
An 80 lb bag of pre-mixed concrete yields about 0.6 cubic feet, a 60 lb bag 0.45, and a 50 lb bag 0.375. Those are manufacturer published figures. Dividing 27 by each gives 45, 60 and 72 bags per cubic yard respectively — the numbers worth committing to memory.
The most common residential pour. A two-car garage slab, 4 inches thick over a prepared base.
Order 8 cubic yards
Plants sell in whole or half yards, so 7.82 becomes 8. The extra 0.18 yd³ is cheap insurance against a short pour, which is the expensive failure here.
A small patio — right at the boundary where bagged concrete stops making sense.
1.63 yd³, or 74 × 80 lb bags
74 bags is nearly 6,000 lb to buy, move and mix. This is well past the point where ready-mix wins — but check the short-load fee first, because at 1.6 yards it may be significant.
A continuous footing 16 inches wide and 8 inches deep, running 40 feet.
Order 1.5–2 cubic yards
Footings use 15% rather than 10%. A trench dug by hand or by machine is never exactly to line, and every inch of over-excavation along 40 feet is concrete you will pay for.
Four round piers, 12 inches across and 4 feet deep.
0.47 yd³ — about 24 × 80 lb bags
Under half a yard. Bags are clearly right here, and mixing on site avoids a delivery charge that would exceed the material cost.
Non-structural guidance. Anything carrying load should be sized by an engineer.
| Use | Typical thickness | Notes |
|---|---|---|
| Walkway, garden path | 3–4 in | Foot traffic only |
| Patio | 4 in | Standard for furniture and foot traffic |
| Shed base | 4 in | Thicken to 6 in for heavy equipment |
| Car driveway | 5–6 in | 4 in will crack under repeated vehicle loading |
| Garage floor | 5–6 in | 6 in if a vehicle lift is planned |
| RV or truck parking | 6–8 in | Reinforcement usually required |
| Bag size | Yield per bag | Bags per yd³ | Approx weight |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 | 3,600 lb |
| 60 lb | 0.45 ft³ | 60 | 3,600 lb |
| 50 lb | 0.375 ft³ | 72 | 3,600 lb |
Raw volume before waste. Add 10% and round up to order.
| Size | Area | Cubic yards | Order |
|---|---|---|---|
| 8 × 10 ft | 80 ft² | 0.99 | 1.5 yd³ |
| 10 × 12 ft | 120 ft² | 1.48 | 2 yd³ |
| 12 × 16 ft | 192 ft² | 2.37 | 3 yd³ |
| 16 × 20 ft | 320 ft² | 3.95 | 4.5 yd³ |
| 20 × 20 ft | 400 ft² | 4.94 | 5.5 yd³ |
| 24 × 24 ft | 576 ft² | 7.11 | 8 yd³ |
| 30 × 40 ft | 1,200 ft² | 14.81 | 16.5 yd³ |
| Condition | Allowance |
|---|---|
| Flat, well-prepared, screeded subgrade | 5% |
| Typical residential slab | 10% |
| Uneven subgrade or irregular shape | 12–15% |
| Hand-dug footing trenches | 15% |
| Machine-dug trenches in soft ground | 15–20% |
Concrete costs split into three parts that behave very differently: the material, the delivery, and the labour to place and finish it. A quote that looks high on material may be cheap overall, and the reverse is just as common.
Figures below are national US averages for 2026 and move with cement prices, fuel and local labour rates. Treat them as a planning range.
Typically $140–$180 per cubic yard delivered for a standard 3,000–4,000 psi mix. Higher-strength mixes, fibre reinforcement, accelerators for cold weather and coloured concrete all add to that. The 24 × 24 garage slab above is 8 yards, so roughly $1,120–$1,440 in material alone.
Most plants charge a surcharge below 3–4 cubic yards, commonly $60–$150, because the truck is tied up either way. This is what makes the 1–3 yard range genuinely awkward: a 2 yard order can cost nearly as much as a 4 yard one. If you are close to the threshold and have somewhere useful to put the extra, ordering up is sometimes cheaper per yard.
An 80 lb bag runs $5–$8, so a cubic yard in bags is $225–$360 — meaningfully more expensive per yard than ready-mix. Bags win below about one cubic yard because they avoid delivery entirely, and because you can pour at your own pace rather than racing a truck.
Labour typically runs $3–$8 per square foot depending on finish. A broom finish is at the low end; stamped, coloured or polished concrete is several times that. On most residential slabs labour is the larger half of the total, which is why material-only comparisons between quotes are misleading.
Excavation and grading, a compacted gravel base (usually 4 inches, sometimes more), formwork, reinforcement mesh or rebar, control joint cutting, and disposal if you are removing an old slab. Removal alone commonly runs $2–$6 per square foot.
Concrete is heavy and cannot travel far economically, so haul distance from the nearest batch plant matters more than for almost any other material. A site 40 minutes from a plant pays noticeably more than one 10 minutes away, and rural pricing reflects that before labour rates are even considered.
The single most expensive error on this page. Typing 4 in a field labelled inches when you meant 4 inches is correct; typing 0.33 because you converted first, then having the calculator convert again, gives you a twelfth of what you need. Enter the number as it appears on the drawing and let the tool handle the units.
Geometry assumes a perfectly flat subgrade and forms that do not move. Neither is true. A subgrade that dips half an inch across a 24 ft slab is nearly a quarter of a yard of extra concrete, and running short mid-pour means a cold joint — a permanent weakness — or an emergency second delivery at full price.
Most slabs sit on 4 inches of compacted gravel. That is a separate material, a separate order and a separate cost, and it is easy to price the concrete carefully while overlooking a base that may add several hundred dollars.
Many slabs are thickened at the perimeter — a 4 inch slab with a 12 inch thickened edge holds noticeably more concrete than the flat calculation suggests. Work the edge out as a separate footing and add it, rather than hoping the waste allowance absorbs it.
Waste covers spillage and minor subgrade variation. It is not a substitute for measuring properly, and it will not save an order based on a wrong dimension. Check the measurements first, then add waste.
The crossover is around one cubic yard. Below it, bags avoid a delivery charge. Above it, you are mixing thousands of pounds by hand — and a large slab poured from bags will not go off at a consistent rate, which shows in the finish.
At 4 inches thick, a 24 × 24 ft slab is 192 ft³, which is 7.11 cubic yards. Add 10% waste and you would order 7.8 — call it 8 cubic yards.
An 80 lb bag of pre-mixed concrete yields about 0.6 ft³, so it takes 45 bags to make one cubic yard. A 60 lb bag yields 0.45 ft³, so 60 bags per yard.
Around 1 cubic yard. That is 45 eighty-pound bags — roughly 3,600 lb to haul and mix by hand. Above that, ready-mix is cheaper and far easier, though most plants add a short-load fee below 3–4 yards.
5–10% on a well-prepared flat subgrade. Go to 15% if the ground is uneven, the forms are irregular, or you are pouring footings into hand-dug trenches.
Five to six inches over a compacted gravel base for cars. Four inches is enough for a patio but will crack under repeated vehicle loading. Go to 6–8 inches for RVs, trucks or anything with a heavy axle.
$140–$180 delivered for a standard 3,000–4,000 psi mix in 2026, before labour. Most plants add a short-load fee below 3–4 yards, commonly $60–$150.
No — this calculator covers the concrete only. The compacted gravel base underneath is a separate material and a separate order, usually 4 inches deep. Work it out with the cubic yard calculator.
You get a cold joint where the first concrete has begun to set before the rest arrives — a permanent plane of weakness through the slab. This is why ordering slightly long is standard practice: surplus costs a little, a cold joint costs the slab.
Concrete Slab Cost per Square Foot — A standard broom-finish slab installs at $6–$12 per square foot in 2026. This guide shows where in that range your project lands, and what the per-square-foot number hides.
How to Calculate Concrete for a Slab — Length × width × thickness, ÷ 27, plus waste. Four steps, two unit conversions, and one worked example you can follow with a pencil.
Embed the Concrete Calculator anywhere — it is free, stays in sync with this page, and sizes itself to fit. The only condition is the credit line in the snippet, which links your readers to the full version with the formulas and worked examples.
This calculator uses published constants from named industry sources, each asserted in an automated test that runs on every build. The formula is printed with the result above, and any correction we make is logged on the changelog.
It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.