What Is Concrete?
Concrete is a mixture of cement, water and aggregate that is placed as a liquid and bought as a volume. Both halves of that sentence matter here: the mixture is why the result is one figure rather than a shopping list, and the volume is why the calculator asks for an area and a depth.
Concrete, Plainly
Concrete is stone and sand held together by hardened cement paste. Cement and water react chemically — hydration, not drying — and the paste that results locks the aggregate into a solid mass. That reaction is why concrete cures under water perfectly well, and why a slab is still gaining strength months after the finishers have gone home.
Proportions vary by job and by region. The National Ready Mixed Concrete Association puts the specified strength for most applications between 3,500 and 5,000 psi (25 to 35 MPa), with a nominal maximum coarse-aggregate size of ¾ or 1 in. None of that touches the arithmetic on this page: a cubic meter of 3,500 psi mix and a cubic meter of 5,000 psi mix fill exactly the same cubic meter.
Cement is not concrete. Cement is one ingredient, the binder, and it is sold by the bag or the tonne. Concrete is the finished material and is sold by volume. Confusing the two is the most common reason an order comes out wrong.
Why Concrete Is Measured by Volume
NRMCA's guidance states it flatly: concrete is sold by volume, in cubic yards or cubic meters, in a freshly mixed and unhardened state, and the basis of sale is set by ASTM C94/C94M. What is being sold is the volume discharged from the truck — not the weight of the ingredients, and not the volume of the hardened slab.
That is why a supplier will ask for cubic yards or cubic meters and nothing else. A truck mixer holds 8 to 12 cubic yards, which is 6.12 to 9.17 cubic meters, so the volume figure also settles how many loads your pour needs.
- Cubic meter (m³) — this calculator's output unit, and the ordering unit through most of the world.
- Cubic yard (yd³) — the US ordering unit. One cubic meter is 1.308 cubic yards; one cubic yard is 0.7645549 m³, which is 27 cubic feet.
- Cubic foot (ft³) — the unit bagged concrete yields are printed in. One cubic meter is 35.3147 cubic feet.
What This Calculator Returns
One line: the words Concrete Volume followed by the figure in cubic meters, rounded to two decimals. There are no breakdown rows beneath it, because nothing else is being computed.
- No waste allowance. Spillage, an over-excavated subgrade and formwork that bows all take concrete the geometry knows nothing about.
- No deduction for reinforcement. Rebar and mesh displace concrete; the result ignores them.
- No deduction for voids, blockouts or drainage sleeves — take those out of the area before you enter it.
- No mix proportions. The answer is finished concrete, not cement, sand and aggregate.
- No cubic yards, no bag counts, no truck loads, no price. One figure, in cubic meters.
Every one of those omissions is deliberate, and each is dealt with further down this page — the unit conversions two sections below, the ordering margin under How to Read Your Result.
How Do You Calculate Concrete?
Volume is area times depth. Everything else is getting those two numbers into consistent units and knowing what the answer does and does not include.
The Concrete Formula, Written Out
Two conversions, then one multiplication:
area_m2 = area × 0.092903 (only if the area unit is square feet) thickness_m = thickness × 0.3048 (only if the thickness unit is feet) volume_m3 = area_m2 × thickness_m displayed = "Concrete Volume: " + volume_m3 rounded to 2 decimals + " m³"
Entries already in square meters and meters pass straight through — no conversion runs on them. Before any of it, both boxes are checked: blank, zero, negative or non-numeric and the calculation stops, answering Invalid input rather than guessing.
Step by Step
- Work out the plan area yourself. The calculator takes an area, not a length and a width, so a 20 ft by 20 ft slab is entered as 400.
- Set the area unit to match — square feet or square meters.
- Convert the thickness to decimal feet or to meters. A 4 in slab is 4 ÷ 12 = 0.3333 ft; a 150 mm slab is 0.15 m. There is no inches box and no millimeters box.
- Set the thickness unit. It is read independently of the area unit, so a mixed pair is legal.
- The tool converts anything imperial to metric, multiplies the two, and rounds the product to two decimals.
- Read the result as cubic meters of placed concrete at plan dimensions — then add an ordering margin before you phone the supplier.
Converting Inches to Decimal Feet
This is where most entry errors happen, because slab thickness is quoted in inches and the box wants decimal feet. Divide inches by 12:
| Slab thickness | Enter as (decimal feet) | Metric equivalent |
|---|---|---|
| 3 in | 0.2500 | 76.2 mm |
| 3½ in | 0.2917 | 88.9 mm |
| 4 in | 0.3333 | 101.6 mm |
| 4½ in | 0.3750 | 114.3 mm |
| 5 in | 0.4167 | 127.0 mm |
| 6 in | 0.5000 | 152.4 mm |
| 8 in | 0.6667 | 203.2 mm |
| 10 in | 0.8333 | 254.0 mm |
| 12 in | 1.0000 | 304.8 mm |
Round that decimal too hard and the shortfall is real money. A 1,200 sq ft floor entered at 0.33 ft returns 11.21 m³; the same floor at 0.3333 ft returns 11.33 m³ — 0.12 m³ apart, which is a wheelbarrow-and-a-half you would be mixing by hand at the end of the pour. Four decimal places is enough; two is not.
Worked Example: a 20 × 20 ft Garage Slab
A two-car garage slab, 20 ft square, poured 4 in thick, worked exactly the way the calculator does it:
- Area: 20 × 20 = 400. Enter 400, area unit square feet.
- Thickness: 4 ÷ 12 = 0.3333. Enter 0.3333, thickness unit feet.
- Area converted: 400 × 0.092903 = 37.1612 m².
- Thickness converted: 0.3333 × 0.3048 = 0.10158984 m.
- Multiplied: 37.1612 × 0.10158984 = 3.7752 m³.
- Rounded to two decimals for display.
- 20 × 20 ft slab at 4 in — calculator output
- Concrete Volume: 3.78 m³
3.78 m³ is 4.94 cubic yards. Applying the 4 to 10 percent ordering allowance puts the actual order between 3.93 and 4.16 m³, or 5.14 to 5.44 cubic yards — comfortably one truck either way.
The Same Calculation in Metric
Set both selectors to metric and the conversions drop out entirely. A 25 m² slab at 150 mm — entered as 25 and 0.15 — returns 3.75 m³, because 25 × 0.15 is 3.75 and nothing is converted on the way.
Millimeters still have to become meters: 100 mm is 0.1, 125 mm is 0.125, 150 mm is 0.15, 200 mm is 0.2. A 50 m² garage floor at 150 mm returns 7.50 m³, which is 9.81 cubic yards.
Mixed units are legal and occasionally useful. Entering 100 sq ft with a thickness of 0.1 m returns 0.93 m³ — a shade under the 0.94 m³ the same slab gives at 0.3333 ft, because 0.1 m is 3.94 in, not 4.
Odd Shapes: Split the Pour, Then Add
One area and one thickness go in, so an L-shaped patio, a slab with a step in it, or a driveway that flares at the apron has to be broken into rectangles and run in pieces.
Take an L made of a 20 × 12 ft leg and a 12 × 8 ft leg, both at 4 in. Run separately, 240 sq ft returns 2.27 m³ and 96 sq ft returns 0.91 m³, for 3.18 m³. Add the areas first instead and enter 336 sq ft at 0.3333 ft, and the answer is 3.17 m³. The 0.01 m³ gap is two-decimal rounding compounding across two runs, not an error in either method — adding the areas first is the more accurate of the two.
Deduct voids before you type the area. A 400 sq ft slab with a 4 × 5 ft blockout in it is 380 sq ft, which returns 3.59 m³ rather than 3.78 m³: 0.19 m³ you would otherwise have paid for and had nowhere to put.
The Conversion Factors Behind the Result
The Two Constants
Two numbers do all the unit work in this tool, and both are fixed in the code:
1 square foot = 0.092903 m² (exact value 0.09290304) 1 foot = 0.3048 m (exact by definition) 1 cubic foot = 0.092903 × 0.3048 = 0.0283168344 m³ 1 cubic yard = 27 cubic feet = 0.7645549 m³ 1 cubic meter = 35.3147 cubic feet = 1.308 cubic yards
NIST Special Publication 811 lists the foot as exactly 0.3048 m and the square foot as exactly 0.09290304 m². The cubic foot (2.831685 E-02 m³) and the cubic yard (7.645549 E-01 m³) are published as rounded values, because they inherit those two exact factors and then have to be truncated somewhere.
Turning the Result Into Cubic Yards
The calculator prints cubic meters only, so a US order needs one more step: divide the figure by 0.7645549, or multiply it by 1.308. Every row below is one of this calculator's own outputs converted that way.
| Calculator result | Cubic yards | Cubic feet | Inputs that produce it |
|---|---|---|---|
| 0.94 m³ | 1.23 yd³ | 33.2 ft³ | 100 sq ft at 0.3333 ft |
| 1.36 m³ | 1.78 yd³ | 48.0 ft³ | 144 sq ft at 0.3333 ft |
| 3.78 m³ | 4.94 yd³ | 133.5 ft³ | 400 sq ft at 0.3333 ft |
| 5.44 m³ | 7.12 yd³ | 192.1 ft³ | 576 sq ft at 0.3333 ft |
| 9.06 m³ | 11.85 yd³ | 320.0 ft³ | 640 sq ft at 0.5 ft |
| 11.33 m³ | 14.82 yd³ | 400.1 ft³ | 1,200 sq ft at 0.3333 ft |
Suppliers sell in their own increments and will tell you what those are. The point of converting is to open the conversation holding a number in their unit rather than yours.
Where the Rounding Shows
Three roundings sit between your tape measure and the displayed figure, and only one of them deserves any attention.
First, the square-foot constant. The code uses 0.092903 where the exact value is 0.09290304 — short by about 4.3 parts in ten million. It cannot move the second decimal of the answer until the pour passes roughly 11,600 m³, which is a large bridge deck, not a driveway.
Second, the two-decimal display. Tiny pours lose proportionally the most: 1 sq ft at 1 ft deep — one cubic foot — shows as 0.03 m³ against a true 0.0283 m³. At the scale of a real slab it is invisible.
Third, the decimal you type for an inch measurement. That one is yours, and it is the only rounding on the list large enough to change what arrives on the truck.
Concrete Volume Chart for Common Slab Sizes
Every figure below is this calculator's own output for the inputs shown. The cubic-yard columns convert the displayed cubic-meter figure at 1 m³ = 1.308 yd³.
| Pour | Area entered | 4 in (0.3333 ft) | in cubic yards | 6 in (0.5 ft) | in cubic yards |
|---|---|---|---|---|---|
| 3 × 3 ft equipment pad | 9 sq ft | 0.08 m³ | 0.10 yd³ | 0.13 m³ | 0.17 yd³ |
| 10 × 10 ft patio | 100 sq ft | 0.94 m³ | 1.23 yd³ | 1.42 m³ | 1.86 yd³ |
| 12 × 12 ft shed pad | 144 sq ft | 1.36 m³ | 1.78 yd³ | 2.04 m³ | 2.67 yd³ |
| 16 × 20 ft single garage | 320 sq ft | 3.02 m³ | 3.95 yd³ | 4.53 m³ | 5.93 yd³ |
| 20 × 20 ft two-car garage | 400 sq ft | 3.78 m³ | 4.94 yd³ | 5.66 m³ | 7.40 yd³ |
| 24 × 24 ft garage | 576 sq ft | 5.44 m³ | 7.12 yd³ | 8.16 m³ | 10.67 yd³ |
| 16 × 40 ft driveway | 640 sq ft | 6.04 m³ | 7.90 yd³ | 9.06 m³ | 11.85 yd³ |
| 30 × 40 ft shop floor | 1,200 sq ft | 11.33 m³ | 14.82 yd³ | 16.99 m³ | 22.22 yd³ |
Doubling the area doubles the volume; going from 4 in to 6 in multiplies it by one and a half. The two-car garage is the clearest case — 3.78 m³ at 4 in against 5.66 m³ at 6 in, half as much concrete again for exactly the same footprint.
Metric Slab Chart
Metric entries skip both conversions, so these rows are straight products of the two numbers typed in.
| Area | Thickness | Entered as | Result | In cubic yards |
|---|---|---|---|---|
| 10 m² | 100 mm | 10 and 0.1 | 1.00 m³ | 1.31 yd³ |
| 12 m² | 100 mm | 12 and 0.1 | 1.20 m³ | 1.57 yd³ |
| 20 m² | 100 mm | 20 and 0.1 | 2.00 m³ | 2.62 yd³ |
| 30 m² | 125 mm | 30 and 0.125 | 3.75 m³ | 4.90 yd³ |
| 25 m² | 150 mm | 25 and 0.15 | 3.75 m³ | 4.90 yd³ |
| 50 m² | 150 mm | 50 and 0.15 | 7.50 m³ | 9.81 yd³ |
| 100 m² | 200 mm | 100 and 0.2 | 20.00 m³ | 26.16 yd³ |
The 30 m² at 125 mm row and the 25 m² at 150 mm row land on the same 3.75 m³ from different dimensions, which is the whole point of the calculation: only the product matters, never the shape it came from.
What Another Inch Costs
The same 400 sq ft footprint, run at every thickness a residential slab is likely to be specified at:
| Thickness | Entered as | Volume | Change from 4 in |
|---|---|---|---|
| 3½ in | 0.2917 | 3.30 m³ | −0.48 m³ |
| 4 in | 0.3333 | 3.78 m³ | — |
| 4½ in | 0.3750 | 4.25 m³ | +0.47 m³ |
| 5 in | 0.4167 | 4.72 m³ | +0.94 m³ |
| 6 in | 0.5000 | 5.66 m³ | +1.88 m³ |
| 8 in | 0.6667 | 7.55 m³ | +3.77 m³ |
Every extra half inch on a 400 sq ft slab is roughly half a cubic meter — about 0.6 of a cubic yard. Thickness is a structural decision, set by what the slab has to carry and by what the local code requires, not by the budget: the International Residential Code puts the minimum for a concrete floor slab on ground inside the building envelope at 3½ in, and 4 in is what residential work is normally poured at. The table is here so you know what the decision costs, not so you can shave it.
Slabs, Footings and Columns: What to Enter for Each
The boxes say area and thickness, but what they mean shifts with the shape. Area is always the face you are looking at; thickness is always the depth behind it. Get that mapping right and one calculator covers most of a residential concrete package.
Slabs on Ground
The straightforward case: plan area in, slab depth in. Measure the formed area rather than the excavation, since the excavation is usually larger and its overcut belongs in the waste margin, not in the geometry. A 12 × 16 ft porch at 4 in is 192 sq ft at 0.3333 ft, which returns 1.81 m³.
A thickened edge or a perimeter grade beam is a second, separate entry. Eighty feet of perimeter with a beam 1 ft wide and 8 in deeper than the slab is 80 sq ft at 0.6667 ft, or 1.51 m³ on top of the slab figure. Run it, then add it — the calculator will not carry two shapes at once.
Footings and Pads
For a footing, the area is its plan area and the thickness is its depth. A strip footing 1.5 ft wide running 60 ft is 90 sq ft; at 1 ft deep it returns 2.55 m³.
Pier footings are small enough that batching them saves time. One 2 × 2 ft pier 3 ft deep is 4 sq ft at 3 ft: 0.34 m³. Four identical piers share that depth, so 16 sq ft at 3 ft returns 1.36 m³ in a single run. The condition is that the depth really is identical — the moment one pier goes deeper for a soft spot, it needs its own entry.
Columns and Walls
Turn the shape on its side. For a column the area is the cross-section and the thickness is the height: a 1 × 1 ft column standing 10 ft is 1 sq ft at 10 ft, or 0.28 m³. A 12 × 18 in column — 1.5 sq ft — standing 9 ft returns 0.38 m³.
A wall reads either way round, because multiplication does not care which face you call the area. Enter the elevation area with the wall thickness as the depth, or the plan cross-section with the wall height as the depth; both produce the same product. Round columns work too, provided you compute πr² yourself first — the area step is the one thing this tool cannot do for you.
How to Read Your Result
The Number Is Plan-Dimension Volume
What comes back is the volume your drawing says the pour occupies: the geometric volume of the formed space, on the assumption that the forms hold their shape and the subgrade sits exactly where you cut it. It is not the volume that will leave the truck, and it is not the quantity to order.
That distinction is not pedantry. ASTM C94 carries a note, quoted in NRMCA's guidance on yield, that the volume of hardened concrete may be — or may appear to be — less than expected because of waste and spillage, over-excavation, spreading forms, some loss of entrained air, or settlement of wet mixtures, none of which are the producer's responsibility. The gap between geometry and reality is expected, documented, and yours to plan for.
How Much to Actually Order
NRMCA's advice is specific: order 4 to 10 percent more than the estimate calculated from plan dimensions, to cover contingencies. Take the tight end for a formed, level pour on a prepared base; take the wide end for footing trenches cut in soft ground, or any pour where the subgrade is rough. Applied to this calculator's own outputs:
| Calculator result | Pour | +4% | +10% | Order range in cubic yards |
|---|---|---|---|---|
| 0.94 m³ | 10 × 10 ft at 4 in | 0.98 m³ | 1.03 m³ | 1.28 – 1.35 yd³ |
| 1.36 m³ | 12 × 12 ft at 4 in | 1.41 m³ | 1.50 m³ | 1.85 – 1.96 yd³ |
| 3.75 m³ | 25 m² at 150 mm | 3.90 m³ | 4.13 m³ | 5.10 – 5.40 yd³ |
| 3.78 m³ | 20 × 20 ft at 4 in | 3.93 m³ | 4.16 m³ | 5.14 – 5.44 yd³ |
| 5.44 m³ | 24 × 24 ft at 4 in | 5.66 m³ | 5.98 m³ | 7.40 – 7.83 yd³ |
| 6.04 m³ | 16 × 40 ft at 4 in | 6.28 m³ | 6.64 m³ | 8.22 – 8.69 yd³ |
Every row there still fits in one truck. The failure worth avoiding is the opposite one: NRMCA is explicit that you should not order excessive concrete or call for small clean-up loads, the second of which can leave a finished crew standing around waiting on a part-load after the plant has closed for the day.
Why the Truck Can Look Short
Depth is almost always the culprit. NRMCA's worked example: an eighth of an inch of extra depth on a 4 in slab is a 3 percent shortage, or one cubic yard in a 32-cubic-yard order. Run the same idea through this tool and it comes out the same size — the 20 × 20 ft slab entered at 0.3438 ft (4⅛ in) instead of 0.3333 ft returns 3.89 m³ against 3.78 m³. That is 0.11 m³, about 0.14 cubic yards, from an eighth of an inch nobody intended to pour.
Forms that bow under the weight of wet concrete, one corner of the subgrade dug an inch low, and the concrete that stays behind in the drum all pull the same direction. So does the material itself: NRMCA puts the in-place volume of hardened concrete at about 2 percent below its fresh volume, from air loss, bleeding, paste volume change and drying shrinkage. On the 3.78 m³ garage slab that is roughly 3.70 m³ of finished concrete from a full order — normal, and not a shortfall anyone owes you for.
Limits: When This Calculator Does Not Apply
The honest boundary of one area multiplied by one depth:
- Anything without a constant thickness. A ramp, a crowned driveway, a tapered footing or a sloping pool floor has no single depth to enter — average the depth by hand, or split the pour into strips and run each one.
- Curved and circular shapes, until you have done the area yourself. The tool multiplies whatever area you hand it, so a round pad is fine once you have worked out πr²; it is the area step it cannot perform.
- Heavily reinforced sections. Rebar and mesh displace concrete and nothing here deducts for them. In ordinary slab work the displacement is far smaller than the ordering margin and simply disappears into the 4 to 10 percent.
- Voids, blockouts, drain sleeves and pipe penetrations — subtract them from the area before entering it, the way the 380 sq ft example above does.
- Mix design. The output is finished concrete, not a cement, sand and aggregate list, and no dry-volume or bulking factor is applied. Multiplying a ready-mix order by a bulking factor would simply inflate it.
- Grade and strength. M20, M25, 3,000 psi and 4,000 psi describe what is inside the mix, not how much space it fills — which is why there is no grade selector on this page.
- Cost. No prices are built in and none should be, because ready-mix rates are local and they move.
Two of those limits belong to the calculator and the rest belong to the method. None of them is a reason to distrust the number; they are a list of the things the number was never measuring.
Volume First, Materials Second
Volume is the first number you need, not the last one. If the concrete is arriving on a truck, volume is the entire conversation and this page has finished its job. If you are batching on site, the split into cement, sand and aggregate starts from a mix ratio and a dry-volume factor rather than from geometry, and that is a genuinely different calculation — it lives in the Cement Calculator.