What this calculator does
It works out how much concrete a slab, footing, column, pier or stair flight actually needs — and then does the part most volume calculators skip: it turns that geometric volume into an order. You get the net volume, the volume with your over-order allowance added, the mass in tonnes, the number of truck loads at your supplier’s capacity, and the number of bags if the pour is small enough to do by hand.
The net volume is the easy half. The reason pours go wrong is that people order the net volume, and the net volume is never what turns up on site.
The formula
Pure solid geometry, with every dimension in metres and every result in m³:
Rectangular slab V = L × W × T
Strip / pad footing V = L × W × D
Rectangular column V = W × T × H
Circular column / pier V = (π/4) × D² × H
Stair flight V = N × (½ × R × G × W) ← the step triangles
+ N × √(R² + G²) × T × W ← the waist slab
For the stair, N is the number of steps, R the rise, G the going, W the flight width and T the waist thickness measured perpendicular to the slope. Each step is a triangular prism sitting on a sloping slab, and the slab’s length along the slope is N × √(R² + G²) — Pythagoras on one step, repeated. If your flight has no waist (steps cast on fill, or a solid mass), set the waist thickness to zero and only the step triangles count.
Then the ordering arithmetic:
V_net = V_unit × quantity
V_order = V_net × (1 + wastage/100)
mass = V_order × ρ
loads = ⌈V_order / truck capacity⌉
bags = ⌈V_order / bag yield⌉
The cubic yard figure uses the exact international definition — 1 yd = 0.9144 m exactly, so 1 yd³ = 0.764554857984 m³ and 1 m³ = 1.30795 yd³. Ready-mix is sold in m³ in most of the world and in yd³ in the United States; the tool gives you both so you can talk to whichever supplier you have.
Reading the result
Order the wastage-inclusive figure, not the net one. The tool sizes trucks and bags from V_order for exactly this reason. Concrete disappears between the batching plant and the finished element in half a dozen ways, none of which are anyone’s fault:
- Formwork deflects under the wet pressure. A 300 mm wide beam side that bows 5 mm over its height is carrying more concrete than the drawing says.
- Subgrade is never flat. A ground slab nominally 150 mm thick averages 155–165 mm over real hardcore, and the difference is 3–10% of the pour.
- Concrete stays behind — in the drum, on the chute, in the pump line and the hopper, in the barrow.
- Spillage, over-dig at the edges of a trench-fill footing, and the bit you scrape off the top when the level is wrong.
- The last barrow is the one that finishes the surface, and running out of concrete mid-pour creates a cold joint you did not design.
Typical wastage is 5–10%. Use the low end for a formed element on a clean prepared base with tight tolerances, the high end for a ground slab. Trench-fill footings in soft or collapsing ground can justify 15–20% because the excavation itself is oversize — but if you find yourself typing 25%, the problem is the excavation, not the allowance.
Short loads cost money. Most suppliers charge a premium below about 1 m³ and many will not deliver under 0.5 m³ at all. When the tool warns you the volume is small, check the bag count before you book a truck: 0.3 m³ is around 30 bags, which one person can mix in a morning.
Rounding up is the whole point of the truck output. You cannot order 2.4 truck loads. The tool tells you both the exact figure and the whole number you actually book, and warns when the last truck comes out nearly empty — that is a part-load charge, and it is often cheaper to increase the allowance and fill it.
Typical values
| Quantity | Typical range |
|---|---|
| Normal-weight concrete density | 2200–2600 kg/m³; 2400 kg/m³ is the standard design value |
| Lightweight concrete density | 1400–2000 kg/m³ |
| Wastage allowance | 5–10% formed work, 10–15% ground slabs, 15–20% trench fill |
| Ready-mix truck capacity | 6–8 m³ (8–10 yd³), limited by local axle-load rules |
| 20 kg bag yield | ≈ 9–10 L |
| 25 kg bag yield | ≈ 12 L |
| 80 lb bag yield | ≈ 0.6 ft³ ≈ 17 L |
The 2400 kg/m³ figure for reinforced normal-weight concrete appears in essentially every loading standard — EN 1991-1-1 Table A.1, AS/NZS 1170.1, ASCE 7 — but it is a nominal value, and your supplier’s actual mix will differ by a percent or two. Enter theirs if you have it.
Reinforcement is deliberately not deducted. Steel occupies well under 1% of a typical member’s volume and ordering practice ignores it; the wastage allowance swamps the difference many times over.
Worked example
A ground-floor slab, 6.0 m long × 4.0 m wide × 150 mm thick, one of them, with an 8% allowance. Normal-weight concrete at 2400 kg/m³, 8 m³ trucks, 10 L bags.
V_net = 6.0 × 4.0 × 0.15 = 3.600 m³
V_order = 3.600 × 1.08 = 3.888 m³
= 3.888 / 0.764554857984 = 5.09 yd³
mass = 3.888 × 2400 = 9331.2 kg = 9.331 t
loads = 3.888 / 8 = 0.486 → 1 truck
bags = 3.888 / 0.010 = 388.8 → 389 bags
One truck, comfortably. The bag figure — 389 bags, roughly 7.8 tonnes of dry material — is the answer to “should I do this by hand?”, and the answer is no. Note also that the 0.288 m³ of allowance is about 11 mm of extra thickness spread over the slab: that is the realistic tolerance on a 150 mm slab poured on hardcore, which is exactly why the allowance exists.
FAQ
Is the volume the wet volume or the finished volume? The finished, in-place volume. Ready-mix is sold and delivered by compacted in-place volume, so the geometry you enter is what you order. Bag yields printed on the packaging are also wet, in-place yields — that is why a 20 kg bag of dry material makes only about 9–10 L.
Should I deduct openings, box-outs and rebates? Yes, if they are significant — subtract them from the dimensions you enter, or run the openings as a negative-quantity element and take the difference. A single 100 mm service penetration in a slab is not worth the arithmetic; a 2 m × 1 m stair void is.
Why does the mass matter? Two reasons. Access: a 3.9 m³ pour is 9.3 tonnes arriving on one vehicle, which the site access and any suspended deck or scaffold below have to take. And formwork and propping design, where the wet concrete load is the governing case.
How do I handle a slab on uneven ground? Raise the wastage, but also consider whether the base needs regulating first. If you are allowing 15% on a 200 m² slab you are buying 6 m³ of extra concrete to fill hollows that a load of blinding would have levelled far more cheaply.
Does this work for stairs with a nosing? It ignores the nosing, which is the right call for ordering: a nosing overhangs, it does not add concrete beneath. Enter the going as the horizontal step depth excluding the overhang.
This tool provides indicative quantities for ordering and estimating. Confirm delivery volumes, minimum load sizes and mix density with your supplier, and confirm dimensions against the issued-for-construction drawings before you place an order.