Calculate exactly how much concrete you need for any pour — slabs, footings, columns, stairs, and more. Results update instantly in cubic metres, cubic yards, cubic feet, and pre-mixed bag counts.
Select the shape that matches your pour from the tabs above — slab, column, tube, curb, or staircase. If your project has multiple shapes, run the calculator for each one separately and add the totals.
Type in your measurements. You can work in metric (metres and centimetres) or imperial (feet and inches) — switch the toggle at any time. The Quantity field lets you calculate for multiple identical pours at once.
Hit Calculate and your results appear instantly below the calculator — in cubic metres, cubic yards, cubic feet, litres, kilograms, and pre-mixed bag count. Add a wastage factor (we recommend 10%) to account for spillage and formwork variation.
Calculating concrete volume is one of the most fundamental tasks in construction. Whether you are pouring a residential driveway, a commercial warehouse floor, or a set of column footings, the basic principle is the same: you multiply the three dimensions of your pour — length, width, and depth — to arrive at a volume in cubic metres or cubic yards.
The basic formula for a rectangular slab is straightforward: Volume = Length × Width × Depth. For example, a concrete slab that is 4 metres long, 3 metres wide, and 0.15 metres deep (15 cm) requires 4 × 3 × 0.15 = 1.80 m³ of concrete. This is the net volume — the exact amount needed to fill the formwork with no room for error.
In practice, you should always add a wastage factor to account for spillage, over-excavation, formwork imperfections, and the concrete that remains in the truck or pump line. A wastage factor of 5–10% is standard for straightforward pours. For complex shapes, uneven ground, or remote sites where re-ordering is expensive, many engineers use 15% or even 20% as a buffer. Under-ordering concrete is far more costly than over-ordering — a cold joint caused by a delayed second load can compromise the structural integrity of the entire pour.
The rectangular slab is the most common concrete shape on any construction site. The formula is simply V = L × W × D, where L is the length, W is the width, and D is the depth or thickness of the slab. This formula works equally well for walls — treat the wall height as one dimension and the wall length as another. Ensure all measurements are in the same unit system before multiplying. A common mistake is mixing centimetres with metres, which produces results that are off by a factor of 100.
For round columns and circular post holes, the volume is calculated using the cylinder formula: V = π × (d/2)² × h, where d is the diameter and h is the height or depth. Accuracy of the diameter measurement is critical — even a small error in diameter is squared in the formula, so it has a disproportionate effect on the final volume. Always measure the diameter at the widest point and use the formwork opening rather than the design diameter if there is any discrepancy.
When you need to calculate the concrete volume for a pipe encasement, a hollow column, or a ring-shaped footing, you use the hollow cylinder formula: V = π × h × ((D₁/2)² − (D₂/2)²), where D₁ is the outer diameter, D₂ is the inner diameter, and h is the height. This formula gives you the volume of the concrete shell only — the hollow centre is excluded. This is commonly used for manhole surrounds, turbine bases, and decorative columns with a hollow core.
Concrete stairs are more complex because each step forms a triangular prism. A practical approximation treats the staircase as a series of right-angled triangles: V = 0.5 × rise × run × width × number of steps, plus the volume of any landing or platform slab at the top. This method gives a reliable estimate for standard straight-run stairs. For curved, spiral, or dog-leg staircases, break the structure into sections and calculate each one individually, or consult a structural engineer who can model the geometry precisely.
The weight of concrete depends on its mix design, aggregate type, and reinforcement. The industry-standard figure for normal-weight reinforced concrete is approximately 2,400 kg/m³ (150 lb/ft³). This is the figure used by this calculator and is appropriate for most structural and non-structural applications.
Lightweight concrete, made with expanded clay or pumice aggregate, typically weighs around 1,800 kg/m³. It is used where reducing dead load is important — for example, on upper floors of multi-storey buildings or in precast panels. Heavyweight concrete, which uses barytes or magnetite aggregate, can weigh 3,200 kg/m³ or more and is used for radiation shielding in hospitals and nuclear facilities.
Knowing the weight of your pour is essential for several reasons. First, you need it to calculate the load on your delivery truck — a standard transit mixer carries 6–8 m³, which means 14–19 tonnes of concrete per load. Second, the weight determines whether your formwork and falsework can support the wet concrete before it cures. Third, the delivered weight is sometimes used to verify the volume if there is a dispute with the batching plant.
For small jobs — fence post holes, minor repairs, small pad footings — pre-mixed bags of concrete are the simplest option. A single 25 kg bag of standard concrete mix produces approximately 0.012 m³ of concrete When mixed with water. That means you need roughly 40 bags of 25 kg concrete per cubic metre. For 20 kg bags, the count rises to about 50 bags per m³, and for larger 40 kg bags, you need approximately 25 per m³.
Pre-mixed bags are convenient but expensive per cubic metre — typically two to three times the cost of ready-mix concrete delivered by a transit mixer. For any pour larger than about 0.5 m³, it is almost always more economical (and faster) to order ready-mix. A transit mixer can deliver 6–8 m³ in a single load, which would take 240–320 bags to replicate by hand — a huge amount of manual labour.
The break-even point varies by region, but a good rule of thumb is: if you need more than 15–20 bags, get a quote for ready-mix delivery. You will save time, labour, and often money.
Ordering concrete for the first time can be daunting. Here are the key points that experienced site engineers always check before placing an order:
1. Mixing units within a calculation. This is the single most common error. A builder measures length in metres, width in metres, but enters thickness in centimetres without converting. The result is a volume that is 100 times too large or too small. Always convert all dimensions to the same unit before multiplying — or use a calculator like this one that handles the conversion automatically.
2. Not adding a wastage factor. Theoretical volume and real-world volume are never exactly the same. Ground is uneven, formwork flexes, concrete sticks to the inside of the pump line, and some material always spills. Ordering exactly the calculated volume guarantees you will run short. A 10% buffer is the industry standard and costs very little compared to the expense of a second delivery.
3. Measuring outside formwork dimensions incorrectly. If you measure the outside of your formwork rather than the inside, your volume calculation will include the thickness of the formwork boards themselves. For a large slab with 25 mm plywood forms, this can add several percent to the calculated area — and therefore to the cost.
4. Using volume for weight without checking density. Not all concrete weighs 2,400 kg/m³. Lightweight mixes, aerated concrete, and mixes with recycite or expanded polystyrene aggregate can weigh significantly less. If you are calculating structural loads or transport weights, always confirm the density of the specific mix you are using with the batching plant.
5. Ordering ready-mix without checking truck access. A standard 8 m³ transit mixer is 8–10 metres long, 2.5 metres wide, and weighs over 30 tonnes fully loaded. If it cannot reach your pour location, you will need a concrete pump — which adds cost and requires advance booking. Always visit the site with the delivery company before placing your first order.