Concrete Volume Calculator
Estimate how many cubic metres of concrete to order for a rectangular slab, footing, or path. Multiply length × width × depth for the in-place volume — add a 5–10% waste allowance before placing the order.
Last updated: May 2026
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About this calculator
The formula is V = L × W × D, where L is length, W is width, and D is depth (or thickness) of the concrete element, all in metres. The result is the geometric in-place volume in cubic metres (m³), which is how ready-mix concrete is sold. For non-rectangular shapes, decompose into rectangular pieces and sum: an L-shaped slab is two rectangles; a circular footing is V = π × r² × D. The calculator handles only the rectangular case, which covers most residential slabs, garage floors, driveways, sidewalks, footings, and pads. Variables: L, W, D all in metres; output in m³. Edge cases: real-world pours never use exactly the geometric volume. Ready-mix suppliers expect you to order 5–10% extra for waste — spillage, ground absorption, formwork gaps, and surface levelling — so a 1.00 m³ calculated volume should be ordered as 1.05–1.10 m³ from the supplier. For very small jobs (< 0.5 m³), bagged concrete is usually cheaper than ready-mix; one 20 kg bag yields about 0.009 m³ of placed concrete, so a 0.1 m³ job needs roughly 11 bags. For deep footings on uneven ground, the actual concrete needed can exceed the nominal volume by 10–20% as wet concrete settles into excavation irregularities — increase your waste factor accordingly. For very thick mass pours (over ~1.5 m), heat-of-hydration concerns drive pour sequencing in lifts and the simple volume figure understates planning complexity.
How to use
Example 1 — Garage floor slab. You're pouring a 6.0 m × 4.5 m garage slab, 0.12 m (12 cm) thick. Enter length = 6.0, width = 4.5, depth = 0.12. V = 6.0 × 4.5 × 0.12 = 3.24 m³. ✓ Order ~3.5 m³ from the ready-mix supplier (about 8% waste allowance). Verify by units: m × m × m = m³. Example 2 — Strip footing for a small extension. A footing 8.0 m long, 0.45 m wide, 0.30 m deep. Enter 8.0, 0.45, 0.30. V = 8.0 × 0.45 × 0.30 = 1.08 m³. ✓ Order ~1.20 m³ (10–11% margin) since deep narrow footings often lose more to formwork gaps and uneven excavation than a shallow slab. Total cost at ~$160/m³ ready-mix: roughly $190.
Frequently asked questions
How much waste should I add to the calculated concrete volume?
For a simple rectangular slab on level, well-prepared ground, 5–7% extra is enough — small spillage, surface levelling, and minor formwork irregularities. For footings or excavations on uneven or soft ground where the wet concrete can settle into irregularities, increase to 10%. For complex shapes (multiple footings poured in sequence, formwork with corners and recesses), 10–15% is realistic. The cost of ordering slightly too much is much smaller than the cost of stopping mid-pour to call back for another truck — that almost always exceeds the cold-joint risk it would create. Most ready-mix suppliers also have a minimum order (typically 1 m³) and charge surcharges for partial loads, so for small jobs the waste percentage matters less than meeting the minimum.
When should I use bagged concrete versus ready-mix?
Bagged concrete is typically the right choice for jobs under about 0.5 m³ — small footings, post holes, fence anchors, and repairs. A 20 kg bag yields roughly 0.009 m³ of placed concrete, so each cubic metre needs about 110 bags. Above 0.5 m³ the labour to mix that many bags by hand or with a small site mixer starts to exceed the savings versus ready-mix delivery. Above 1–2 m³, ready-mix is almost always cheaper and faster, and produces a more consistent mix because all of it comes from one supplier-controlled batch. The threshold also depends on access: tight urban sites where a concrete truck cannot reach favour bagged work over ready-mix despite the higher per-m³ cost.
How do I calculate concrete volume for non-rectangular shapes?
Decompose into simple shapes and sum. For a circular footing, V = π × r² × D where r is radius and D is depth. For an L-shaped slab, split into two rectangles and add their volumes. For trapezoidal cross-sections typical of retaining walls, V = ((b₁ + b₂) / 2) × h × length, where b₁ and b₂ are the top and bottom widths. For irregular shapes that don't decompose cleanly, take measurements on a grid and approximate as a sum of small rectangular prisms. Most residential and light commercial concrete work decomposes into rectangles and circles — only specialty architectural work needs more complex shapes. Always sketch the geometry before computing, and double-check that you're using consistent units throughout.
What are the most common mistakes people make estimating concrete volume?
The first is mixing units — entering depth in centimetres while length and width are in metres produces results 100× too small. Stay in metres throughout, or convert everything to a single unit first. The second is forgetting waste allowance and ordering exactly the geometric volume; running short mid-pour creates a cold joint that weakens the slab. The third is treating the calculator's output as a precise spec rather than a planning estimate; real pours have natural variation from formwork, ground irregularities, and finishing. The fourth is ignoring reinforcement, which displaces some concrete volume — typically less than 2%, but worth noting on heavily reinforced elements. The fifth is computing slab volume using outside-of-form dimensions when the form will define the inside face — your concrete fills to the form's inside face, so use inside dimensions. The sixth is forgetting that formwork compression and ground absorption reduce the effective placed depth slightly on soft ground.
When should I not use this calculator?
Skip it for circular, trapezoidal, or irregular pours — decompose into rectangles or use a shape-specific calculator (cylinder calculator for circular footings, etc.). Avoid it for prestressed or post-tensioned concrete elements where reinforcement displacement and dimensional tolerances matter at the percent level; consult the structural engineer's takeoff. It is the wrong tool for shotcrete (sprayed concrete) work where overspray and rebound add 20–30% material loss that the simple volume formula does not capture. Do not use it as the only input for cost estimation; total concrete cost includes delivery fees, pump rental for hard-to-reach pours, surcharges for short loads, weekend rates, and finisher labour, all of which scale poorly with the raw volume. And for any pour over about 10 m³ in a single load, consult the supplier directly for mix design, delivery schedule, and finishing approach.