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Free Concrete Calculator — Slabs, Footings & Post Holes
Work out how much concrete you need for a slab, trench footing or post holes — in the quantities suppliers actually deliver. Enter your dimensions and the calculator gives you an exact volume, adds a waste allowance, and rounds the order up to real ready-mix 0.2 m³ increments so you're never caught short on pour day.
How to use this calculator
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Choose what you're pouring — a slab, a trench or footing, or post holes.
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Enter your dimensions: length, width and depth for slabs; linear length, trench width and depth for footings; hole diameter, depth and number of holes for posts.
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Adjust the waste allowance if you want a bigger or smaller buffer.
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Read off your order quantity, rounded up to the next 0.2 m³ — the increment ready-mix suppliers work in.
What this calculator assumes
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All measurements are in standard Australian units, with volume in cubic metres (m³).
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A waste allowance is applied to the exact volume to cover uneven ground, spillage and over-excavation. You can adjust it.
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Orders round up to 0.2 m³ increments, matching how ready-mix concrete is sold in Australia.
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This is an estimating tool for planning and quotes. Slab thickness, footing sizes and concrete strength (MPa) for any structural work must come from your engineer's drawings — always confirm the final order with your engineer and supplier.
Frequently Asked Questions (F&Q)
FAQ 1
Question: How much concrete do I need for a slab?
Answer: Multiply length × width × depth in metres to get cubic metres, then add a waste allowance. A 10 m × 5 m shed slab at 100 mm thick is 5 m³ exact — this calculator does the maths for you and rounds it up to a real deliverable quantity.
FAQ 2
Question: Why does the calculator round up to 0.2 m³?
Answer: Ready-mix suppliers in Australia typically batch and charge in 0.2 m³ increments. Ordering the exact calculated volume risks running short mid-pour, which is one of the most expensive mistakes on a concreting job.
FAQ 3
Question: How much extra concrete should I order?
Answer: A common buffer is 5–10% on top of the exact volume. Uneven excavation, soft spots and spillage all eat into your order. Running out mid-pour usually means a cold joint or a costly second truck, so it's cheaper to have a little left over.
FAQ 4
Question: Can I use this for footings and post holes?
Answer: Yes — the calculator has dedicated modes for trench footings (linear length × width × depth) and post holes (diameter, depth and number of holes), as well as slabs.
FAQ 5
Question: Does this tell me what strength concrete to order?
Answer: No. Concrete strength (e.g. 20 MPa, 25 MPa, 32 MPa) depends on the application and your engineer's specification. This tool works out volume only — confirm the mix with your engineer and supplier before ordering.