Enter the length and width in metres and the thickness in centimetres, and the tool works out the concrete volume in cubic metres and litres — plus a rough count of 25 kg ready-mix bags. Useful for a slab, path, footing or small foundation. It runs on your device.
Enter a value.
Enter length, width and thickness to see the concrete volume.
Bag count is an estimate (~11 L per 25 kg bag) — check the product. Add 5–10% for waste and order ready-mix above ~0.5 m³.
How it works
Volume is length × width × thickness, with the thickness converted from centimetres to metres. A 4 m × 3 m slab 10 cm thick is 4 × 3 × 0.1 = 1.2 m³. That’s the figure to give a ready-mix supplier, who delivers by the cubic metre.
The bag estimate divides the volume by roughly 0.0113 m³ (about 11 litres) per 25 kg bag of dry mix and rounds up. Yield varies by product and how wet you mix it, so treat it as a guide and check the bag. For anything above about half a cubic metre, ordering ready-mix is usually cheaper and easier than mixing bags by hand.
Practical examples
A garden shed base
A 3 m × 2.5 m base at 10 cm thick is 0.75 m³ (750 litres). That’s roughly 67 bags of 25 kg mix — well past the point where ordering ready-mix makes sense.
A strip footing
A footing 8 m long, 0.4 m wide and 20 cm deep is 8 × 0.4 × 0.2 = 0.64 m³. Add a little for uneven trench bottoms when you order.
A few fence-post holes
For post holes, treat each as its own small slab. A 0.3 m × 0.3 m hole 50 cm deep is 0.045 m³ — about 4 bags each, so bag mix is fine for a handful of posts.
Frequently asked questions
What units does this use?
Length and width in metres, thickness (or depth) in centimetres. A 12 cm slab is entered as 12. The result comes out in cubic metres and litres, the units suppliers and bags quote.
How accurate is the bag estimate?
It’s a guide. One 25 kg bag yields roughly 11 litres of wet concrete, but that shifts with the product and water added. Always round up and buy a bag or two spare — running short mid-pour is far worse than a leftover bag.
When should I order ready-mix instead of bags?
Roughly above half a cubic metre. Beyond that, the number of bags and the effort of hand-mixing add up fast, and a ready-mix delivery is usually cheaper and gives a more consistent result.
How do I handle an irregular shape?
Split it into rectangular sections, calculate each, and add the volumes. For a stepped or L-shaped slab that’s two or three simple boxes. Round shapes need the area (π × radius²) times the thickness.
Should I add extra for waste or spillage?
Yes — add about 5–10% for uneven ground, over-dig, and spillage, especially in trenches where the base is rarely flat. It’s cheaper than a second delivery to top up.
Does the concrete strength or mix ratio change the volume?
No. Volume depends only on the dimensions. The mix ratio (say 1:2:3 cement:sand:aggregate) affects the ingredients and strength, not how many cubic metres the space holds.
Can I use this for a circular slab or column?
Not directly — this assumes a rectangular shape. For a circle, work out the area with π × radius² (our area calculator does this) and multiply by the thickness yourself.
Are my measurements private?
Yes. Everything is calculated in your browser; none of the dimensions you enter are uploaded or sent to analytics.
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