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Concrete Calculator

Find how much concrete a slab needs.

How it is calculated

The volume of a rectangular slab is V = length × width × depth. All three dimensions are entered in metres, so a 10 cm slab is 0.1. The tool shows the volume in cubic metres (four decimals), converts it to cubic feet with V × 35.3147, and estimates how many bags are needed by dividing by 0.018 m³ per bag (the yield of a 40 kg bag of mix) and rounding up to a whole bag. All dimensions must be above zero. The figure is the exact geometric volume: it adds nothing for spillage, uneven ground or over-excavation, so many people order 5 to 10% extra.

Worked example

A patio slab 4 m long, 3 m wide and 0.1 m (10 cm) thick. Volume = 4 × 3 × 0.1 = 1.2000 m³. In cubic feet: 1.2 × 35.3147 = 42.38 ft³. Bags of 40 kg: 1.2 / 0.018 = 66.7, rounded up to 67 bags. If you add a 10% allowance for waste, order about 74 bags.

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Common mistakes

  • Entering the depth in centimetres (10) instead of metres (0.1), which gives a volume 100 times too large.
  • Ordering exactly the calculated amount, without an allowance for waste and uneven subbase.
  • Using the same value for a sloped or stepped slab; break it into rectangles and add them.
  • Confusing cubic metres with square metres, since slabs are often quoted by area only.
  • Applying the bag count to a ready-mix delivery; truck loads are ordered by m³, not bags.

About this calculator

This calculator computes the volume of concrete a project needs by multiplying the length, width, and depth of a slab, footing, or similar pour, then converts that volume into practical purchasing units such as cubic yards, cubic meters, or number of pre-mixed bags. Getting the volume right matters because ordering too little means a delayed second pour and a visible seam, while too much wastes money.

DIY builders use it for small projects like a patio, shed base, or fence post footings, while contractors use the same math on a larger scale when ordering ready-mix concrete for a job. It's typically one of the first calculations done when planning a concrete pour, before materials are ordered.

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Comments (1)

  • Editorial team

    The bag count here assumes 0.018 m³ per 40 kg bag. Bag sizes and yields vary between products, so check the yield printed on the bag you plan to buy and divide the cubic-metre figure by it yourself. A 25 kg bag, for example, yields less, so you would need more of them. Measuring the depth at several points on the prepared ground also helps, since that is where the volume usually drifts. Which bag size is common where you buy your mix?

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